Wire size to charging battery? [Archive] - Ausfish fishing Australia, Australian Angling Forums

TheRealAndy

05-01-2008, 10:10 AM

the resistor isnt realy a good idea.

(remember lead acid batteries are a "voltage charge" device as oposed to nickle cad or NiMHi which are current charged and often thru a resistor in cheap chargers.)

It will reduce NOT limit the currect and will do nothing much at all for the end of charge terminal voltage......... if the resistor is big enough to stop the battery overcharging it will make the charge rate unacceptably slow.

Regardless
You will get only a very small charging effect when the engine isnt running and the state of charge of the car battery will determine the charge rate of the small SLA battery.any charging of significance will occur while the engine is running.

yess you can do this BUT

don't leave the battery connected for too long
most definitely watch the temperature
even better watch the terminal volts.

one other and slight better idea ist to run a series diode or two, each diode will reduce the terminal voltage by .6 of a volt, the diodes will also prevent back flow of the current into the cars electrical system when carnking & such.
If the vehicles charge voltage is 15 Volts two diodes will bring that back to 13.8 which is a safer long term charge voltage for SLA batteries, if the charge voltage is a bit lower in the car one diode would be more appropriate.

there are many cheap and nasty ways of doing this that will work a purpose designed regulator would be the best choice.
cheap & cheerfull one or two 20 amp diodes would be the best choice

cheers

Actually a resistors prime purpose in life is to limit current. The only problem with this is that they are a fairly crude current limiting device, and one of the side affects is the the develop a voltage drop across them. So if the resistor mention before (3.3 ohm) is limiting the current to 1amp, then there is actually 3.3V drop across the resistor. The biggest problem with this is that as the battery becomes charged, there is less current drawn and the voltage dropped across the resistor decreases, which means you battery has a higher charge voltage. This is how the crude chargers work in the suprecheap et al rechargeable (they use SLA) laterns and lights.

The diode is good to prevent current flowing back into the battery.

I will draw up a little circuit later than cam be built for under $10 bucks that will solve the problem. Stay tuned.

oddbudman

05-01-2008, 10:34 AM

driveon

05-01-2008, 01:39 PM

Hi Bungie, for charging small batteries like yours, why not just use the same simple method used by most domestic home security systems use to charge their backup battery.

Simply fit a 5 or 10 watt 24v festoon globe in series in the positive wire and it will control the charge going into the battery.

It’s a simple system and the globe automatically self regulates the current based on the state of charge of battery.

The lower the state of charge of the battery, the higher the current going into the battery because of the larger voltage difference across the globe.

As the battery charge rises, the voltage difference across the globe reduces, in turn reducing the amount of current going into the battery.

The only thing you have to take into account is that this is a slower ( but safer ) way to charge a small battery.

Just a suggestion.

TheRealAndy

05-01-2008, 10:34 PM

Heh,

Lets keep this thread interesting....

Interested to see your circuit.... I'll throw my hat in the ring.

Much depends on the terminal voltage of the battery in the car when the car is running. Unless you want to get ultra tech and boost / regulate the voltage.

oddbudman

I normally use a program called protel, but I have not used it for 3-4 years, so I cant draw up a circuit. If I find the CD i will install it, its around somewhere.

Anyhow, the circuit i was going to suggest was based around an LM317 voltage regulator configured as a constant current regulator as in here:
http://diyparadise.com/yhlmccs.html

if you want to get real tricky you could even add another LM317 after the first configured as constant voltage regulator.

oddbudman

05-01-2008, 11:19 PM

TheRealAndy,

The problem you will find with the lm317 is it has a high dropout voltage (ie the voltage it needs to drop to accurately regulate). So when the available charging voltage is so low (as is the case with a car trying to charge a 12V battery), its always going to be an uphill battle. In the circuit you posted you are dropping 1.25V across the resistor and also voltage across the regulator (at least 1.5v) itself, so this sort of circuit would really only work well if the charging voltage was up around 16V.

This is why i originally suggested a simple resistor to do the charge, as the charging voltage is likely to be at voltages a SLA battery can handle to begin with. The resistor was just a simple way to stop too much current going into the battery in the early stages of charge (many SLA batteries have this max charging current and voltage printed on them). The resistor could still cause problems as oldboot suggested such as overcharge, but this would depend on the terminal voltage in the car, how long the battery was left on charge and the SLA battery charge state at the time.

Anyway, I guess we have now bamboozled bungie, without giving him a clear answer.

Bungie -
were you planning on running the car when charging the battery?
do you know the voltage at the car fusebox?

oddbudman

TheRealAndy

06-01-2008, 07:57 AM

YEah goid point oddbudman, didn't even think of that, all I was thinking was simplest way to to make a constant current regulator!

Its been in the back of my mind for a while to do this properly. I was asking about on an electronics forum cheap microcontrollers with free c compilers the other day for this exact purpose. There is a few semi manufacturers that make some shit hot parts for battery charging. My biggest problem would be the design of the magnetics for step up/step down as I have never don it before. You seem pretty clued up oddbudman, whats the story? You in the field? Its been about 4-5 years now since I have done any design, maybe longer. <-- this still does not help bungie!!

As for bungie, I would still be hooking the battery directly, and only using a resistor if the battery draws to much current.

TheRealAndy

06-01-2008, 08:48 AM

I forgot to say to the driveon's solutions is a great one too, and I have seen this used before. The more current you draw through a lighbulb, the hotter it gets and the higher the resistance. These make great simple regulators. Any bulb will do, but a headlight bulb will allow higer charge current, as opposed to an indicator bulb which will be a lower charge current. One could even use a 240 incandecsent bulb, however these are a bit fragile.

driveon

06-01-2008, 10:38 AM

Hi Andy, if your coming off an alternator then you can use just about any size globe.

If you are powering the globe from a power supply that is higher than the maximum tolerable voltage of the battery then it is safer to use the smaller globes as suggested, the smaller globes not only limit the charge current but once the battery is fully charged, the internal resistance of the battery will be sufficient to counter the current coming from the power source and the battery will simply remain fully charged but if you use a larger globe then the current coming through globe will be too high for the battery to take.

So while you use a small globe, you can leave the battery connected to the power supply all the time with no risk of cooking the battery but if you use a large globe, you will either have to remove the charge supply once the battery is fully charged or risk cooking the battery if you don’t remove the charge supply.

Cheers.

davez104

06-01-2008, 12:23 PM

Wouldn't the power from the car alternator be regulated anyway? I must be missing something because I can't see how you would cook a 12V battery by hooking into your car power supply.

Dave.

TheRealAndy

06-01-2008, 06:36 PM

Wouldn't the power from the car alternator be regulated anyway? I must be missing something because I can't see how you would cook a 12V battery by hooking into your car power supply.

Dave.

Its regulated but can supply a lot of amps. Most cars i think have around 75A altenators. IF you let a small battery draw 75amps it would probably explode! SO the trick is to regulate, or limit the current that the smaller battery can draw when charging

oldboot

06-01-2008, 10:48 PM

I have edited my original post here because a lot of the information was either irrelivent or wrong.:-/

cheers

oldboot

06-01-2008, 10:59 PM

yep I remover this one too.
cheers

oddbudman

07-01-2008, 01:03 PM

One point I need to make seperately.

There is no danger of damage to a sealed lead acid battery from being short term charged at 15 volts.............I have done it many times.............many of the clever chargers do it in boost charge mode.

cheers

Yes they can do it in boost mode, but they do set a current limit into the battery.

You just stated ohms law, so if we follow this connecting a 11V battery to a 15V source with basically a short circuit (the hookup wire) is likely to cause a big current spike into the 11V battery. Get a meter and test it. A small 2.5Ah battery is likely to have a inital input current specification of around 0.3C or in this case 0.75A, connecting a empty battery to 15V will get the charge up, but it will damage the battery if you do it.

A series resistor will help to reduce this input current rush, as it can bias the current that is fed into the battery.

Your point on excessive charge voltage into a battery causing overcharging is 100% correct.

oddbudman

finga

07-01-2008, 03:19 PM

I think you fellas have got way over the top of the purpose of the thread.

Bungie just wants to charge his SLA battery from the car.

He wants to put and auxiliary fuse panel in (and yes 4mm wire is dokky dokky but use a fuse to protect the wire from over current situations from the main battery to this extra fuse panel).

Charging of the SLA is easy then.
Just bung one of these in.
http://www.jaycar.com.au/productView.asp?ID=AA0258&CATID=&keywords=charger&SPECIAL=&form=KEYWORD&ProdCodeOnly=&Keyword1=&Keyword2=&pageNumber=&priceMin=&priceMax=&SUBCATID=
A couple of wires to the axillary fuse panel (so it's protected with a fuse and you could even put a switch in if you want to leave it in the ute permanently) and then a couple of wires on the output so you can attach it the your little bubba SLA battery.
Jobs done. All automatic, cost effective, no ifs and no buts about it.

oddbudman

07-01-2008, 05:11 PM

Finga,

The input voltage on that unit is 16-20VDC (solar panel).

The jaycar blurb is a little misleading, it should probably say... "a rectified and filtered ac input of 13.8-15VAC".

http://www.kemo-electronic.com/pdf/m083/m083.pdf

oddbudman

TheRealAndy

07-01-2008, 06:25 PM

Actually, for cyclic use 15v (14.9 actuall) is fine according to this reputable manufacturer:http://www.panasonic.com/industrial/battery/oem/images/pdf/Panasonic_VRLA_LC-R122R2P.pdf

I assume based on the description from bungie that the battery is going to be used for cyclic use.

So the problem then comes down to current limiting. Short of buying some expensive 12v - 12v charger, then i think the light bulb has merits. Sure it wont prevent overcharging, but small SLA'a are cheap and thats the tradeoff between spending hundreds on a proper charger as oppose to throwing away a $20 battery once in a while.

Using the lightbulb and looking at panasonics website, dont leave the battery on for more than 12hours and you will be fine

oldboot

07-01-2008, 06:39 PM

this one too.

cheers

oldboot

18-01-2008, 03:48 PM

Due to the great amount of " strong opinions" including my own, I thought it would be "interesting" to do some actual testing.

And the results were surprising......... in short none of us " experts" got it all right.

So
I statred up my old faithfull corona fishing waggon and ran the motor to let it charge the battery for an hour or so......I checked the battery terminal voltage.
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.come on guess
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13.78 Volts:o ........ I was firmly convinced it would be more......... so I give the motor a good long rev........ nup doesn't move.

So to confirm that the alternator has finished charging & itsn't playing a trick I put my high res DC clamp meter on the alternator lead....5 & bit amps..... consistent with an ignition load....... clamp the battery lead..... diddly squat.

HMMMMMM

I take a well used 12V 7Ah gell cell and flatten it proper with a 50 watt dichroic lamp & let it cool....... resting terminal voltage under 8 volts way below recommended...... the halogen light was well yellow.

I hook this directly to the battery terminals and measure the current with the clamp as it is connected and observe.
initial current just over 7 amps and it reduces rapidly to about 4 amps in under a minute... to drops to under 5 within about 20 or 30 sec.

As ve speak it is charging I expect it to be fully charged in about 2 hours, give or take.

This is all looking a lot simpler than we all thaught, certaily much safer and decent than I expected.

Further testing will insue later today.....I have a near new 1.2Ah 12V cell that I have flattened.

will report later.

So this all looks very viable.......I have hesitated to recomend or use this method myself for many years because of the "accepted problems".

I've been arround car electrica and this sort of stuff but never realy bothered to measure running terminal voltages....... or more likely believed what I have been taught and read & not what I measure.

To a certain extent what I am measuring actualy makes more sence..... or we'dd be stewing battereies left right & centre

I would be most gratefull if various member that have the instruments could measure the terminal voltage of their car & boat battery with the engine running after a good long run.....

I want to establish what is correct and normal.......! decimal place ... or better two.

I now know what I expect to see......

. the alternator on the "rona" is a reasonably bew bosch with inbuilt reg...which is pretty common.

cheers

oldboot

18-01-2008, 05:14 PM

All right testing completed, I find my self red faced, I'm happy to say when I am wrong.

so I found

Yes these SLA battereies will draw excessive current when charged directy from a car charging system.

There seems to be quite a bit of variability between different batteries, and the smaller the battery the bigger the problems.

I tested a couple of cells arround the 7 & 8 AH & they seemed to stablaise at arround half their rated capacity in charge rate....... which is still too high....1/3dr is the recommended maximum rate.
This size of battery warmed up, more than I would like...this wouldnt be wise long term.

The 1.2 AH battery I had on hand drew quite high currents to start with and stabalised at arround 2.5 amps..... which is way too high......and because of the small size the battery heated up very quickly causing me to treminate the test.

So my conclusion on direct connection to charge.......batteries larger than 7Ah you will probably get away with it. but it wont be good for the battery......I would consider it only a viable emergency measure.

Anything smaller.....I would strongly recomend against it.......little 1.2 or 2.5 Ah cells are likely to get very hot and vent acid or worse...... don't do it.

I recon once you got over 20 AH you would be pretty safe....... but I have no proof.

Now the diode thing is totaly useless, because if all the alternators are as I have found & I dont see why they wont be....... terminal voltage isnt a problem.

I tested both the resistor and light bulb options....

I tried 5 watt and 21 watt light bulbs......both reduced the current but neither allowes sufficient current to achieve a rapid enough charge and insufficient current was passed to allow the light bulb to do its "thremal dynamic thing"

The light bulb will work but It isnt all that convienient and you would have to try several light bulbs to get an effective result, and no dynamic action is achieved the light bulb is simply acting as a resistor.

The resistor has my vote, a 3.9ohm resistor reduced the current to arround 300mA which is border line acceptable for the 1.2 ah cell.
The same resistor was tested with a variety of batteries & seemed to result in similar charge currents on all cells.

The problem is the resistor will need to be selected for each different capacity cell, & I would argue by trial and error measuring the charge currents.
A small value resistor say under 4 ohms is unlikely to have any significant effect on end of charge terminal voltage as the current flowing should be very low at this point.

As recomended earlier by someone else a 3.3ohm 5 watt resistor is as good as it will get for a 2.5AH gell cell.........the resistor tested didn't seem to get hot at all.... but one should assume that it will for safety sake..

I intend to do more testing ith different batteries as getting this all down would be very usefull to a lot of people , myself included.

I appoligise for my earlier incorrect asumptions and misleading recomendations.

As to the wire size...... for charging anything up to 10 AH 1.9mm2 wire should be fine, I think that equates to 4mm auto electrical wire. as long as the length was less than say 3 metres.

cheers chaps

TheRealAndy

18-01-2008, 06:40 PM

If there is enough interest I will design a small charging using a topology that makes input voltage irrelevant. I am not confident enough in my abilites to do something for larger batteries, but small batteries less than 5AH or so is no problem. You can probably do it all using one IC these days :). I have had intentions to do this for sometime now, but I have been to lazy.

oldboot

18-01-2008, 08:46 PM

One of the electronics magasines published a dc to dc converter based unit specificlay designed to charge sealed lead acid batteries at one stage.
Jaycar used to stock it.....I haven't looked for it in the catalogue recently.

For small to medium batteries the series resistor isn't wonderfull but it is cheap and works.

The light bulb also works but by the time you buy a big enough light bulb to be effective it starts to look expensive and cumbersome.

Cheers

oddbudman

18-01-2008, 09:27 PM

Interesting reading. I may as well explain a little about what i do.

I design electronic products, many of which use battery chargers.

My most recent product involves a Microprocessor controlled 3 stage Buck converter for charging SLA batteries. Using the processor is great as you can easily set the battery charging parameters using software.

I'm happy to see you did the tests oldboot, as you got to learn something new :) .

Be interested to read your circuit suggestion TheRealAndy. Here's a simple one i thought was quite nifty without going into dc:dc conversion.

http://www.siliconchip.com.au/cms/A_109625/article.html

oddbudman

oldboot

18-01-2008, 10:33 PM

I am a technician...I learn therfore I am.....The day I cant learn something new is the day I need to curl up & die.

Designing a simple workable charger isnt too dificult a thing where you have plenty of voltage to spare, give me a good solid 18V dc & its a straight application notes job.

The problem here is that we are already at ideal charge voltage when the motor is running.......when the motor stops so does all effective charge.

Even putting a diode into the situation brings you back to 13.5V........ the only reasonable option would be a fet based, switching, series pass sort of thing & that is starting to get complicated.

I had a look at that note book application & it looks over complicated to me for what it achieves.
I can take or leave the series diode..... low & behold there is a 3.3 ohm series resistor & another switched to double the charge current.
I dont see any point in reducung charge current as the battery nears charge.... it will do that it's self.

Why not just select a resistor for maximum permisable initail charge current or a bit less and let nature take its course.
I cant see why have series resistors and a PTC thermistor....unless it was glued to the battery as an over temp device

It occurs to me that the circuit would not be able to deliver the permisable 2.5 amps for a fast charge.
In theory the two 3.3 ohm resistors should limit the charge current to about 2 amps assuming battery at 10v and charge terminal at 13.8..... but in reality the treminla voltage of the battery will rise rapidly in the first few minutes & the charge current will end up being more like 600mA after not very long.
so you are going to take at least 12 hours to charge the battery..... that is a long drive in any terms

The use of a PTC thermistor as a series limiting eliment (this one IS a limiter) would seem worthwhile.... however i've had bad experiences with these things.
They arent fast....in fact they are slow as a wet week
their spread of tolerance particularly with temperature is very wide
and they come in a limited range of values.
And they are expensive for what they are.

The more I look at a properly selected resistor, the better it looks.... a 5 or 10 watt resistor will cost watt $1- $1.50 if you pay retail.....Crikey..I've paid more than that for 2 crimp lugs in a time of despiration.

In reality how often do we need to charge a SLA in a vehicle......most of the time we can charge on 240......and there are plenty of good cheap chargers available.
It would be often simpler & cheaper to take either a larger cell or a couple of charged replacements.

Now if a dc to dc converter allowed charging from a stationary larger battery....AHH now that is a lot more use.

cheers

driveon

19-01-2008, 05:42 AM

Hi old boot, with this type of battery you will find the light bulb does work and works better than a resistor.

I started working with domestic security installations a little over 30 ago and at that time it depended on what brand of security panel you bought as to whether you got a resistor or a globe as the charge regulator.

Within the next couple of years every panel you bought had a globe fitted as the charge regulator.

There are a number of advantaged to using a globe but the main one over a resistor, relating to this thread and some findings you have posted, is that you don’t have to match the globe to the size of battery. One size globe fits all size batteries up to say about 15 A/H.

The one thing that you do have to take into consideration is that a globe will take longer to fully charge the battery but this is intended as the battery can be left indefinitely connected to the power supply.

This is the main reason resistors were eventually phased out. If the resistor was not matched exactly, based on both the battery size and the charging voltage, the battery was either never fully charged or was over charged and in both cases, the battery had a short life.

Using a globe simply gets around all these problems, particularly when relating to the varying voltages that are going to be a common place in the type of situation being covered in this thread.

Cheers

oddbudman

19-01-2008, 10:03 AM

driveon, you raise some good points about using a globe,

The globe would have a positive temperature coefficient. This would be useful in both the high current stages of charging and the low current stages towards the end of charge.

Also each globe is likely to have a reasonable level of tolerance.

I would be interested to know the charge voltage used in the security systems as well as the globes that are used.

oldboot,

I'm interested to know what type of technician you are? You are right about that circuit not lasting very long in the high current stage. With a charging voltage of 13.8V, its likely to switch over at around 12.7V on the sla battery.

cheers,
oddbudman

driveon

19-01-2008, 06:16 PM

Hi oddbudman, I can give you exact info because from 1988 to 2000, I manufactured my own Security panels.

My panels were specifically designed for domestic installations and as such were designed to use very little power as compered to the commercial panels that were and still are available.

Commercial panels are designed to last between 8 and 24 hour on their backup battery, mine were designed to last between 1 and 2 week on the same size 7 A/H 12v backup battery.

The panel was supplied by a two wire plug pack that supplied 15 VAC to a rectifying bridge and then to a 12 DC volt regulator, tricked up to 13.7 DC volts.

This 13.7 DC supplied power to the detectors and siren driver and to a 5 volt reg that powered the PCB which included a wireless remote control receiver.

13.7v was also fed via a 5w 24v festoon globe, to the 7 A/H backup battery.

A low battery would take up to a week to fully recharge but as posted earlier, was connected permanently to the power supply and as many of the systems I not only made but also fitted, are still powered by the original batteries, the globe set up obviously works.

Cheers.

oldboot

19-01-2008, 11:11 PM

my background is orriginaly Telecom working in an obscure section that dealt with lots of weird stuff including modems when they were as big as 2 phone books.
But thats a long time ago. They called it electronics but I seemd to do a lot of electro mechanics as an apprectice.

I've done all sorts of stuff, security , intercoms, mining phones, nurse call, fire evac, parking equipment TV antenna / matv, closed circuit TV, wired specilaised truck bodies, and lots of stuff no one else would touch.
These days I work mostly in pro audio and commercial AV, & do a bit of related timber & metal work.......the weirdest thing Ive done recently is some electro mechanical support for an Illusion builder.

Yeh I remember servicing some of those early alarm panels, ( OH there was some horrible stuff) they were notorious for being hard on batteries and not comming up to charge quick enough after black outs...( remember the SEQEB dispute).
Fortunatly all the recent stuff arround my time was 3 terminal reg based & charged at 1 to 1.5 amps & relied on the fold back current limiting on the chip to keep the plug pack in one piece. That is were all the panels sit today...straight simple application notes 3 term reg. That configuration will take a full 10 hours to come up back to full charge from flat.

drive on, you describe precisely what I don't like about the light bulb thing.....slow charging..... unless you specificlay select the bulb for the battery AND the the lamp will be very large.
If you want a slow charge that will work with anything, you might as well spend under a dollar an a 3.3 ohm resistor.

Light bulbs were very popular in the sixties, seventies and early eighties for all sorts of uses...It was a cheap dynamic eliment when we didn't have VCA's and a whole pile of other components we have now.
But all of these designs required some fiddling with different lamps to get good results.

My favorite use is speaker protection, 50 watt 8 ohm speaker, 12 V 21 W lamp in series.....near impossible to blow it up even with a 500 watt amp....Why is there a light shining inside me speaker???

The problem in this application is that unless you use a low voltage high wattage lamp it is not going to function dynamicaly....the best part its postiive temperiture coeficient is when the lamp glows a bit more than half bright under load.

Also a low voltage high wattage globe is needed to achieve a low enough cold resistance to get a practical charge....Id be thinking a 6 volt 18 watt globe would be probly of use... but it would need testing.

The other issue with using lamps as a dynamic ( quasi active) component is that we have far less variety of lamps easily available to us now than we did 20 years ago, and frankly the quality and reliability is very poor these days.

In this situation we don't have a full week to charge a battery......we'd be back home by then...... we need something that will give us some usefull charge in the hour or so run back to camp or along the highway to the next days fishing.

The fastest practical charge with SLA is 4 hours from flat at the 1/3 rate.

If the item is recharged long before it is flat then we are in the race for the short drive or boat run.

cheers

driveon

20-01-2008, 01:05 AM

Hi oldboot, I don’t know what your going on about, having to match the globe. The whole point of using a globe is that you DON’T have to match it, one globe fits all, no matter what size globe it is.

The lower the battery is, the higher the voltage difference across the globe, the greater the current flowing through the globe.

As the battery charges, the voltage difference reduces and so does the amount of current flowing through the globe.

Varying the wattage of the globe will vary the amount of current going to a low charged battery but once the battery is near fully charged, no matter what size globe you have the trickle current will be nearly the same whether it’s a small wattage globe or a large wattage globe.

As originally posted, a light globe used as like this is self regulating.

Next, if you fast charge these types of batteries you are going to shorten their operating life, mine were specifically designed for slow recharging but anything quicker than a 10 hour recharge is not recommended for this type of battery.

Last but not least, you can’t use less than a 12 volt globe in a 12 volt charging circuit or the first time the battery goes dead flat ( and is usually stuffed ) or if you reverse the leads, your 6 volt globe is going to have 12 volts through it and it’s going to be stuffed at the same time.

The joys of having a 24 volt globe is that if a customer did reverse the leads all that happens is the globe lights up. No damage to the charging circuit and no damage to the battery but it will still charge the battery with out any problems.

If you want fast charging then you should not be using this type of battery in the first place, they are just not designed to take a fast charge with out doing damage to the battery.

Cheers

TheRealAndy

20-01-2008, 10:01 AM

Oddbudman, I was thinking about using SEPIC or flyback topology, as this means input voltage is irrelevant. If the input battery is at 12V, it is still possible to generate 13.8 volts or more for charging. I had in my mind using a LT1513 from linear, but this may be a tad difficult to come across, will need to look at this further. The great thing about Linear Technology parts is that they come with free simulation models and LT supplies a free simulation program. That means i can nut out the best part before buying (and smoking) prototype parts!

The microcontroller design is right up my alley these days, as I do software for a quid now. The only problem with this means that you can no longer just take a part of the shelf and solder it to a board. Most ppl dont have the tools to program a micro. In saying that, manufacturer like Microchip now have (and have for some time) devices that have low voltage programming, making it possible to download to a device from a serial port. Problem there is that serial ports on PC's are getting hard to come by these days. Catch 22.

Oldboot, the problem with SLA batteries is they dont take to fast charging. Fast charging is typically reserved for chemistries such as NiCad and NiMh where you can set the charger current to C1 or greater. I have even seen C4. So charging an SLA to capacity is alwasy going to take around 12hours, that is one of the shortfalls of this chemistry.

As for me, since everyone else has spilled there guts on their profession i better do the same. I started out as an electronics tech. As a tech i did a lot of remote environment monitoring gear, most of it used SLA and solar panels as chargers. I started to design some of the gear and got the bug, so i decided to study. I moved into electronics engineering and did that for quite a few years. I did mainly digital design and power supplies, both linear and switchmode. Also did some RF, but that stuff made you use to much of your brain. Toward the end, was doing a lot of assembler and c programming, and I decided that there was more money in programming, so thats wht i do now.

Interesting reading. I may as well explain a little about what i do.

I design electronic products, many of which use battery chargers.

My most recent product involves a Microprocessor controlled 3 stage Buck converter for charging SLA batteries. Using the processor is great as you can easily set the battery charging parameters using software.

I'm happy to see you did the tests oldboot, as you got to learn something new :) .

Be interested to read your circuit suggestion TheRealAndy. Here's a simple one i thought was quite nifty without going into dc:dc conversion.

http://www.siliconchip.com.au/cms/A_109625/article.html

oddbudman

driveon

20-01-2008, 11:27 AM

the problem with SLA batteries is they dont take to fast charging.

Hi TheRealAndy, my point exactly, the general rule for these types of batteries is charging and just as importantly, discharging of these batteries should be no more than 1/10th the battery’s total capacity, so for say a typical 7 A/H battery, 700ma is the maximum recommended charge and load current.

BTW, I use a USB to Serial converter to program some of the micros I use.

Cheers.

oldboot

20-01-2008, 09:45 PM

Lets look at the lamps.

If we have a 24 volt lamp regardless of wattage in series with a 12V battery of any capacity.....unless the battery is faulty...the maximum voltage you will get across it will be 5 volts....assuming the battery is discharged to 8.8 volts which is way below recomended..........the lamp filament MIGHT barely glow... consider the battery will rise in terminal voltage rapidly in the first minute or so to arround 12 volts probly more like 13.4/5 volts once charging has stabalised.
there will be less than 2 volts across the lamp.... it will be nowhere near glowing & will be acting as a static resistance.

for a lamp to be an effective regulator it needs to be operating in a area of its characteristic that causes it to glow at some time.

Ok we have 20AH battery.... if we want to charge at the 10 hour rate.. we want 2 amps......insert a 6 watt 12 volt lamp.......the maximum current it can possibly pass is less than .5 amps........ too little, the lamp is clearly inapropriate... the lamp needs to be selected for battery capacity..... not any lamp will work properly.

While some may consider charging SLA batteries at the 10 hour rate is required...specks sheets I have quickly to hand do not support this... for cyclic use.

Typiclay a 7 ah battery will have a specified maximum initial charge rate of somewhere arround 2 to 3 amps depending on the manufacturer and the detail of the technology..... which is arround the 3 or 4 hour rate.

Talking about slow charge rates may be all fine and beaut for stationary or float applications but is completely unworkable for an on demand cyclic application were it is necessary to charge a battery in a vehicle whilst it is being driven.

Afterall we can't run our cars engine for 14 hours evrey time we want to charge a battery.

so there is a nesesity to charge at the 4 hour rate at least for any practicality..... therfore we need to get optimum charge rates ..... therefore we need to select the charge control eliment for the battery capacity used.

BTW I have never seen a one size fits all component of any type that actulay worked optimumly, I dont care if its a shirt, a hammer or a charge controller.

It is necessary to select the lamp for the size battery, or the lamp will not allow sufficient charge current or the lamp will be acting as a static resstance and you might as well use a resistor.

cheers

driveon

21-01-2008, 09:07 AM

Hi oldboot, the problem here is that we don’t know what brand of battery bungie has, so to say that some of these types of batteries can be charged with a higher current is not the way to go about giving safe charging info.

Your info is based on a specific brand while the majority of these types of batteries would be stuffed in quick time if you charge them regularly at too high a current rate.

I will be more precise about one thing I posted, in my reference to one globe fits all, as posted earlier, I was referring to batteries up to 7 A/H and if you want to charge a 20 A/H battery you would need to increase the number of globes to do the same job but it would still be on the basis of a SLOW CHARGE.

Most of these batteries just can not tolerate fast charging.

Also as a an additional point of interest to how the globe regulator can be used, if you have a smallish solar panel ( 10w to 20w ) and want to keep automotive batteries topped up with out cooking them, you can use globe(s) to regulate the current and voltage going to the batteries and again leave them set-up indefinitely.

With larger solar panels you need to go to proper solar regulators but with smaller panels, this can be a cheap and easy way to trickle charge conventional automotive batteries.

Cheers.

oldboot

21-01-2008, 10:22 AM

Lets look at current practice and what is currently available. ( I'm excluding the expensive & exotix batteries like the so called AGM and Gates Cyclon and the like.) we are talking about what you would commonly buy at the local allarm supplier or at an electronics supplier.

Almost without exception the SLA batteries we are finding commonly used now are starved electrolite batteries, most of them are pretty similar in performance and capacity, because the industries that use them demand some sort of comodity status.... The better brands will typiclay perform marginaly better and spec marginaly better.... but we re looking at a product that performs THE SAME with in reasonable engineering limits.

Gone are the true gell cells, gone are the calims of some sort of exclusive process or claims of overwhelming superiority the SLA bayttery is a comodity.....unlike in the eighties when they became a real alternative to small wett cell batteries.

I use the 7 AH 12 V battery because it is the buggest volume battery sold.
In the past it would have been a 6 or 6.5 Ah battery the case is the same and consistent between all brands withing a couple of mm
Almost without exception this battery form is now claimed as arround 7 AH and has been for the last 10 years...this reflects an improvement ( or perhaps a change in the way they are specified) and also reflects a comonality across manufactuers
just on spec sheets I don't have to look for
Panasonic...
capacity rated at the 20 hour rate 7 AH
maximum initial charge current for cyclic use 2.8 amps
Quoted initial charge current float use 1.05 amps

Dia mec
capacity also rated at the 20 hour rate 7.2 AH
Maximum initial charge current 2.1 amps
no float charge quoted.

Look to standard industry practice......there are all sorts of off the shelf specific SLA chargers available, there are also lots of current pieces of equipment with SLA chargers incoprirated in them.
It is very common no almost a defacto standard to charge 7Ah gell cell at 1 to 1.5 amps at 13.8 volts...this is predominlatly because stock application notes 3 terminal reg designs are being used and they rely on the foldback current limiting in the chip to provide a limit to prevent the 16V 1.5 amp plug pack from being overloaded.
Most of the dedicated small multistage chargers follow a similar form.
I have been servicing chayo and mipro portable PA systems now for over 15 years... they have either two 7AH SLA (in the earlier models) or 4AH in the later models.... these typiclay charge at arround 1 amps and typiclay charge in between 6 and 10 hours forom flat.... when chayo reduced the battery size from 7AH to 4 AH to save weight they did not change their charging circuit.
All of these units now ship with 4AH batteries.
In schools these things get a hammering, 5 days a week, they get run down till they wont go any more and the charge cycles are not supervised by anybodyu that knows much at all.
While the clients can easily kill the batteries by leaving the unit turned on over the weekend, if they avoid this and charge the units reasonably regularly they get 5 years out of the batteries....i've seen 7.

Now the maximum recomended charge rate on panasonic 4 AH is1.68 amps and DiaMec 1.2 amps. These units usulay come with some obscure brand of chineese SLA. I have unsed both the above brands as replacments as well as others... performance is similar.

So I see no problem repeatedly charging a 7AH battery at 2amps.

You simple dismission that tw lamps would be needed for a 20 AH confirms that the lamp needs to be selected for the battery capacity.

cheers

cheers

oldboot

21-01-2008, 10:29 AM

Oh and
As far as discharge both panasonic and DiaMec
quote capacities
at 20 , 10, 5 and 1 hour rates.

Current SLA technology ( that has been with us for at least 15 years) in a substantial improvement on the early gelled electrolite batteries of the early eighties and before and a small improvement on the early starved electrolite.

On the matter of a solar panel
If you were to spend $150 to 300 on a solar panel why woulndt you send the extra $20 on an off the shelf solar regulator, that will give you maximum output for you money.........the other option being a lamp that will give you an indeterminate low charge rate..........sorry not if its my money.

Remember also that the terninal voltage of a 12 volt solar panel can exceed 18 volts....an entirely different story.

cheers

driveon

21-01-2008, 11:40 AM

Remember also that the terninal voltage of a 12 volt solar panel can exceed 18 volts....an entirely different story.

And a combination of the globes current limitation and the internal resistance of the battery, even a fully charged battery will guaranty that the battery is never over charged, even when the solar panel output goes up to 18+ volts.

As posted above, this is not a means of fast charging but a cheap trickle charger, that works.

Cheers.

oldboot

21-01-2008, 07:07 PM

The point I was making about a solar panel is that with a maximum of 18 volts you have up to 4.2 clear volts to play with... in the original senario we only have........ none.... desired charge voltage 13.8 volts....voltage available 13.8 volts.

Again you will have to select a globe for the purpose, even with more than 4 volts latitude available, a 24 volt lamp is still going to be a static resistance.
You might as well sepnd less that $1 on a resistor.

I keep comming back to the issue that regardless of what eliment it is it will have to be selected for battery capacity, for acceptable results.

The combination of a solar penel a lamp and a battery, will most certaily not prevent overcharging....once the battery has reacged full charge, charge current will reduce considerably and the lamp will definitely be a static resistance at its minimum resistance..........this resistance will be insignificant and the battery will overcharge as it might as well be connected direct to the pannel.

Unless you are relying on the lamp resistance to make the charge current so low that the battery doesn't have a chance to charge in a reasonable time scale.

.
cheers

driveon

21-01-2008, 07:59 PM

Oldboot, I don’t know where you get your “THEORIES” from but you have no idea how small solar panels and automotive batteries work together.

For a starter, I have already pointed out this is not for charging the battery but for maintaining the battery with a trickle charge.

But some info based on equipment I make for one of our customers.

I produce a microprocessor based control module for an agriculture device. I not going into what it is as it is marketed under the name of the customer but they install my controller in their device which contains amongst other things, a 90 A/H wet cell battery and a 20w solar panel and NO REGULATOR of any form.

We have been supplying the controllers for these devices for over 10 years and to date I’ve not heard of any of the batteries being over charged.

So far you’ve put up a lot of theories but as I’ve pointed out, these light globe regulators are used in a number of devices and have been for 30 to 40 years but suddenly, after a few days experimenting, you have come to the conclusion that, despite all the evidence that they DO work, you say they don’t.

Oh well, each to his own.

TheRealAndy

21-01-2008, 10:13 PM

Well i think this is gone right off topic now. The OP has solved the problem by using a solar charger. I think we all agree that both a resistor and bulb are crude solutions.

OTOH, I have done a spice simulation around an LT1513 and I have got it throwing 1.9A into a battery with about 10V of charge, from a battery with 5V (Assuming the 5V battery can suply the 4A peak needed)! I need to make a few more adjustments, but the float voltage is currently about 13.1V from a 5 volt source.

oldboot

21-01-2008, 10:29 PM

Drive on

Now lets remember what the post was originaly about.
A bloke wants to go fishing and run some stuff off a SLA battery.
He then wants to charge that battery from his car.

We dont need to maintain the battery with a trickle charge, we need to charge the battery from a discharged state.

To achieve this in any sort of practical time we need to do this at the maximum permissable charge rate.........

You persist with your trickle charge philosophy.
Even your piece of equipment with its 90 AH battery and 20 watt solar panel ilustrates this.

there is strong evidence that long term trickle charging of lead acid batteries isn't good for them...........even on this forum there have been vigourous dicussions about the causes of sulphation........One of the stated causes is failing to bring batteries to a healthy state of charge quickly. ( well documented )
Do not confuse "trickle charging" with "float charging"...they look similar but the rersult is different.

I'm hardly surprised that a panel capable of delivering a long way less that 2 amps is not capable of overcharging a 90 amp hour battery. It will be barely able to charge it.

by my rough calculations it would take something like 90 hours of bright sunlight to charge that battery from flat. considering a BP solar 20 watt panel delivers a maximum of 1.19 amps..... assuming little or no laod from the equipment over that time it would take a fortnight to charge the battery from flat... & thats not accounting for leakage or losses......do the maths on the basis of 8 hours of sunlight & thats generous.

This is the sort of panel /battery mis matches we were seeing in solar installations in the early eighties.....when panels were very expensive and batteries were cheap......it is obvious to anybody with even cursory understanding of modern solar practice that the panel is far too small for the battery.......

I had mates that worked on some of Telecom's early "solar failures".... yep thats what they looked like.... big batteries... small panel arrays.

modern solar design would match a 20 watt panel with somewhere between 7 & 20 AH battery. that will be capable of overcharging the battery. It would recharge the 7AH from flat in a little over a day and the largere extreem in about 3 days assuming good sunny days.

When you consider that a 20 watt solar panel will cost arround $300 retail an extra $20 for a regulator is a wise investment.

Being able to fully charge the batteries within 12 to 16 hours of sunlight is considered reasonable

I have had quite a lot to do with filament lamps as quasi active eliments and running lamps above and below their rated voltages......

Yes I remember the era of the light bulb and the relay I even worked on equipment with baretta tubes in my apprentiship.
( a baretta tube is a light bulb like device with perdictable characteristics, they were used as current regulators.... sound familiar)

I have not once said that a light bulb can not be used to charge a battery.....read my posts again.

I have said that, for it to be effective in terms of charging the battery in a reasonable time period the lamp needs to be selected for the battery capacity.

Also I have said that in many cases the lamp will not be acting as anything more than a static resistance.

I have also stated that a light bulb will not prevent long term overcharging...... neither will a resistor.

Unless of course the light bulb is allowing so little current to flow that it can not charge the battery in a timely manner.

I have supplied references from data to hand to support my views and I have illustrated several issues with example that can be proved.

If you think any of my examples arent correct..... go on build it up and do the measurements.... lets have it give me figures.......

Yep light bulbs were used for all sorts of things "30 years ago" and some of them were very clever.

I am also old enough to remember some of the horrible abominations that abounded in times past..........

We have moved along from charging regulators that consisted of a 2N3055, a zenner diode, a resistor & a light bulb.....even by the standards of the day that was rough. Yes I remember alarm panels built up from dodgy modules.

That makes no difference to my point

I admit that I made a couple of specific incorrect asumptions.....nobodies memory is 100%....... BUT do not underestimate my understanding.

Thruout I have confined my comments to technical discussion
Think twice before you insult me agian......it will not be tolerated.

cheers

driveon

21-01-2008, 11:30 PM

Oldboot, as The RealAndy has posted, this is now way off subject but please show me where I have made a statement about you that you yourself have not already posted earlier in this thread.

My original post in this thread was a SUGGESTION for solving a potential problem posted by bungie.

My suggestion is based on sound principals that, as I have shown, are in use TODAY in hundreds of thousands of devices.

You have done nothing but post replies based solely on your opinions and unproved theories.

If you know of any established situation, where the suggestion I put forward, caused a genuine problem, not your theory of a mythical problem, please post away as I would be very interested to hear about it.

In the mean time, try reading back through your own posts, as the criticisms you have posted, all based on theories not hard core evidence of a real problem, far exceed anything I have posted but unlike you I’m happy to tolerate it, this is how people learn.

My Suggestions are based on many years of hands on testing and in the field experience and are well founded practices.

Cheers and my apologies to bungie and others.

oldboot

22-01-2008, 11:02 AM

Ok 1 issue at a time

there is nothing unproved or theoretical about the specifications quoted from battery manufacturers.

Relativly high charge rates and discharge rates are permisable.
Charaging a 7AH SLA battery between 1 & 2 amps will cause not a scrap of problems.
The specifications say this in black & white

using discahrge rates of 4 amps or more will not cause a scrap of problems in fact I have in black and white a quoted maximum discharge rate of 40 amps..for 5 seconds....

These specifications are born out by examples of current technology That use this specific battery, which I posess and use daily or service for others, such as UPS units, burgular alarm panels, halogen lighting packs, portable PA systems and other weird stuff.

By the way I date all batteries on instalation So I know how long they last.

Panasonic's application note on charging SLA batteries states that initial charge rate should be less than .4 times the quoted capacity......in a 7Ah battery that means op to 2.8 amps.

Furrther under precautions it warns that, when long term charged, " voltage should be controled within a narrow range.

You insist ( repeatedly) that relativly high charge & discharge rates cause reduced life in these batteries.......
Niether the specifications nor my experience with theses batteries agrees with that insistance.

It is an out dated perception based on old technology.

If you have any recent documentation or evidence to the contary I would be pleased to see it.

cheers

driveon

22-01-2008, 12:17 PM

It is an out dated perception based on old technology.

If you have any recent documentation or evidence to the contary I would be pleased to see it.

The technology may be old but is anything but out dated and as globe type charging is still used by the vast majority of alarm panel manufactures, to say this is out dated is purely and simply a perception of yours and not a fact.

I posted numerous ( easily checked ) examples of current use of the suggestion I posted, doing exactly what I posted it would do, and even though you continually post that it won’t work and I have asked for any relevant examples of where my suggestion is unable to do what I have suggested it WILL do, you still persist with your theories and have yet to post any form of relevant examples.

Your pretty quick to criticise suggestions posted but your short on evidence to back your criticisms.

Some genuine facts would be nice.

Sorry folks for this is now going around in circles, you can take or leave my suggest but I'll not take up any more of your time.

oldboot

22-01-2008, 05:19 PM

You have not addressed the issue of my last post and you have quoted me out of context.

Do you or do you not conceed that the manufacturers spec's support charging at levels up to the C4 rate and discharges at rates at least as high as the 1 hour rate

And that this is a very common occurance in contempory equipment, and further that nw damage or loss of battery life results.

Yes or No

oldboot

24-01-2008, 01:25 PM

I've just been checking some facts.
I rang arround some of my contacts in the alarm industry.
Yes there are some manufacturers still using light bulbs in their charging circuits but it certainly could not be considered a majority.

Direct Alarm supplies... the largest distributor ( some would say dominant) of alarm equipment in the country. To the knoweledge of the technical dude there have never used light bulbs in their charging curcuits, most certainly not in any current product. Standard 3 terminal regulator. ( I have one I looked )

Altronics, a minor player in alarms wholsale AEI brand.... no light bulbs, a multi stage charger with battery supervision

Mainline security.. one of the larger independed distributers, distribute BOSCH ( aka SOLUTION), no light bulbs, but a weird sealed component that flashes & glows during charging & it isnt a LED or a light bulb.

NESS, at one time a dominant player....not as strong these days.
Yes current Ness Panels do have two light bulbs in the charging circuit. They also quote charging from flat as 24 hours.

I spoke to a couple of fellows I know wha have been in the industry recently and they can not remember seeing light globes in alarm panels in recent years..... looks like they don't use NESS panels.

Ed, commented......light bulbs in charging circuits.... I thaught that went out with window tape & preasure mats.

The problem is there is an expectation that the panel should be able to charge the battery from flat in 8 to 10 hours, a 1 amp 3 terminal regulator configuration will do that, the NESS panel by their own statemants will not.

Now, I have never said, "you can not charge a battery thru a light bulb", I have never said "it wont work".

I have said multiple times
1/ for effective charge in a reasonable time the lamp needs to be selected for the battery capacity......

2/ If the lamp is too small an inappropriately chosen the charge rate will be very slow and it will in many cases be acting as a static resistance.

3/ The light bulb will not prevent overcharging, unless it supplies such a small charge current that it is incapable of overcharging the battery and there for the charging will be too slow to be practical....in the case originlay in point.

Note the NESS example ilustrates this just fine....... further NESS consider it adequate to charge over a 24 hour period in their situation....... they have two globes in theirs....... you can bet your boots they will have been selected specificlay to achieve their design goals.Light globe selected for battery capacity.

I still maintain that light globes in charging circuits is outdated technology.
As we see, there are panlels with multi stage chargers comming on the market ,and simple 3 terminal regulator charging circuits are very common & effective.

facts is facts.
cheers

driveon

24-01-2008, 03:38 PM

Mate you need help, all you have done is confirm what I and others have posted.

I stated that the maximum recommended charge current for these types of batteries is 1/10 of the battery’s total capacity and for a 7 A/H battery which is the standard size used in security panels, is 700ma.

Now you have posted that the expectation is to recharge a flat battery in 8 to 10 hours. Not only is this a far cry from the drivel you have been posting but to do an 8 hour recharge you need 875ma and to do a 10 hour recharge you need, guess what, 700ma and remember, a flat battery in an alarm panel is not one at 0% capacity but somewhere around 25% capacity, depending on the brand of panel and the low voltage setting, so those current rates would be even lower.

So the FACTS you are ranting about just help the validity of my posts.

By the way, as to the question in your second last post, if you read back through this thread, to post 41, you will see that I had already acknowledged the high charge current capabilities of SOME batteries before you even asked the question but also in that post I stated why it is safer to slow charge, the safety factor is something you just can seem to get your head around.

Last but not least, as bungie has never posted what brand of battery he has, we do not know if it can be fast charged.

Cheers

bungie

24-01-2008, 06:54 PM

So ....... 4mm is ok ?? :devilish:

TheRealAndy

24-01-2008, 08:55 PM

Nah bungie, 4.75mm :P

driveon

24-01-2008, 09:57 PM

Hi bungie and sorry mate for highjacking your thread,.

4MM is fine considering the small currents you will be needing, no matter which type of charger you use.

BTW, did you finally resolve your charging requirements to your liking?

Cheers.

bungie

24-01-2008, 10:48 PM

Yeah mate, got a solar cell :)

oldboot

24-01-2008, 11:26 PM

Sorry bungie, but I'm sick of this insulent driveon character.

Drive on you can not help but make personal insults.

And you cant agree with facts when they are there in black and white from the manufacturers specifications.

You insist that charging at the 10 hour rate is maximum when the manufacturers state that 4 hour rate is permissable. further that there are plenty of brand name items out there that do with no reduction of battery life. Using a variety of well known and lesser known brand cells

Further you fail to factor that charging at any specific rate, you need to account for charging losses of 20% or more.
So charging at the 10 hour rate will typiclay take 12 hours or more.

So to bring a battery up to full charge in 8 hours will require about 1 amp which is a bit different to 700mA.

you claim to know how alarm panels work, but you also claimed that " globe type charging is still used by the vast majority of alarm panel manufactures", which is simply not true.
You claim hunderds and thousands of products using lamps in their charging circuits, but you do not name them.

You also fail to acknoledge that the SLA battery has become prety much comodified and the spec's between brands are prety damn close, even the cheapies come up pretty well. Afterall lots of them are manufactured in asia and china, so there are no secrets in this technology

You fail to be interested in learning, you fail to be friendly and courtieous.
You have not produced one single shred of evidence to support your position.

I have encountered attitudes like yours many times in the past.

It is attitudes like that, which preserve enginering expectations of past technologies.
Atitudes that have lost one of the major alarm panel manufacturers its market leader status long ago. Its products simply do not peform as well as its more progressive competitor.
The company persists in preacing its engineering philosophy, customers persist in buying oposition product.

I think I have finished
I thank other readrers for their tolerance.
cheers

driveon

25-01-2008, 12:29 AM

oldboot, I’ll keep this short and sweet, if you care to go back through this thread, you will see that YOU were the one who started the slinging match and yes I wrongly responded but again, you were also the one trying to make out the info I was posting was incorrect but you have yet to post one shred of legitimate info that does that.

I based my input to this thread on hands on experience, your own posts show that you have been making statements that only after you made them, did you actually make some enquiries as to whether they were correct and even then their still based on half truths.

And just to show how far you are prepared to go, in your last post you claim that "there is a need to account for a 20% charging loss".

BASED ON WHAT, this is nothing more than one of your mythical theories.

The industry accepted rate of charge needed for these and most lead acid batteries is 21 amps of charge current for every 20 amps used, thats 5% loss, not your mythical 20% loss.

You know the old saying ”People who live in glass houses, should invest blinds”

Cheers all.

TheRealAndy

25-01-2008, 06:50 AM

SOrry oldboot, just out of curiosity can you point be to the manufacturers that quote 4 hour charge times please. Most of the material I have seen points to about C/3 and about 12 hours. I would like to factor this into my charger design.

BTW. I have dropped the LT1513 from my design, they are about $20 a peice one off, so going to use a PIC or similar microcontroller at about half that cost.

oddbudman

25-01-2008, 09:16 AM

Wow, this thread never dies.

I have a fair bit of experience with fast charging SLA batteries, so let me try and explain why fast charging SLA is a struggle.

When you put a decent amount of current into the battery the cell voltage will quite quickly get over the safe cycle charge voltage. So when this happens you really need to switch into constant voltage mode to get the rest of the charge into the battery otherwise the SLA battery will start to make a hissing sound which is gasses venting from the battery..

With 0.3C charge rates you will find the cell voltage will quickly get above the safe cycle cell voltage,

The chargers I have made manage to almost fully charge a SLA battery within 16hours. (I charge a 6V 4.5AH battery using a microprocessor - starting current is 0.1C - I limit this due to the limited smps output power i have on tap). It would be possible to go faster, but not too much as eventually the cell will hit the point where Constant voltage is the best way to charge (as the cell voltage has reached the safe cycle voltage limit). Once you hit constant voltage the battery regulates its current intake and it drops relatively quickly getting the last of the charge into the battery takes some time- this is why fast charge times are difficult to achieve on SLA.

To help me explain, attached is some data from the one of the chargers I have made (I have added in the current line to explain what the current does during this type of charge also)(please also note you should terminate the cycle charge voltage after extended periods of charge, also the charge current limit on this graph was dictated by me and is by no means applicable to all batteries):

oddbudman

oldboot

25-01-2008, 10:35 AM

Excuse ME

Mythical hmmm.

Why is it then that manufacturers state in black and white a 10 hour charge rate, then state for between 10 and 14 hours.

It may indeed be possible to charge a new battery at the 10 hour rate in less that 12 hours, but as the battery ages the charging efficeincy will be reduced.
In addition even at the 10 hour rate, as the battery approaches a state of increased charge the charge rate will naturaly reduce ( unless constant current charging) as the battery terminal voltage approaches the charging voltage.

This is 1st year electrical theory.

If you want it in black and white I refer you to panasonic application note "Panasonic VRLA charging methods".
Where it provides charging curves and states with regaard to constant voltage , current limited charging, which it refers to as "constant - voltage and constant current charging method" which closely approximates what we are talking about.

" This method charges the battery by controlling the current at 0.4CA and controlling the voltage at 2.45 V/per cell. Proper charging time is 6 to 12 hours depending on discharge rate"

Most of the manufacturers have similar data and application notes on their web sites, they are very easy to find.
For their data just google " panasonic SLA batteries" & wade in.

While 4CA is specified and permitted in specifications, personaly I would be happier to charge at a lower rate, while charging a 7Ah cell at 2 to 2.5 amps may be quite fine, I would be happy to charge at 1 to 1.5 amps. Most of the equipment I encounter would do this mainly because it is a convinient thing to do with available components.

I am in the business of dealing with facts, not myths or outdated preconceptions.
If I am incorrect, I am happy to be corrected, provided I am supplied with reasonable evidence.

I thank oddbudman and therealandy for their learned responses and his encouragment to check my facts, I have learned things important to me.

I have checked my facts by refering to manufacturers data, making enquiries and performing a couple of simple experiments.

I shall be continuing with my investigations into matters regarding SLA batteries as I feel the need to update and refresh my knoweledge.

In this day and age, things change, somtimes rapidly.
I also often find many perceptions and ideas old and new that have been preached as fact, were flawed or incorrect from day one.

cheers