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Cheech
10-12-2007, 10:43 AM
Just wondering how long you can make your battery leads before they become an issue with dropping current (or is it amps or volts).

Thinking of moving my 2 batteries forward to about midships for better weight distribution. The leads would then be about 5m to the motor instead of about 1 - 1.5m.

The boat is 21ft, and the position would be under the port seat if that makes any difference.

Would the extra pounding that happens the further forward you go make any difference to the batteries?

If I do not move them forward, at least I will be moving them from the starboard side to the port side because with them on the same side as me, the boat has a lean to the right that makes it go on the trailer at an angle. Am hoping that this will help as I cannot adjust the lean out by adjusting the rollers no matter what I do.

Cheech

Vitamin Sea
10-12-2007, 01:20 PM
Hi Cheech

I did exactley what you are thinking about, moved 2 batteries to a box in front of the starboard seat, boat is a 19C Haines.

The leads I used were about 3.5 metres long for memory, used about 1/2" cable ( don't remember technical spec )

Have not had any problems at all.

Regards

Bill

Noelm
10-12-2007, 01:42 PM
if it is the starting battery and you move it further away and use thicker cable, then there is no reason why you cannot extend them twice as far, but remember that a battery up the front will cop a pounding, so only "proper" marine batteries should be used, and even then I reckon they may have a shorter life span

Cheech
10-12-2007, 02:06 PM
Hi Cheech

I did exactley what you are thinking about, moved 2 batteries to a box in front of the starboard seat, boat is a 19C Haines.

The leads I used were about 3.5 metres long for memory, used about 1/2" cable ( don't remember technical spec )

Have not had any problems at all.

Regards

Bill



Good to hear. Do you recall noticing an improvement when you did it getting the weight out of the stern.

Vitamin Sea
10-12-2007, 03:09 PM
Good to hear. Do you recall noticing an improvement when you did it getting the weight out of the stern.

I moved them when I put the new Opti on, the Opti is about 50kg heavier than the previous 175 Johnno, so it more or less balanced out.

Thick cables, good marine batteries as Noel mentioned, will not be a problem.

Cheers

chris_s
10-12-2007, 09:40 PM
Not a problem mate. There will be nearly nil voltage drop over such a short run. Just make sure that the cable you use has the right current carrying capacity, and use SDI (single double insulated).

finga
11-12-2007, 07:44 AM
It is very important to have the right current carrying capacity of the cable to do the job but it's also very important to work out the voltage drop over the length of the cable taking into account the maximum current the cable will have going through them as well as some other variables.
Since maximum current is when you kick the starter over this current will cause a voltage drop and if the cables are too small then it will be a considerable voltage drop and will do damage even if the current and voltage drop is exhibited for a very short time.

An example of this is my old Merc.
It comes with 10mm cables (not really flexible) as standard.
These cables are about 1.5m long and the current going through them on startup is about say 100-120amps.
I don't really know as I've never tonged them but just say 100-120amps.

Voltage drop (Volts)=millivolts/amperemeter (a calculated figure usually found in various tables) X length (metre) X current (amps) / 1000

So therefore for the example of mine with the cables at 10mm, 1.5m long and with a current of 100amps at startup the voltage drop will be
Voltage drop = 4.4mV/AM (roughly) X 1.5M X 100A / 1000
Voltage drop = .66Volts (roughly)
If you loose .66 of a volt when you start out with 13.8 volts this is pretty good

Now take take the same current and same cable over 10m you'll get
Voltage drop = 4.4mV/AM X 10m X 100A / 1000
Voltage drop = 4.4Volts
When you loose 4.4 volts when you start out with only 13.8 volts is very bad

Now if you went up to say 16mm welding cables then the voltage drop would work out to be roughly 1.24 volts....not too bad but you could go better
If the cable was 25mm then the voltage drop would work out to be roughly .795 volts.
And so on and so on until if you used 50mm cables the voltage drop over the same 10m with the same load would work out to be roughly .393 of a volt.

Now these figures are worked out if there was continuous current of 100A. Hopefully this will not happen so duty cycles are worked out as well.

Factors like temperature and duty cycle play an important part in selecting cable size but after a while you get to know roughly what's needed.

A fairy easy way to see the impact various factors have on your cables (in the case in the link welding cables) the link shows different variances on the cable and cable size and the net result.
http://www.cse-distributors.co.uk/cable/0361TQ-welding-cable.htm
It also explains a bit better duty cycle etc etc a lot better.

So the moral of the story.....if in doubt...go big

PADDLES
11-12-2007, 08:21 AM
Where did ya get the values of mV/A.m from finga? remember that the ones in 3008 are for 50Hz AC, is there a standard with the data for DC? best way for DC drops is to get the resistance data out of the back of an olex book and just use ohm's law (V=IR) where I=your load/current draw and R=resistance worked out from your length times the ohms/km data. if you want the numbers and don't have the book let me know what cable size you've got (in sq.mm) and i'll chuck the figures on here.

finga
11-12-2007, 08:24 AM
Where did ya get the values of mV/A.m from finga? remember that the ones in 3008 are for 50Hz AC, is there a standard with the data for DC? best way for DC drops is to get the resistance data out of the back of an olex book and just use ohm's law (V=IR) where I=your load/current draw and R=resistance worked out from your length times the ohms/km data. if you want the numbers and don't have the book let me know what cable size you've got (in sq.mm) and i'll chuck the figures on here.
MM cables book for the 10mm and that website for the others.
They wrong are they matey??
If they are just let me know and I'll do a modification of the figures.
It's hard to get all the cable sizes from one source though.
Especially all size combo's and configuration of flexible cables.

There was a standard....AS/NZS 1125:2001 "Conductors in electric insulated cables and flexible cords"
and also AS 2218-1978 which was "Cables for use in automotive vehicles - PVC insulated cables having copper conductors"

I just went and had a gander in the 2006/7 Olex book and the DC general data in the back (pge 88) is for solid and stranded conductors not flexible.
Is there another table for flexible??
I tend to steer towards welding cable because of the ease of data and if they vary a tad from AC- DC then the difference aren't that great. Well not great enough to worry about.
But if there's another source of information then I'm more then willing to accept the source....and will be accepted with many thanks :)
Cheers then
Scott :)

Owen
11-12-2007, 09:35 AM
There's a data sheet for welding cable here
http://www.liquidarc.com.au/pdfs/Access-Consum.pdf
Bear in mind though that we aren't all that concerned with a voltage variation of a couple of volts in welding.
As finga says, if in doubt go bigger.
Any welding supply house will cut to length and crimp the lugs on for you at no extra charge if you ask nicely ;)
For the price difference I personally wouldn't bother with less than 50mm2.
After that you may struggle to get lugs that will fit the cable and still have a small enough mounting hole for your terminals as typically they are designed to filt welder lugs of 12mm or bigger.

PADDLES
11-12-2007, 10:43 AM
nah, dunno if they're wrong finga. i was just wondering where you got your numbers from because a mV/A.m figure will only relate to a given voltage and usually thats 415V in most books. haven't seen the mm cables book myself, we just use the resistance in ohms/km method here for dc circuits when we've gotta do them. spot on with the "if in doubt, go bigger" statement though. olex reckon (sq.mm : ohms/km @ 20 degrees c) - 2.5:7.41 , 4:4.61 , 6:3.08 , 10:1.83 , 16:1.15 , 250.727 , 35:0.524 , 50:0.387 , 70:0.268. ps. thanks for the heads up on those standards. yeah you're right it isn't for flexibles, i just assumed that the data for stranded would be pretty close to a flexible, that's another thing we do here called a "fudge factor" ;D

finga
11-12-2007, 11:20 AM
Mate, we were taught the only differences were between single and multiphase voltage drops.
Working voltage should not be a factor.
AC-DC differences should not matter either to the mV/Am figure as well.

Fed
11-12-2007, 11:42 AM
What about the return path voltage drop, obviously the SAA wiring rules for AC factors in the return path voltage drop but I'm not so sure about the welding cable charts listed in the above post and of course when using the resistance values directly for cable to calculate the voltage drop the length would have to be doubled.
Top info in a top thread gents.

PADDLES
11-12-2007, 12:06 PM
yep fed, for single phase systems you have to allow for the return distance (ie. neutral conductor) in your calc. this should be the same for a 2 wire style dc system.

good question finga, now you've got me wondering, i always thought that the voltage mattered but maybe it doesn't. i'll find out. i thought that ac and dc figures were different because of the reactance in the cable. again i'll find out.

Owen
11-12-2007, 12:17 PM
In the case of welding cable the chart says combined length of electrode & earth lead.

As a welder I know all about this electric stuff.
It says in my handbook.

"Smoke is the real thing that makes power circuits work because every time you let smoke out of something electrical, it quits working. I have verified this with thorough testing."

"Of course! Smoke makes all things that are electrical work. Remember the last time smoke escaped from a transformer? Didn't it quit working?
“See, it's the conductor that carries the smoke from one device to another. It starts at a power plant where the stuff is burned to produce smoke. The smoke we see coming from the stacks is excess that the system doesn't need. The smoke is then sent down the conductors to transformers.

“Transformers are big and require lots of smoke to work properly. That's why the conductors are so big. If those conductors spring a leak, it lets the smoke out of everything, and then nothing works! Forget about electron theory!"

PADDLES
11-12-2007, 12:48 PM
ahhhhhhh owen, you've exposed us electrical people for the frauds we are. i mean we carry on about all this theory but in reality you're right, it's smoke, you can even see the excess stuff coming out of the power station chimneys when it's not being used.

you're spot on with the voltages finga, it doesn't matter, frequency and conductor temperature (which changes with load) are the big variables. dc is different to the ac mV/A.m figure (because of cable reactance) but not by massive amounts and the most accurate way to measure volt drop in dc circuits
is to simply use the total conductor resistance. i asked an old fella here, who at 79 should have given up electrical engineering a long time ago, but like most old blokes are a top source of info because he could probably write what he doesn't know about electrics on the back of a stamp.

Cheech
11-12-2007, 01:30 PM
Good info, So,,,,,, the short story is to go to a welding supplier and buy standard (15mm'ish) welding cables? The ones currently in use are probably only about 10mm, so it sounds like 15mm would be more than enough.

Owen
11-12-2007, 02:17 PM
Cheech,
Cables are measured in mm squared, not the diameter of the copper.
Plus they are double insulated, so make sure you have a look before you get them to cut it off.