Dual Battery Issue

[Winf]

Ok, Just want to make sure I have this right. Recently had a bit of a drama with the dual batteries.

I have a dual battery systemwith a simple 1, 1+2, 2 or all off isolator switch arrangement. The boat was idle for a long time while I did trailer repairs so the batteries were down. I charged them up in thedays before I went out. Motor (115 Suzi) started fine at the ramp and thought tomyself I’ll set the switch so both batteries on charge so they get a full charge while cruising.

Pulled up to do a bit of jigging and it was just gettinginteresting with a couple of hits and a hookup, go to start up to get back onmy drift and nothing but clicks and whirring of the starter. Check allthe terminals, connections, the usual stuff, still nothing. Try eachbattery via the isolator switch. Nothing. Notice that one of the batteries (the main) isreally hot to the touch so disconnect it completely in case it blows up orsomething stupid.

Try to pull start the 115 – that was never going tohappen so had to limp home on the aux. Back at the ramp and the main batteryis still hot to the touch.

My reasoning, cell had collapsed with an internalshort in the main so because the isolator was connecting both batteries, the shorted battery pulledthe other one down to no charge . So the take home message for me is alwaysrun with the isolator switch on either 1 or 2 but not both.

Both batteries now replaced. The old ones were 5 years old so not a bad run.

Am I right here in my thinking.

Cheers

Winf

[WalrusLike]

Yes if both in parallel a crook one will suck down the good one and you will have nothing.

It wasn't clear to me.... Did you switch to both while the motor was running?

That is a big no no because it can stuff the rectifier in your motor. (that's what I was told from multiple sources... Think it was the rectifier.) In any case never switch with motor running. Not saying you did... Just clarifying.

[Winf]

Not switched to both while running. Started the day with switch on 1 and 2 to charge both up. My thinking was that the main battery shorted internally sometime after startup and killed the other one.

[fat-buoy]

Or it was already dead and you put it to both batts to start so it used the good battery while the charge lasted.

Interesting... I have a battery that is running down over time so that when I start the motor I get a second beep.. (sign of low battery power) but then after a few minutes of running will be fine.
So I did the usual test of starting with batt 1 and then with batt 2 and both gave the low signal.. I have to say now that when I went to the boat after it has been sitting for a couple of weeks I had forgotten to isolate the batteries and it was set for 1 & 2

The way the batts are wired to the isolator they will have the common ground wiring but the positive cables run to the isolator individually so there should be no way that if one is run down it would affect the other unless they are both linked in together on the isolator ..

This sounds like the same affect that you had with your batts except that yours was more dramatic as when 1 failed it basically took out batt 2 with it as the second battery would have been passing its charge on to the dead battery.

[Dignity]

Yes if both in parallel a crook one will suck down the good one and you will have nothing.

It wasn't clear to me.... Did you switch to both while the motor was running?

That is a big no no because it can stuff the rectifier in your motor. (that's what I was told from multiple sources... Think it was the rectifier.) In any case never switch with motor running. Not saying you did... Just clarifying.

I keep hearing this but I don't know the ins and outs of it. In older days I would have said maybe, but these days you would think that there would be protection against ............. what would you need protection against .............. you can generally disconnect a battery after you have started a motor or is this a no no also and you are not throwing a spike by chaging from 1 battery to 2 so long as the are not in series, or this is also old hat ...............I wish some one with good technical knowledge would clear this up.

[shy guys]

I'm not a genius but have been told by my auto electrician that it is no good to disconnect any battery while a motor is running it causes the altinator to spike or something or rather

[cormorant]

There are good and bad quality battery switches and there is also ones which "make before break" connections when switching between 1 and 2 or both. That means the motor as long as you don't go past off will always be connected to a battery and it should minimise a problem to the alternator. That said I am old schol and don't switch battery switches with motor running or ignition on.

As a side comment I am always amazed that mates don't understand the basic config of their battery and electrical system , where fuses are etc as they are asking for a offshore issue in the future as a lot run on both batteries all the time and yet think because they have 2 batteries all will be OK.

[WalrusLike]

... older days I would have said maybe, but these days you would think that there would be protection against ............. what would you need protection against ...........

I am just a layman but I am pretty sure that the common isolator switches don't have any circuitry in them to protect against anything. And a battery is still a battery like it was in the old days. So if it was risky then, it still is.

As I understand it you can treat electricity like water.... Voltage is pressure, ampage is flow, resistance is pipe restriction.

If you have a bung cell in a paralleled battery setup it's like two water tanks with a connector pipe and one has got a hole. They both are going to go down.

Or if ones flat then it's like two different water levels... Join em and they will both reach the same height(voltage).

Keeping them isolated means one can't drain the other.

What do you gain if you have two in parallel? If they are both healthy then you've doubled your cranking amps but in that case you didn't need to because either would've started you.

My original thinking was if one was low and the other good then starting on both meant they would both be fine after a bit of a run.

But as I was told above... If the flat one was actually sick then it could drag them both down. And you'd be out there with no good battery to get you home.

So I now start on 1 and at destination switch to 2. Then if I flatten it with fresh water pumps and lights etc no harm... I can switch back to the healthy. If instead its not flat, it will start me and get charged on the run home.

If any of this is wrong I am definitely open to learning more about it... Don't hesitate to correct me.

[cormorant]

Start at ramp on 1. After stopping and putting trailer away switch to 2 via off and start. That way you know both batteries and switch are good before you leave the ramp . Your sounder on will tell you the standing voltage before you crank each battery and voltage afterwards will confirm alternator is working. As you have said run out on one battery and back on the other but make sure you are putting plenty of charge back into your batery that has teh accesories ( sounder, radio) running off it. One of the VSR is a assett if set up correctly and if the owner knows how to use it means 2nd battery will automatically be charged as required after the main battery is up to voltage.

[WalrusLike]

Start at ramp on 1. After stopping and putting trailer away switch to 2 via off and start. That way you know both batteries and switch are good before you leave the ramp . ....

Thanks for those good tips.

I am often wrong, so my thinking here is probably flawed... but isn't there a possible downside to that method?

Havent you then pulled one of your starts out of the first battery without recharging? What if it was not at full charge and now it's almost out of oomph?

( "oomph" is a technical term we experts use to describe the battery's capacity to wake up a sleeping motor)

So I very much like the comprehensive checkout that your method gives, but am unsure about the price to pay. What do you reckon?

[Dignity]

I am just a layman but I am pretty sure that the common isolator switches don't have any circuitry in them to protect against anything.

Walrus, I was refering to the motors having circuitry to protect them and not the isolator switches which I have pulled apart and am aware of their limitations. I have heard all of the arguments mentioned and when talking to people with lots of technical knowledge they tell me that most of them are urban myths but once again I do't know enough about electronics ( read that as negligible or non existant) to argue the point. For example what is a "spike" to the system?????? often referred to when discussing switching from 1 to both to 2 on the isolator switches, this is one area that I think is very clouded by non technical geniuses.

[Fed]

Although those 4 position changeover switches are 'make before break' I wouldn't trust them at all. Low voltage, high current plus a salt water environment is a recipe for failure.

[WalrusLike]

Walrus, I was refering to the motors having circuitry to protect them and not the isolator switches.....

Ah.... Sorry. I see what you mean now.

I too don't know the technicalities but am just parroting what I have been told. One salesman and two mechanics have told me not to switch batteries while the motor is running because it can bugger the rectifier.

Don't know of that's true but I am not going to test it.

I understand your frustration with the cloudy mishmash of stories but I just reckon I will stick to the safe side of the street and follow the advice.

We could say that the first concern about paralleled batteries dragging the good one down was proven by Winf.

Now has anyone broken their rectifier by switching while the motor was running? Does anyone here do that routinely and so can prove for their motor it's safe?

[cormorant]

Yes older motors stufed rectifiers regularly with bad battery connection or bad switches or switching between batteries while running. It as never a case of every time you did it cost you a rectifier but it did happen a lot. Ever wondered why there are no alternators and rectifiers in motors at the wrecking yards - they used to be like gold for some models

Have I switched over with new near silent Modern outboards by accident - never happened with a smokey 2 stroke - yep - got away with it a few times over the last few years but not going to push myluck. Yep new motors electronics should be better but ???????? but they aren't designed to not have a battery on the other end.

The sudden change in voltage , current can also screw with your sounders and radios when moving battery switch while motors are running.

It is just a bad idea and probably outside the design of most alternators regulators and rectfiers. I know that you are pushing crap upill trying it on as a warranty claim.


In reply to above about the lost start juice my thoughts are by the time you have it idled over to the ramp and warmed up a little most would be back in with the quick way modern motors crank and fire. I see your point .
With twin motors and 4 batteries I have a bit of spare juice and can cross over to start other motor if both batteries were dead on one side with a solar panel to keep em all in good nick. .

[WalrusLike]

Great post Comorant. Puts all the questions in my mind to bed. Thanks very much.

It's why I love this forum... Helpful folk with deep knowledge.

Thanks again. (I can't use the "like" and "thanks" buttons because I access the forum by mobile phone using Tapatalk. My mobile phone stupid-bloody-autocorrect is also the reason why my posts sometimes heave 'ergo' instead of having 'rego' ). 😄