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Battery pairing problems

2K views 15 replies 6 participants last post by  sgjfrods 
#1 ·
I have the facility to have two battery's on the boat and can alternate with a switch to be drawing power from battery one, battery two or both at the same time. However when there is only one battery hooked up to say to battery ones terminals there is still a potential difference at battery two's terminals. Is this normal or is something wrong here?

I found this out when fitting a simple battery charge indicator. When I only had one battery hooked up I was still getting reading for both battery one and battery two.

Any help is much appreciated. Thanks in advance!
 
#3 ·
Thanks very much for your reply.

Okay would the isolator you mention in layman's terms be the switch that selects which battery you wish to draw power from?

I assume this problem means that no matter what battery I select I am still drawing power from both batteries.
 
#4 ·
I'm not sure what you mean by potential difference. Even if battery two was not connected at all, it would still have voltage, assuming it was charged. If both are connected, but only one selected, your charger may be putting voltage into both batteries separately, even though your selector can power your panel/bus with only one battery at a time.

I think I need a better understanding of the question.
 
#5 ·
I'm saying the circuits may be combined by something - an automatic gadget - other than your manual switch.

If this is only happening while they're getting charged, that's how it's supposed to work. Google those terms with "dual batteries" to start learning.

And yes, we need details on exactly what you're measuring (in volts & amps) and under what conditions.
 
#6 ·
Thanks for your replies, Okay when I have a battery connected to battery ones terminals there is 12 volts across battery two's terminals (without a battery connected connected to battery two's terminals) so only one battery connected in total.

There is no charger connected either. What I was talking about before was a charge indicator so just a battery "tester" which you flick a switch and it will move an analogue dial to tell you how much charge is in each battery (battery one and battery two separately). How I found this out was that I was getting a reading for battery one and battery two, with only one battery connected.

This happens regardless of whether the battery switch is on battery one, battery two or indeed both

I hope this is more clear, please let me know if you require any more info. Thanks alot.
 
#9 ·
Yes both positive cables go to the 1/2/both switch.
With regards to what charges the battery there is a battery charger that constantly tops up the battery when the boat is plugged into shore power. This charger has two wires going to each batteries positive terminal could this be linking the circuits? But as the boat is on hard standing with no power I take the battery off and charge them at home currently.

I know the switch works atleast for shutting the power off as when it's on "off" none of the electrics work on the boat.

Thanks alot
 
#11 ·
#12 ·
What position was your 1-2-both switch in when you measured voltage at the missing battery's connections?

Are you sure your battery voltage monitor inputs aren't connected directly to each battery? You should follow those two pairs of wires back to their source.
 
#15 ·
sg-
There are many ways to waste time and money on a boat, and trying to diagnose electrical problems without a complete understanding of the equipment is one of the finest.
It sounds like you have a used boat. That is a problem because you have no way of knowing what the previous owners did. How the boat was built is meaningless because of that.
And, you don't necessarily want to find out "what is wrong" but rather, you want to find out "What would be right for me?" and possibly rewire the entire system to make that so.
You DO need to understand basic boat DC circuitry. There are many good books, inexpensive, that go over that. A book is better than the internet for this, because it is comprehensive and you are less likely to miss something that is important.
And as part of that you need to buy an inexpensive multimeter (DMM) that would be $20-$50 in the US, should be the same there, and know how to use it.

Given the DMM and some simple knowledge of how circuits might be connected, you will be able to spend one afternoon on the boat and get it fixed--for certain. Anything else will just eat your time and money, and perhaps set the boat on fire as well. (Boats built 20 years ago often did not have fuses and other devices considered essential for safety today.)

Sometimes the fastest way to fix things, is to slow down and start from the very basics.
 
#16 ·
I have been doing some reading as suggested.

There is a battery charger on the boat as shown on the attached image. One that is constantly wired into the batteries. It is wired in by two separate wires going to each positive and negative terminal respectively.
When I disconnected one of the wires from the battery charger to the positive battery terminal the voltage across the eyelets of battery 2 when only battery 1 is connected was 0V. Which is what we want.
Would a conceivable explanation of this be that the battery charger is stuck on "charge" even though there is no power to it as the boat is on the hard stand with no AC power? I.e completing the circuit linking the batteries? Could have been caused by cutting the power to the charger at the mains without turning it off at the switch correctly.
Or is this not conceivable?
 

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