voltage back flow?

Stu Jackson · May 2, 2012

Did you check the voltage at the battery?

Dantodd · May 2, 2012

As Stu said check the voltage at the battery and use a good DVM. Don't rely on the built in voltage meter.

There are a number of possibilities: (in order of likelihood)

1) you are only charging one of the banks with your charger and/or alternator

2) your battery is dead because of age, poor maintenance, excessive discharge etc.

3)you have a short somewhere in your system

Where do your alternator and charger connect to the batteries? The most common place is the "C" post on the switch which means if you always leave the switch on "1" the bank on "2" never gets charged.

kloudie1 · May 2, 2012

above all true.. another thing is that the bilge pump is often connected to the hot side for one of the batteries on the 1-2 switch.. check that side carefully.. It would be rare for the switch itself to short without going up in smoke..

JohnShannon · May 2, 2012

What kind of meter? A digital type will have an input impedance of 20megaohms, which is a lot. Call it 10.7 megaohms to make the math easier. Now if you had 1microamp of leakage, that is a millionth of an amp leaking from the energized side, then you would show 10.7 V on your voltmeter.

That is still pretty much off.

Try loading the 10.7V side with a small load and see if the voltage goes away, alternatively go try an old timey voltmeter (with a needle) and see what you get.

navigator97 · May 2, 2012

Why would you not connect one battery to #1 and one battery to #2? That way if one battery goes down on its charge, you can switch to the other and have full voltage (at least initially).

Don S/V ILLusion · May 2, 2012

navigator97 said:
Why would you not connect one battery to #1 and one battery to #2? That way if one battery goes down on its charge, you can switch to the other and have full voltage (at least initially).

I assume there is a dedicated (separate) engine start battery not connected to the 1/2 switch. If not, you are correct.

kenn · May 2, 2012

Does your battery meter have its own #1-#2 switch? What's the position of your main battery 1-2-B switch when you get 10.7v on #2?

Just a guess -You're probably seeing the residual voltage stored in the capacitors of some of your electronic equipment. To prove this, switch all of your DC panel switches or breakers off; at some point that voltage will disappear.

Stu Jackson · May 2, 2012

OP asked about mysterious voltage and we're talking about whether he should separate his battery banks? C'mon...

To the OP, I'd look behind your panel and find out what your switch and voltmeter are connected to. You may have already looked at the switch, if I read your post correctly.

centerline · May 2, 2012

if you have a solid 10 volts showing where there are no batteries connected, then you are getting a back feed.... period.
unhook any and all wires from the #2 term on the batt switch, then check the post itself to see if the switch is leaking across.
if it is a capacitor that is still loaded, you should see the voltage on the meter drop slowly as it remains hooked up over a 2 to 3 minute period of time. (this drop will even occur with a digital meter) sometimes an alternator will show a voltage back feed, but it has no real current and measuring it will start draining off the stored voltage. hook a test light to it (smaller is better) in addition to the voltmeter and see if it makes the light glow. if the light has any glow at all, and you still have a slight voltage reading, you have something linking up somewhere in the system that is allowing you a bit of current to seep thru. its highly possible it could be a gauge. less likely, but possible would be corrosion? the green stuff can carry a lot more voltage away than a person would think.... if you find it to be a real back feed with current flow, you just need to start disconnecting things one at a time until you find the corrupted wire.....

Dantodd · May 2, 2012

Jack said:
Hi,
thanks for the quick responses...
My apologies for not explaining myself better...

both batts are connected to #1. I got 12.7 volts at meter.
Volts at the batteries is good, 12.5 & 12.7
There are NO batteries connected to #2.
When the 1-2-B switch is in the #2 position I am getting about 10V at the meter.

Just to make sure, all of the electrical on the boat stops working once you switch over to #2 correct?

As someone else has suggested it could be a capacitor discharging is it still showing this voltage after half an hour of being on #2?

If it is, measure the voltage at post #1 and #2 directly using a good digital meter. It is certainly possible that the problem is in your meter or the meter wiring and eliminating that as a variable is the first step.

Stu Jackson · May 2, 2012

For those of you who mentioned it, where would a capacitor be on a boat?

Mike B1026 · May 2, 2012

Electronic equipment