Mysterious battery drain... where to begin?

[carambo46]

The only unswitched thing I would have on my boat would be the bilge pump and the VHF radio good luck

[carambo46]

The way I figure out what an unidentified wire runs I turn everything on and then pull that wire off and see what with what went off

[ramblinrod]

Originally Posted by Jim Cate

Umm... this does not make any sense. A voltmeter between the pos battery post and the positive cable should read zero, because there should be no voltage drop between them... even if there were a significant current flowing.

Perhaps you meant he should disconnect the cable from the battery and then, having set his multi meter to an AMPERE scale, connect it between the two points. That would indeed measure any current flowing. I'd start out on whatever the highest ampere scale the meter offers, and if nothing shows, work down to the most sensitive scale. Do be careful NOT to get the meter between a positive point and ground. The ammeter is essentially a short circuit, a large current will flow and the meter will be toast (some have a fuse in that circuit, and it may save the meter. Still not a good idea!).

Jim

Original post was correct.

If the battery circuit is open (no load) and one disconnects the battery positive terminal and positive post, the voltmeter will read 0 Vdc. There is no potential between the two, because the circuit is open elsewhere.

If the battery circuit is not open (loaded) and one measures a potential between the disconnected positive terminal and battery post, that indicates there is a closed circuit (load on the battery) and the only thing currently disconnecting it, is the gap created by removing the battery terminal.

[Cadence]

Originally Posted by Jim Cate

Umm... this does not make any sense. A voltmeter between the pos battery post and the positive cable should read zero, because there should be no voltage drop between them... even if there were a significant current flowing.

Perhaps you meant he should disconnect the cable from the battery and then, having set his multi meter to an AMPERE scale, connect it between the two points. That would indeed measure any current flowing. I'd start out on whatever the highest ampere scale the meter offers, and if nothing shows, work down to the most sensitive scale. Do be careful NOT to get the meter between a positive point and ground. The ammeter is essentially a short circuit, a large current will flow and the meter will be toast (some have a fuse in that circuit, and it may save the meter. Still not a good idea!).

Jim

Jim, if something is presenting a load, voltage would be seen between the disconnected lead and battery anode. With the high impedance of the meter any resistance down the line will essentially show a ground.

[senormechanico]

Originally Posted by OrangeCrush

Then I tested the smaller cable marked "unswitched panel" and it showed a rhythmic fluctuation between 30 and 60 mA, like a metronome.

Do you have a small fan from a computer being used as a small ventilation or exhaust fan? That sounds like the kind of draw you would expect if it was trying to start with too much series resistance.

Also, an ammeter in series with the battery lead is the correct way to check for leakages.
Any load, no matter how small, will cause a high impedance voltmeter to read almost full battery voltage if used in the manner described.
Cadence is correct.

[OrangeCrush]

I connected only that wire and the only thing that regained power were the bilge pumps and the battery monitor. I'm guessing the pulsating amperage I saw when I tested that wire was the battery monitor.

Haven't done the math yet but it seems unlikely that a draw of ~30mA would be draining my 440ah bank in a week or two.

I disconnected the batteries and I'll test their voltage after a week or two in case they are indeed the culprit. If it's not I'm not sure what the next step would be...


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[Bleemus]

Put the battery monitor on a switch? Nobody is looking at it when your off the boat right?

[Jim Cate]

Originally Posted by ramblinrod

Original post was correct.

If the battery circuit is open (no load) and one disconnects the battery positive terminal and positive post, the voltmeter will read 0 Vdc. There is no potential between the two, because the circuit is open elsewhere.

If the battery circuit is not open (loaded) and one measures a potential between the disconnected positive terminal and battery post, that indicates there is a closed circuit (load on the battery) and the only thing currently disconnecting it, is the gap created by removing the battery terminal.

Originally Posted by Cadence

Jim, if something is presenting a load, voltage would be seen between the disconnected lead and battery anode. With the high impedance of the meter any resistance down the line will essentially show a ground.

Originally Posted by senormechanico

Do you have a small fan from a computer being used as a small ventilation or exhaust fan? That sounds like the kind of draw you would expect if it was trying to start with too much series resistance.

Also, an ammeter in series with the battery lead is the correct way to check for leakages.
Any load, no matter how small, will cause a high impedance voltmeter to read almost full battery voltage if used in the manner described.
Cadence is correct.

The problem is that he neglected to mention disconnecting the battery cable. As you all state, with it disconnected, any load will generate a voltage reading between battery and cable. Not a very good analytical tool, IMO, but the ammeter, as he has now demonstrated, is. He's on the right track...

Jim

[OrangeCrush]

Well by my calculations (please feel free to check me) it would take over 500 days of a 35mA draw to exhaust a 440aH bank. That combined with the fact that the previous owner never had problems losing charge while in storage with everything connected makes me think it might indeed be the batteries.

Beside checking how much voltage they lose while disconnected, is there any other way to know if my AGM batteries are toast?


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[hellosailor]

Divide and conquer. Start with what RW said: Charge the batteries, then disconnect them all and go away for a week. When you come back, see if they all have the charged voltage, or if one in particular is way down. If any of them did not hold voltage, sitting disconnected that way, it/them are toast and the others were just being puled won by it. On internal short on one battery can pull down an entire bank, these things happen. While you are off the boat, get "the 12 volt doctor" or "the 12 volt bible" or Charlie Wing's book on boat electrics. You've got three choices: Read up and learn to DIY. Pay an alleged pro beacoupe bucks to fix things. Or just screw around, which ever works best for you. Attacking it piecemeal without understanding the whole system (which is what forum questions pretty much result in) won't really do the same job. It just patches one problem, and when there's been one or more PO's working on the electrics...there's usually more than one thing wrong.

[tkeithlu]

You can do the amps drawn test anywhere along a circuit; at the battery, just before the piece of equipment, just after it, at the ground. Knowing that should make it easier for you to avoid tracing hidden wires. You are just inserting the amp meter in any part of the 12v+ to ground circuit and seeing how much current is flowing. BUT... if you have any reservation about the amperage used not being below the milliamp ranges on your meter face, switch the lead over to the ten amp socket or there's going to be a little puff of smoke....

[Cadence]

Originally Posted by OrangeCrush

Well by my calculations (please feel free to check me) it would take over 500 days of a 35mA draw to exhaust a 440aH bank. That combined with the fact that the previous owner never had problems losing charge while in storage with everything connected makes me think it might indeed be the batteries.

Beside checking how much voltage they lose while disconnected, is there any other way to know if my AGM batteries are toast?


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Get them topped off and disconnect them. If you are on the hill nothing is probably needed if your topsides are tight Check them after a period of time. They may drop a little but shouldn't be a bunch.

[Symphony]

do you have an inverter? Some have an auto function which detects any call for AC. they check off and on, off and on - that will definitely draw your batteries down over time.

sounds like a Xantrex system. the link 10 will pull a little current, also current feeds the memory chips on AM/FM radios, stereos, A/C units. all these vampire loads add up with the cold and deplete more than what the perfect-condition calculations might have you believing.

I lift off the negative side from all my batteries over the winter.

[Cadence]

Originally Posted by hellosailor

Divide and conquer. Start with what RW said: Charge the batteries, then disconnect them all and go away for a week. When you come back, see if they all have the charged voltage, or if one in particular is way down. If any of them did not hold voltage, sitting disconnected that way, it/them are toast and the others were just being puled won by it. On internal short on one battery can pull down an entire bank, these things happen. While you are off the boat, get "the 12 volt doctor" or "the 12 volt bible" or Charlie Wing's book on boat electrics. You've got three choices: Read up and learn to DIY. Pay an alleged pro beacoupe bucks to fix things. Or just screw around, which ever works best for you. Attacking it piecemeal without understanding the whole system (which is what forum questions pretty much result in) won't really do the same job. It just patches one problem, and when there's been one or more PO's working on the electrics...there's usually more than one thing wrong.

I believe you have nailed it. Not just for the OP but for anyone with a boat. Get a book on DC fundamentals. When I see questions about dock AC and polarity lights, a one on AC would be a good read also.

[mike d.]

I talked quite a bit with the tech guy at the company that built the sears PM1, as I have 1 in our boat ! He said the problem, that most people don't understand is that AGMs have to be hit hard charging wise initially, IE high volts/amps, because of their design ! So the question would be, does my charger understand which battery is being charged, can it be set up for multiple battery designs ? The fact that you stated, 1 of your bats. has corrosion on the neg. term. says that, that battery is bad, because the acid is weeping up the post, past the lead/stainless stud ! Forgive me for assuming there are s.s. cable/wire terminals, and regular automotive studs. I have a battery just as you describe. It won't hold a charge, no matter what I do, compared to the other AGMs. Going back to the top where I said does your charger understand which battery is being charged. The tech said AGMs need chargers that will maintain high voltage . I have a Gulfstar 37 with reg bats., but my bass boat has AGMs, which I was having the problem with ! I had to go with the big MinnKota charger, putting 2 charge leads on 1 battery to give it a close 2nd to the amount of voltage it actually needed. These are not my thoughts, just what I was told, by those, who I assume would know!