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MG MGB Technical - Battery drain

My '69 BGT went dead sitting for a couple months without being driven. I tried charging the battery, and it worked for about a day then sat overnight and went dead again. Alternator seems to be working. I replaced the battery and drove around for a couple days, including a couple 20 minute drives using headlights and fan. The car started right up even after the night time trips. Then I let it sit for a few more days and it went deader then dead again.. So is there anything I should check for that might be mysteriously putting a drain on my battery?
-Bill
Bill Mertz

Bill - Check that the light above the rear hatch is not left on. If that is not the problem then after getting the battery charged up, connect a multimeter, set for amps between the negative post of the battery and the ground cable. You should have some kind of a reading on the meter even with the iginition off. Now start pulling fuses one at a time and disconnecting things until you see that reading drop, that will tell you where the problem is located of at least on which circuit in the case of a pulled fuse. Don't over look the alternator. There is a diode in the alternator that can go bad and cause a drain on the battery even with the ignition off. Good luck - Dave
David DuBois

Dave thanks for the advice.
Someone else told me the alternator diode thing today as well.
I'll give it a look tomorrow.
I don't think my dome light even works, and if it does I don't know how to turn it on. But I'll pull the bulb out just in case.
-Bill
Bill Mertz

Bill,

A dirty, wet battery can drain itself over a period of months as a small current can travel through the crud between the terminals. Best option is to have a switch in the line to stop parasitic losses and clean the top of the battery when laying it up for a period.

John
John T

The PO of my 73 MGB had the same problem with the battery draining over night. They finally just parked the car.

When I got the car and went through the electrical system, it turned out that the front right turn signal wiring insulation had cracked and fallen away, leaving bare wire that was shorting out on the inside of the fender. It was simple fix, but hard to find. So your problem may be something as easy or as hard a simple short somewhere.

I had another car that was doing the same thing and it turned out that the wire to the license plate light was shorting out against the bumper.

David has described the correct solution, using a volt meter and pulling fuses to isolate the drain.

I prefer not to have them, and I am sure that I will get a lot of grief for saying this, but electrical shorts are like a big jigsaw puzzle. They can be very frustrating, but it makes you feel good when you solve the puzzle.

Robert
Robert Browning

Since I only drive the car weekends I put in a battery disconnect and a 'Battery Minder'. That way I keep the battery fully charged. Short circuits can pop up in mysterious places.
P.J.F. Rogers

Just FYI, I hooked up the multi-tester between the battery and ground cable and was getting a current draw. I got lucky and unplugged the alternator first and the draw went away. So it looks like I have a bad diode in the alternator.
Also installed a battery cut-off switch while I was in there.
Thanks again
-Bill
Bill Mertz

Bill

You did not say how much the reading was. A drain from the alternator of 10-20 mA is normal. A small drain from the battery can be tolerated.
werner haussmann

I wanna say it was 75mA.
And with the alternator disconnected there was no drain whatsoever. With the battery disconnected the car was able to sit for several days, then start up fine with the battery reconnected. Where it would die overnight, or a couple nights if I left it connected. So I'm pretty sure I found the problem.
-Bill
Bill Mertz

25 uA for leakage in a diode. 25 mA is 1/3 watt at 12 volts!
Stan Best

Stan,
Son't know what uA is, but assume its a much smaller unit of measure than mA. I also realized it may not have been 75mA, but possibly .75mA. Just got this new digital meter and am not very familiar with it yet. I do know it pegged my old analog meter on the mA setting.
-Bill
Bill Mertz

a micro Amp is of of course 1/1,000,000 amps or 1/1,000 mA by definition. Silicon diode leakages will be down around this sort of number. I can imagine locomotive traction diodes leaking mA, and will see if I can find some data.
Stan Best

http://www.dynexsemi.com/assets/DNX_RD65FV.pdf If you can't be bother to click on it this kiloamp plating diode has a reverse leakage of 150 mA. Although while soaking up all those carriers when it is being turned off can take 230A!
A 1A 1N4000 is specified as 5uA at 25C and 50v. So 75 uA seems a reasonable number for an alternator.
Stan Best

Normal alternator diode leakage is in micro-amps and the battery can tolerate this for months. The best way to detect a leakage is to disconnect the battery ground strap and connect an analogue (digital instruments may give varying results) multi-meter on its 12v scale in its place. Alternator leakage will typically display as a few volts and can be ignored. If you see a full 12v the drain is significant. If removing the plug from the alternator drops this to zero then the alternator is at fault. If it stays at 12v then pull the purple fuse (horns, interior and boot lights), thn the browns from ignition switch, main lighting switch, and starter and ignition relays where fitted until it drops to zero. Of course if you have alarms or radios fitted these can also exhibit a constant drain. On my V8 the alarm would flatten the battery within a couple of weeks, and knacker the battery after a year or two. I fitted a cut-off switch.
Paul Hunt 2

Paul,
Seems like measuring the amperage, like I did, would achieve the same result as measuring the voltage right? If there is something draining the battery its going to be using current ie amps.
Bill Mertz

This thread was discussed between 14/08/2006 and 18/08/2006

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