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MG MGB Technical - Battery discharges almost overnight

I usually try to get out in the B once a day, or at the very least every other day, however lately my priorities have been upset by, of all things, WORK.

Last week, after two days of non use, the B would not start. I charged the battery, went for a run, and all was fine.

Last night,three days after use, she would not start again.

The battery is only four months old and, when charged, checks out ok. I have a clock, which is on power all the time and a CD player.

The battery is obviously being drained from some source. Could anyone advise me where I should start the investigation?

As always, your collective helpfull advice is much appreciated.

Regards from Down Under

Graham
GRAHAM HOYLE

Check the archive, there should be a lot in there as this subject comes up for time to time. Your going to have to check for a drain circuit by circuit. Use a test light, disconnect the ground cable. Conect the test light to the negative terminal and a suitable ground. If the light comes on you have a drain. Then pull you fuses one at a time, check the test light. If its still on, replace the fuse and move to the next until you finally pull one and the light goes off. That fuse circuit has the short causing your drain.
william fox

lamp in trunk????
J Arthurs

William,
Thanks, I'll try the test at the first opportunity.

J.Arthurs
No, I've checked the trunk/boot light but no bulb is fitted.

Thanks

Graham
GRAHAM HOYLE

Graham. The basic test for a bad battery is to fully charge it and let it set, not connected to the car for 24 hours. Then, fit the connections and try to start the car. If the battery is dead, it is a bad cell. If it is good, something is draining it. That is when you use Paul Hunt's technique of hooking a test light between the ground terminal of the battery and the clamp of the ground wire and begin to disconnect things in a systematic manner. Yes, batteries can go bad in a very short period of time. Not often, but often enough to make it worth while to check out the battery as a first step whenever these type of problems come up. Les
Les Bengtson

Bad diode pack in the alternator?

Daniel wong

Adding to Daniel's comment - is the ignition warning light behaving correctly? (comes on when you first turn the key, goes out then stays out) check to see if the IWL is glowing very dimly when you are on the road - worth having the alternator checked anyway if you get through the above suggested checks and are still stumped. In the states, reputable alternator shops will test for free or for a small charge.
Greg Peek

First check it is charging - you should see about 14v on a brown at the fusebox at a fast idle. Then check for a drain. A test-lamp will indicate a significant drain, but there are smaller drains that will flatten the battery (probably take more than overnight though) without being big enough to light a test-lamp. To check for these I use an analogue voltmeter connected in place of the battery ground strap. If this shows 12v you have a drain, and the next thing I do is eliminate the alt by pulling the plug out of the back. It is normal to see a drain of 6v or so from an alternator, this is the normal leakage current of the diodes and is insignificant. Damp on the battery tops (OK, maybe not at the moment) can also cause small drains.
Paul Hunt

Thanks everyone, I'll have a crack at a few of tests that you have recommended this weekend.

Regards

Graham
GRAHAM HOYLE

I had the same trouble about 2 months ago. The posts are in the archives, look under dead battery.

The root cause turned-out to be the voltage regulator. I had thought it was a drain or short somewhere in the wiring. There was a short at the time, which shut down the fuel guage circuit. That threw me off, but again, the problem of battery drain was due to insufficient charging.

I now have a lighted voltmeter mounted under the pull ring for the bonnet. Oh yes, and a new regulator.

Fred
Fred Horstmeyer

Glad I read this thread. Battery in my 79 was dead yesterday, after sitting a couple weeks. Put it on a charger overnight. Still nothing. Took the battery out of my 71 bgt and put it in the 79 and started right up. I'm going to leave the battery on charge until tomorrow and see if that helps. Hope it's not a problem that kills both batteries.
Sam C

I had the same problem in my 71BGT. In this case it turned out to be the CD player. It had malfunction internaly so that the lamp stayed on all the time, thus draining the battery. I wasn't driving the car at night so I never noticed the light staying on. My point is that it may not be the expected!
gerry masterman

This thread was discussed between 05/09/2002 and 07/09/2002

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