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MG MGB Technical - Battery problem

Help !! I'm having problems with battery losing charge. tried fitting new battery but it's only registering 10% charged on my charger - even after several hours of charging. Is this likely to be a short somewhere ? If so are there any likely trouble spots ? It's a 76 roadster. Cheers, Iain
I Mason

Basics, first: Clean and make certain that battery cables are very tight on the posts. Likewise the ground strap attached to the body (both from the battery and from the engine). Same with connections from the battery to the starter (but if you decide to tighten, disconnect a cable at the battery end so your hair doesn't get a collective erection.)

Dan Robinson

If it's cranking OK when fully charged it is more likely to be a drain than bad connections. Are you charging the battery from an external charger with the battery still connected to the car? If so what happens if it is removed from the car? If you still get the same indication then either the battery or the charger (or possibly both!) are faulty. If it now charges OK then again it is probably a drain.

Turn everything off, including interior and boot/hatch lights. Disconnect the battery ground strap and connect an analogue (digital instruments can give unpredicatble results) voltmeter in its place *switched to its 12v scale*. On an alternator equipped car like yours you will probably see a few volts registered (should be zero volts on a dynamo-equipped car), that is a minute drain which is just the reverse-leakage current of the alternator diodes and can be ignored. If you see 12v registered you have a significant drain. Unplug the alternator, if it falls to zero now the alternator is faulty. If still at 12v remove the purple fuse (horns, interior lights) from the fusebox and disconnect the in-line fuse for the hazards, if either of those causes it to drop to zero then the drain lies down that path. If still 12v you will have to start disconnecting things connected to the brown like the ignition switch, main lighting switch, starter relay, ignition relay if fitted, etc. If still there then remove the browns from the starter solenoid and finally the battery cable from the solenoid until the voltmeter reading drops to zero. As long as you have the voltmeter connected in place of the battery ground strap working with the browns and battery cable will be safe.
Paul Hunt 2

Many people, I understand, hook their charger to the battery clamps rather than the posts. I have an added complication due to a cut-off switch behind the driver's seat.
Dan Robinson

I had a persistent battery drainage from the time of purchase from the DPO, and even my mechanic couldn't trace it. He had the starter diode pack replaced as a last resort. No luck, and I was getting POed.

As it happened, before he returned to school, my son replaced the Sony radio/CD player that the DPO had installed with my newer Panasonic that had been sitting on the garage shelf. Took him less than an hour. Voila! No more battery problems. And the speakers sound better as well.

Obviously the DPO had goobered up the previous installation and simply kept the car connected to the floater/bounce charger to keep it charged.

That device is now residing in a drawer in my garage cabinet, as I will now get startup immediately even after two weeks of non-use.

When I informed my mechanic, I detected a hint of embarrassment at his oversight.
Gary Kinslow '69B roadster Pale Primrose

This thread was discussed between 22/09/2005 and 25/09/2005

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