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MG MGB Technical - Battery Discharge

Hi Guys

A friend of mine is complaining that his battery is running flat after a few days. The volt meter on the charging circuite apparently shows a steady charge rate when running, so we suspect a wiring short to earth. Am I right in assuming that if the battery is discharging with everything switched off, it must be in the part of the circuit that is continually on, ie. White or Brown wires. We pulled the engine and replaced the clutch last year and he thinks thats when the problem started.
Any tips or suggestions would be appreciated.
Ken Martin

Ken. A bad battery can do that. First thing to do is to fully charge the battery and disconnect it from the system--i.e. remove both the battery terminal clamps. Then, let it sit for "a few days" and reconnect it. If the battery is flat, it is a battery problem. If not, let us know and we can provide more information. But, first, let us define where the problem is.

Les Bengtson

I agree with Les. It's easy to disconnect the battery and check voltage at intervals over a period of days; if it drops, the batt is DOA.
Other than that, it's not White wires, since they are dead when the key is OFF. browN or Purple are the only choices, and it will not be a short to ground since that will cook wires, unless the short is after the load, which doesn't happen often. That leaves something powered by browN or Purple. It 's frequently the trunk or interior lamp staying on, and sometimes a fault in the alternator. Remove a reverse light to see if the trunk light is ON with the lid closed - common for the switch to be dead or misadjusted. All Purple circuits can be checked by removing the browN-Purple fuse. The alternator can be eliminated by unplugging it and seeing if the thing dies over the appropriate time.
FR Millmore

You can check for and locate a drain by disconencting the battery ground strap and conencting a voltmeter in its place switched to its 12v scale. If it reads 12v there is a drain.

Dynamo equipped cars should register no drain from original equipment, but if clock, radio, alarm or sundry other items have been added they can cause a drain, so they should be disconnected. Alternator equipped cars normally have a very small drain and this may show up as a few volts on the voltmeter, this can be ignored, and if that is all you see there is no significant drain.

If you have a 12v drain, or even if you want to confirm a lower voltage os coming from the alternator, unplug it and it should drop to zero. If a full 12v drain drops to zero doing this then one or more of the diodes in the alternator is suspect.

If you still have 12v shown disconenct the purple fuse, which eliminates horns, interior lights, and sundry other stuff.

If still present you will have to start disconencting browns from the ignition switch, main lighting switch, and starter and ignition relays where provided.

If *still* present disconnenct the browns from the solenoid. When the indication drops to zero the last circuit you broke is the one that contains the drain.

Batteries can also leak across their tops if condensation is present, as well as leak internally i.e. bad battery. A good battery should be able to hold its charge for a couple of months or more in an MGB, although some alarm systems can reduce this significantly.

I'm having a similar problem with my son's BMW. Sometimes I can leave it 4 weeks, starting it occasionally for a fe moments to manouvere it, it it will be fine for four weeks or more. Others left entirely alone it won't start after just three weeks. Battery shows 12.2v (about the same as my other cars after standing a while) after 3 weeks with battery disconencted. Drain measures about 45mA which is supposed to be 'normal'.
Paul Hunt 2


45ma will drain a fully charged average battery in 1000 hours, or 42 days.

What you need is a battery tender, like an Optimate, that plugs into a trailing socket on the car, (where you can see it when you get in). This does leave a visible cable across the drive, and the mains unit needs to live under cover though.
I have one connected to the MGB, and always have a fully charged battery to play with.
Martin Layton

Ken - You state in your post "The volt meter on the charging circuite apparently shows a steady charge rate when running..." What are you considering a steady charge rate? The volt meter should show about 14.5 volts when the engine is running above 1000rpm. If the voltage is only showing 12 volts or less, then the problem is insufficient charge for the battery. If the car is equipped with an alternator, the voltage should remain at 13 to 14.5 volts after the engine has bee reved above 1000 rpm and the regulator has taken over the voltage regulation. If the voltage varries a lot with engine rpm, then the alternator's regulator is bad and should be replaced. If the car instead, has a generator, then the fluctuation of voltage with engine rpm is normal. Cheers - Dave
David DuBois

Ummmmmn, After about three hours of checking, testing and pulling bulbs, he says "would a loose fan belt have anything to do with it"? "Uh yes it would" and it was...very loose. In fact the slider bracket was on backwards and the pinch bolt was actually in the bracket mounting hole so nothing could move and therefore the belt couldn't be tightened. We changed everything around tightened the belt and the battery started charging at around 13.5v I did notice that the meter was very sluggish though. Probably because it was used to being on permanent discharge! Anyway, after a few raps over the knuckels with a torque wrench, he admitted that he thought the battery was discharging because the car kept stopping after a few miles and he would have to jump start it. I can understand that. So now we sit and wait until the snow goes away to see if there is any other discharge. By the way Dave, I did say "apparently shows a steady charging rate" So the moral of the story is, don't assume it's the fiddly stuff, look at the basics first.
Once again, thanks for all he advice.
Ken Martin

This thread was discussed between 26/03/2007 and 07/04/2007

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