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MG MGB Technical - Flat batteries

Had my two 6 volt batteries go flat again, I have had this problem for about a month now. Had them both checked and found one was faulty,these batteries were both about 8 months old, the faulty one was replaced. The alternator charging rate was also checked and found to be ok,everything fine for about a month now they are flat again. Where do I go from here?
D Bates



There is a thread just before this one which most likely will answer your question. It is titled:-

6v battery what do you like

I hope that is helpful, it offers advice from some very qualified contributors.

If you continue to use 6 volt batteries then both should be replaced at the same time.
Ian Buckley

If it was only 8 months old it is presumably under warranty. If on testing it is not the battery, then there is a drain somewhere. Things such as radios, sat navs or even boot lights remaining on can be the cause. Obviously the amount of use against time sitting in the garage determines how long it is before a drain flattens the battery.
Disconnect the ground terminal (probably -'ve)and leave the car for the same length of time as it took for the batteries to run down. If when you reconnect the batteries are flat it is the battery(s), if not you have a drain. Finding a drain can be difficult but a methodical apptoach is vital. 6v batteries are uncommon in mainstream use, so can have been standing on the shelf for a while before you buy them. A specialist such as MGOC should have good stock turn. If you use the car infrequently, a battery conditioner (as opposed to an overnight battery charger) will keep the batteries in good shape.

HTH michael
Michael Beswick

One of the problems of having two batteries connected together is that if one goes down, maybe by being faulty or whatever is that the faulty one will pull power from the good one and loose it, so you finish up with having two flat batteries.
The recommendation of changing both together usually will eliminate this but if one has a fault early in its life (as yours seem to have done) the other one is wasted. The best reason for fitting one 12 volt battery apart from the saving in cost. Check out an 063 type.
Trevor Harvey

As Michael says it sounds like a drain. Disconnect the battery ground strap and connect a voltmeter on its 12v scale in its place. I'd advise using an analogue as I don't know what various digital instruments might show. With an analogue, and everything switched off including courtesy and boot lights and any aftermarket items disconnected, you will probably see a few volts registered. This is expected and is the normal reverse leakage of the alternator diodes which is microscopic. If you see a full 12v with fully charged batteries you have a drain. Unplug the alternator, and if the drain drops to zero the alternator is suspect. If it stays at 12v it is something else, so you will have to remove the fuse in the purple circuit first, and if that makes no difference then start disconnecting brown wires from the ignition switch, main lighting switch, and hazards fuse, starter and ignition relays if provided and finally the solenoid until it goes. The drain will be from the circuit or component last disconnected.

I agree that at 'normal' battery change intervals both should be replaced together, but if one fails very early under warranty then it is fine to replace just that (in fact the supplier almost certainly won't replace both as their own cost) then replace both again in the fullness of time. If one failed just outside the 2 or 3 year warranty then given the typical 10 or so year life I'd only replace one. If more than half-way to 10 years then probably both.
Paul Hunt

"One of the problems of having two batteries connected together is that if one goes down, maybe by being faulty or whatever is that the faulty one will pull power from the good one and loose it, so you finish up with having two flat batteries."

Only with two batteries connected in parallel, not in series like the twin 6v of the MGB. One dead or dying 6v battery may well prevent the engine starting of course, but it won't flatten the good one.
Paul Hunt

Regardless of the voltage of the battery (6 volts or 12 volts), long periods of inactivity will cause them to be come sulfated. Once that happens, it is difficult to revive them. One way to prevent this situation is to attach a battery tender (also called battery manager, battery conditioner or some similar name). These units are much more than just a trickle charger (which will overcharge a battery over a long period of time), they bring the charge of the battery up to optimum level, then shut off and let them discharge to a preset level and then recharge the battery. This is continues for for as long as it is connected, keeping the battery in good condition. If your car sits, undriven for long periods of time, it is well worth the cost to purchase and use one of these items. Cheers - Dave
David DuBois

I have one of these chargers on my '67 B and the batteries last an average of 10 years. I also have one installed on my motercycle and it triples the average life span of the battery. RAY
rjm RAY

Many thanks for the help. I have tried pulling off brown wires as you suggested Paul and found that when I removed the brown wires connected to the starter motor I had no drain at the batteries. Now not being that good with electrics I'm not sure exactly what that shows me, have I got a problem with the starter motor or is it the solenoid.
The car is a 1974 B, the batteries were supplied by the MGB Hive so shouldn't have been on the shelf long I wouldn't think.
D Bates

That's a starter problem and is due to conductive dust from the starter solenoid as the contacts "splash" when closing or opening. The debris from this is mainly a thin layer of copper from the contacts to the body. Take the starter motor from the car and dismantle the contactor mechanism and clean it. You may as well clean the debris from around the starter motor commutator and brushes at the same time. Do a resistance check from the main starter terminal to the body before you replace the staryer in the car.
AG Martin

Thread entitled Starter problem diagnostic might be worth reading
Michael Beswick

Only if the drain was showing with the battery cable connected, but the browns removed, would it be the solenoid. If removing the brown wires from the starter stopped the drain, while the battery cable was still connected, then that shows the problem *isn't* on the starter but somewhere on the brown wires, and as this could have serious implications I'll assume this is the case.

On a UK 74 there should be two brown wires going up to the alternator, probably one thick and one thin, plus another thick one feeding the rest of the cars electrics. You will have to connect the brown wires back to the solenoid stud one at a time, with the battery cable, alternator unplugged, and see which one causes the drain to appear again. When it does test the two browns in the alternator plug and the brown at the fusebox to see which shows 12v. If it's either of the wires in the alternator plug then they shouldn't be connected to anything else so it looks like the insulation has got chafed somewhere. This is potentially (no pun intended) very serious as if it developed into a full short it would burn the harness at the very least and possibly the car. But that is relatively unlikely, the drain is more likely to show when you have 12v on the brown at the fusebox.

The wire up from the solenoid goes to a sealed connection behind the dash, with four spurs leading off it. One goes to the lighting switch, one to the ignition switch, one to the in-line fuse feeding the hazard flasher (probably behind the centre console), and the remaining one goes first to the fusebox and then to the starter relay. If the brown wires are removed from all these components (and the in-line fuse removed) and you still have the drain, then either a PO has connected something else to one of these brown wires somewhere, or again one of them has chafed its insulation and is partially short. And again this is potentially very serious if it should develop into a full short. At the very least you should disconnect the battery ground strap while the car is unattended, but it could still go full short while you are driving it and in fact this is more likely then when parked, and the wires will be burning merrily while you frantically get a screwdriver and spanner out of the boot, remove the battery lid, and undo the clamp. It's one of the reasons why I have fitted a battery cut-off switch to both my MGBs.
Paul Hunt

This thread was discussed between 09/11/2009 and 11/11/2009

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