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MG MGB Technical - Battery

Hello good people, I need some advice please. A year ago (July 05)I put a new 12v battery in my Roadster (1972). The following February after 2 months of storage I went in to sort the car out for it's MOT only to find the battery was completed dead. I took it to a local garage for testing who told me "I've never seen a deader battery". I took it back to Halfords who replaced it.

Having used the car quite alot this year I had it standing for 2 weeks. Again I went in to use the car only to find the battery was completely dead again. I took it back to Halfords who again replaced it. Surprisingly, in no way did they blame the car and blamed bad luck for me having been given two duff batterys.

Now, I didn't say anything to Halfords but I'm wondering if the car could be killing the batterys. I have no radio, not even a cigar lighter and I dont usually use the car at night so not even the lights go on regularly! I do have an alarm but hardly ever arm it. Certainly not while it's in the garage

Any ideas for a total novice?? Any advice would be very much appreciated

Many thanks
Ricardo Vincenzi

Dissconnect one battery cable and connect a voltmeter between the disconnected cable and the battery post it came from. If you read a voltage than there is a drain somewhere. Start disconnecting electrical equipment when the meter drops to near zero, you have found the culprit. Since the battery is going down rather quickly the likly suspect is a shorted diode in the alternator.
John H

I have a similar problem with my 70 MGB if I do not drive it every weekend the battery would not have enough charge to start the car after two weeks. I have fitted a battery conditioner that keeps the battery fully charged (available from Moss).
It is a simple job for an auto electrician to meter the batery with every thing on the car turned of to see if you have a short, visit a friendly electrician at a quiet time of the day and he should be able to check hopefuly for no charge. Having said that my auto electrician could not detect my leak, maybe his meter was not sensitive enough or the leak was two small.
David Levy

Use an analogue multi-meter on its 12v scale as John H advises, digital meters may vary in what they display when used in this way. On a car with an alternator (like yours) you will normally see a few volts registered and this is the normal leakage current of the alternator diodes which is microscopic, you can confirm this by unplugging the alt whereupon it should drop to zero. A decent battery should hold its charge for 2 months or more with the alternator connected. If you see a full 12v regsiterd on the meter then you have a significant drain on the battery. Again first disconnect the alternator as faulty diodes are often the cause, if the meter reading drops to zero then the alternator is definitely faulty. But if it still reads 12v then there is some other drain. Check the door and boot lights switches next - that they are disconnecting the lights. If they are OK remove the brown to purple fuse from the fusebox, and if the meter is still reading remove the browns from the fusebox, main lighting switch, and unplug the ignition switch one by one to see which if any does the trick. If stoll there you will have to remove the browns from the starter solenoid one by one, and finally the battery cable. Whichever one causes the meter to drop to zero you will have to investigate what it is connected to.

You experience was better with Halfords than mine elsewhere, they changed it the first time but were definitely not keen the second time saying the gurantee only applied to the first battery not the 2nd, even though the 2nd battery was clearly labelled 'Guaranteed 2 years'. I insisted and eventually they did change it but said "don't come back". I noticed they also gave me a battery with the terminals the othger way round, but I wasn't going to push my luck any further, just took account of that when I fitted it. I bet they did that deliberately in the hope I'd blow up my alternator.
Paul Hunt 2

If you are looking for a power drain, start with the boot light. Remove the bulb and see if the drain is gone.

I had the same trouble with my Ford Sierra Estate, flat battery after a couple of day, this must have happened 4 or 5 times, I fitted a new batery same thing happened, I then changed the alternator problem solved, the car will now stand un used for three week and then start first time, worth a try.

Colin Lanning

I recently changed the battery on a neighbours car, got the same price on a branded Bosch at the local autofactors as the Halfords own brand one. The Bosch had a better quoted cranking current as well. This combined with the spring loaded toggle switch I bought at Halfords that had a service life of one operation and the fact they no longer carry even metric nuts and bolts means they are now way down my list of useful places to visit.
Stan Best

This thread was discussed between 10/08/2006 and 13/08/2006

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