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MG MGB Technical - Battery going flat

Hi, I have been having some difficulty recently in that my 1974 GT keeps having a flat battery.

When I take it out and charge it up off the car it usually takes an overnight charge to bring it back to life, I pop it back on the car and it works fine.

This time (3rd time) I checked the battery with a volt meter and saw it had 3volts left in it.

It takes different lengths of time to go flat ranging from a fortnight to a month.

The car is not used every day - just sunny and high days - its my pride and joy.

Now, the battery looks okay, the terminals are clear, (they are the sort where a small screw in the centre needs to be screwed down to secure them)and the leads are in good order.

The battery came with the car, I have done the obvious checked its levels etc and they are fine. But I don't know how old the battery is.

Where do I start to find out if there is a constant drain on it?

I do have a multi meter but am not fully okay with it - I tend to use it more for continuity checks.


E McGee

Charge the battery up and leave it un-connected. does it go flat? if so it is toast.check for voltage draw between the post and cable. If there is a voltage showing then you will have to trace it.
I would make a wager that it is the battery as it is an unknown age. Most good shops have a battery checker and they will do a load test.Usually for free.

Edd - Your battery has probably become sulphated from lack of use. This is quite common in vehicles that are not used on a regular basis. Lead/acid batteries need to be charged/discharged on a regular basis to maintain them in good health. Whether or not you battery can be rejuvenated or will have to be replaced is up to a professional, but there is something you can do to protect the battery in the future.

After checking that there is no vampire circuits drawing current with the ignition turned off as suggested above, purchase a battery maintainer, battery tender, battery buddy (these thing are marketed under a variety of names) and hook it up to your battery whenever it is going to sit for more than a week without the car being driven. These units are basically a smart trickle charger, that charge the battery to full charge, then turn off until the battery voltage drops to a predetermined level, the charges it back up. By doing this up and down charge, the battery is kept in good condition and the life span of the battery is actually extended. Just be careful not to get just a plain trickle charger, which is much cheaper, but will ultimately over charge the battery and boil off the electrolyte over time. Cheers - Dave
David DuBois

Unless you have a sophisticated alarm you should be able to leave an MGB with a good battery and no drains for a couple of months or more. My V8 has an alarm so when I moved it inside and it was no longer my daily driver I fitted a battery cut-off switch, which stops even the alarm drain.

To check for drains remove the battery earth strap and connect a voltmeter on it's 12v range in place of the strap. On a car with an alternator and no drains a typical analogue meter will show about 6v, which is the microscopic leakage of the diodes, and unplugging the alternator will drop that to zero. If you get that then you need do nothing more - there are no drains. Digital meters are much more sensitive and will probably show 12v from the alternator. If unplugging the alternator drops that to zero, then you have no other drains, but the alternator diodes could be leaky. So now you have to disconnect the meter, switch to the highest current range (if not auto-ranging), and reconnect. A few micro-amps is fine, milli-amps is too much.

If an analogue meter shows 12v which drops to zero when the alt is unplugged, the alt diodes are probably leaky. Slightly leaky will drain the battery, but if they go very bad they can effectively put a short on the battery which can burn the wiring (hence battery cut-off switches are a good idea anyway ...).

If either type of meter shows 12v, which is still there when the alt is unplugged, you have a drain. Disconnect any clock, alarm, radio etc. first to see if it is one of those. If not check a courtesy/boot/load space light isn't staying on. If not that remove the brown to purple fuse, which feeds the horns, headlamp flasher, courtesy lights. If still there remove the other three fuses, but with the ignition off they should make no difference. If still there you will theh have to start disconnect brown wires from things like the ignition switch, main lighting switch, starter relay, and finally the starter solenoid until it does go. Whatever you disconnect that makes the meter drop to zero, indicates that circuit is feeding the drain.
PaulH Solihull

Wow thank you guys for your replies.

I will have a go at tracing the leak / battery perromance this weekend.

You have given me some excellent advice - good to know that it is there when needed.


E McGee

Can't add anything to the previous postings, but it's not often you see someone with the same surname on here.

And ironically you're not that far away either.

Andrew McGee

To check for minor battery drain caused by a faulty circuit in the wiring, disconnect the positive terminal on the battery and using a multimeter set to lowest Amps DC, connect one lead to the battery terminal connector and the other lead to the battery post. The reading should be zero. If not you have a current leak.
Be sure that the multimeter is set up correctly for checking amps. Repeat the test, by switching the leads in case your multimeter does not read negative amps.

Also, if you buy a trickle charger, I suggest that you buy the Battery Minder brand because the will not only maintain the battery during idle time, but will also desulfate your battery. They have several models from which to choose. Here is their site.
Frank Grimaldi

Never mind positive, it should always be the earth lead, not the 12v lead that is removed first, and that is for either polarity.

If you work on the 12v post with the earth still connected, and the spanner touches the body - very easy on an MGB - you will short out the battery with predictable results. By contrast if you work on the earth terminal, and the spanner touches the body, nothing will happen as the earth post is already connected to it.

Once the earth is disconnected, you can work on the 12v terminal safely, as if that comes into contact with the body now, nothing can happen as the earth is already disconnected.

And there is no point disconnecting the earth, disconnecting the 12v, reconnecting the earth, then connecting a meter between the 12v cable and the 12v post to check for a drain, then go through the whole rigmarole again in reverse to reconnect the battery back up again. Once the earth is disconnected, simply connect the meter between the earth terminal and the body, as previously advised.
PaulH Solihull

This thread was discussed between 11/03/2012 and 17/03/2012

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