Welcome to our resource for MG Car Information.



MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Battery/Alternator


I have a battery/alternator problem and would appreciate some help understanding the readings I have received from my multi meter.

The problem is, I recently replaced a battery because 3 cells were dead as determined by using a hydrometer and a loss of voltage over night (as checked by the store who warranted the battery).

The new battery got a couple of 1 hour driving runs under her belt and then sat for about 10 days, at which time she refused to start the car. I checked the cells and was surprised to discover they were all in the red on the hydrometer.
The voltage of the battery was 9.86 and after a re-charge it was 13.12. With the battery un-hooked it dropped to 12.41 volts over 4 days.

Today it started and I took the car for a run. Upon returning back from the trip with the car running I received a reading which fluctuated from 12.79 to 14.30 volts at the battery or the alternator, it didnít make any difference.
When the car was shut off I got a reading of 12.89 at the battery and the same at the alternator.

I am wondering if the internal regulator in the alternator is fried and has over charged the battery and fried that as well.

And since I have jump started the car using the alternator + post to attach the jumper wire to, I wonder if this could have caused the regulator to go??




Bruce - Time to get your alternator tested, it is not doing its job correctly. Your probably right thinking that the internal regulator is the culprit. Cheers - Dave
David DuBois

The battery looks good, and when it feels like it the alteranator looks good. This has to be either the alternator, or the very last contact to it going intermittent. Slip rings are a possible and its just not gettinga field voltage. Whatever if its inside the effect and cure are the same
Stan Best


Looking at the figures you're getting I would suspect you've got something discharging the battery while left standing. The fact that battery was down to 9.86V after 10 days shows something is discharging it. When I first started using lead acid batteries (a long time ago) it was reckoned that a battery left standing with no load wou1d lose 1% of it's charge per day, yours has lost 100%. The fact that battery when un-hooked for 4 days measured 12.41V is correct. The voltage readings you're getting don't look unreasonable to me, the fluctuations in the charging voltage could just be sampling rate of your DVM, an AVO would smooth these out and give you an average reading.

My suggestion to determine if you have a current drain would be to disconnect one of your battery leads, ensure everything is turned off, put your meter on a current setting (starting with a high range) between the disconnected lead and the battery. DO NOT TURN ANYTHING ON. If you're unhappy about putting your meter in circuit like this you could use a small 12v bulb instead, a 2.2w dash bulb would do. You shouldn't see more than a few milliamps (mA) with everything turned off. Or if youíre using a bulb it should be completely off.

The things I would suspect are:- boot light, alternator, alarm or radio. If you have an electronic ignition it should be isolated by the ignition switch but it's possible it could have a live feed.

Let us know how you get on.

Bob Davis

i had a similar problem with my GT. If i left the car standing it would drain the battery. If i managed to get the car started it would run fine as the battery was being recharged, the meter readings showed a good charge from the alternator. But when switched off i could see the drain on the meter, in about an hour it dropped nearly 2 volts. Turned out the diodes on the alternator were shot. New alternator fixed it right away.
A P New

Bloody hell! You jump-started the engine by connecting the leads to the *alternator*? At the very least that is going to have fried the wires leading down to the solenoid, and maybe has damaged the alternator. Jump leads should only ever be connected to one end or the other of the heavy gauge battery cable, i.e. the battery posts or the stud on the solenoid if you can get to it.

Apart from the low reading of 9.8v and fluctuating between 12.79v and 14.3v with the engine running all your readings are normal.

The 9.8v after being parked up for 10 days, assuming the battery was left connected, sounds more like the effects of a drain than anything else i.e. an interior light left on or some other circuit drawing current. 12.41v after being charged then left disconnected for 4 days is fine.

The fluctuating voltage sounds like a faulty voltage regulator, or a bad connection at or just inside the alternator, depending on precisely where you connected your meter.
PaulH Solihull

Tried to add the following but it didn't take:

With the alternator connected remove the battery earth strap and if you have an analogue voltmeter switch that to 12v and connect that in place of the earth strap. If the meter shows 12v there is a drain. Remove the plug from the alternator and if the reading drops to zero it is the alternator. If you only have a digital meter you will have to measure current, and hope it is less than the maximum the meter can take. But as a first step you can connect a low-wattage 12v bulb in place of the earth strap, and if that glows, even dimly, there is a drain, again remove the alternator plug to see if it goes out. If there is no visible glow from the bulb you can look for current with your meter. If there is still a reading/glow with the alternator unplugged then there is a drain from another source, of course everything should be switched off, doors and boot etc. closed, before doing this test.
PaulH Solihull


I pulled the battery (-) and tested as mentioned above.
The test light glowed.
I pulled the alternator wire and now the test light only pulsated but faintly. It was pulsating in time with the radio light which is not normally on when the key is off.
I pulled that fuse and the test light stayed out.
Hooked the alternator back up and the light came back on.
Guess it's time to have the alternator checked out. And to dig deeper into the 4th fuse down (fuse #7+8)
This fuse is the N -P wires which feed luggage compartment lamp (1975MGB) which is unplugged, the horn push, interior courtesy light, brake pressure warning light and the headlight flasher switch.

I have had some trouble with the brake pressure warning light and the headlight flasher switch. Now that I know how to check it out for a voltage drain (thanks to you guys) I think I should be able figure it out.

And I guess I won't be jumping the car from the alternator any more either. I went through a period where I was having to jump the car frequently. This involved pulling the boot cover, pulling the top up, moving all the spare parts stored behind the seats,then pulling the speaker, carpet and cover plate. I got lazy and jumped from the alternator.

Thanks all for your help



This thread was discussed on 08/11/2010

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.