MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

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 Drain.

Ongoing problem with Dads 1981 MGB. Battery is fine if used every other day or so, but if left for any longer, battery drains, so much that will not even light the interior light.Newish alternator, battery not that old.
Any ideas?
P D Gossland

Hi.

If you have got a multimeter and know how to use it, I suggest that you connect it between the battery -ve and chassis after disconnecting the usual battery-to-chassis strap.

A reading of more than a few milliamps suggests that something is wrong.

If the current is significant, start disconnecting things (one at a time) until the current stops.

Don't forget that if you have the doors (or boot in a BGT) open the interior light will be on.

I would start by removing the plug from the alternator.

If there is a lot of gunge on top of the battery this could be providing a leakage path.

HTH.. Don
Don

I fully agree with Don. However, if you disconnect everything and still have a drain, you could have a wire/connection grounding itself out causing a drain. Hopefully Don's instructions will find the problem. I ended up having a wire going to my front turn signal that had worn through and was causing a slow battery drain. Just have lots of patience. Also, have you worked on anything recently? Go through and look at everything such as the tag light connections, and be sure that the trunk light switch is actually working and the trunk light goes off, etc.
Robert Browning

P D if you dont have a multi-meter you can use a 12volt testlight in series between a battery terminal and the disconnected cable. If you have a draw the light will light dissconect one item/circut at a time until the light goes out, you will have found your draw. RIC
R E L Lloyd

Hi all.

Yes, a test lamp will do the job, you may need to experiment a little with the bulb wattage, if the wattage is too high the bulb won't light if the leakage is small.

I personally like to use a small 12v buzzer when I am 'poking around' for faults, you can apply full visual attention to what you are waggling and listen for a change of noise.

A buzzer also works well for setting static ignition timing.

Don
Don

1981 MGB????
That's like a BJ9, ( Austin Healey. [last model was a BJ8]

Ongoing problem with Dads 1981 MGB. Battery is fine if used every other day or so, but if left for any longer, battery drains, so much that will not even light the interior light.Newish alternator, battery not that old.
Any ideas?

OK, so to the battery drain problem.
1. Check for boot light switch not turning off that light.Or any other electrical consumer. Use Don's test method for battery drain.

2. Test battery voltage with the engine off, it should be at least 12 volts, then start the engine, and it should go to between 12.8 to 14.5 volts. If not, the charging system isn't working.

3. Maybe number 1, check battery connections, they should be clean and dry, shiny on the inside of the clamp and the battery post.

4. Don't discount any 'newish' part, including the battery.
Safety Fast
Dwight
Dwight McCullough

I had a 1981 B. Made in late 1980 and imported to Canada and registered as an 81. One of the last 1600 made. I gues the dealers did not want to sell a 'one year old' vehicle. Does make for strange looks when discussing it with B enthusiasts though. Alas, sh bruned and is now scrap.
Peter

P D, I'm curious if you found the source of the drain. Also check the battery cable at the starter. The other ends of the battery cables can get forgotten and need to be kept clean. I use a wire brush.

kids1

Much safer to use an analogue multi-meter on its *12v* scale in place of the battery ground strap, than an ammeter. If you have a significant drain and use an ammeter it will try and pass the whole of the current, which could damage the instruument or leads as well as causing a spark which right by the batteries is not a good idea. A test-lamp will register larger drains but not small ones, which may still be large enough to cause battery problems over time. On a car with an alternator it is normal for a voltmeter to register a few volts, this is down to the reverse leakage of the alternator diodes and is normal, you can check by unplugging the alt. If you see a full 12v registered that does indicate a drain. Again start by pulling the alt plug, then the purple fuse, then browns from things like the light switch, ignition switch etc. An interior/boot light bulb continually glowing because of a faulty switch can flatten a battery over a few days.
Paul Hunt

Hi all.

Paul is right of course, there is a risk of damage to the meter if you have it switched to a low current range and accidentally switch something on, perhaps I should expand on '... and know how to use it'.

When connecting a current meter you need to :

Connect the meter leads the right way around (unless the meter has auto-reverse, which most digital meters do).

Ensure that the expected current is not more than the meter can handle. (I would expect a car leakage current to be within the range of most multimeters).
A current drain of 1A will completely flatten a B battery in around 2 days, so I would expect the current in question to be well below this.

Set the range switch to the highest range and gradually switch down the ranges until a reading is obtained. You may need to start with the meter +ve lead plugged into a different socket on the meter to get the highest range.

Ensure that the current isn't increased by switching on extra loads.
For safety in this application it is probably better connect the meter only briefly while you note the reading rather than leaving the meter connected.

Don't tinker with high voltage circuits unless you REALLY know what you are doing.

As Paul says, avoid sparks in close proximity to batteries and ventilate the area around batteries to get rid of any accumulated explosive gasses.

Finally, if you are not sure, ask for advice before proceeding.

Don
Don

Also take a look here for multimeter use tips :

http://www.autospeed.com/cms/A_0072/article.html?popularArticle

Don
Don

This thread was discussed between 10/10/2003 and 14/10/2003

MG MGB Technical index

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