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

Hi all,
For some time now I have had a problem with my battery going flat if I leave the car for more than a few days. Today I replaced the battery, which was 2.5 years old and decided to get a bit more methodical about trouble shooting. I measured the current drain on the barrery, ignition off and all doors closed, at 28 milliamps. I think there is always some drain due to the diode in the alternator and I have a clock (doesnt always work properly) and a radio, both of which will use some small current drain. Is that number about right, or should I be searching for the source of the drain?
Regatds
Stephen Elster
SJ Elster

Stephen, the alternator diodes should draw almost nothing, dependant on the type of clock it will either be a small (a few milliamps) continuous or a slightly larger intermittent current draw. The radio depends on age and type, very old radios should draw nothing when off, with more modern radios only drawing a few milliamps to maintain memory and flash an LED.

If your battery is going flat in a couple of days there must be something else drawing current.

Without knowing the capacity of your battery, but lets say 50Ah, a 28mA drain would take approx. 900 hours or 37 days to drain half the battery (and before the pedants tell me I haven’t taken self drain, internal resistance and draining the battery at approx 1/1000 C into account, I know)

I can’t tell you where the issue is but I can tell you that although zero drain would be better than 28mA, I don’t think that’s your problem.

I’d suggest measuring the voltage of the battery with the engine running, you should see high 13V or low 14V dependant on the state of charge. It may be the battery isn’t being fully charged.

Bob
R.A Davis

Good advice from Bob-----

Maybe if there's still a flat battery issue disconnect, alternator, radio, clock, one at a time to see if one is the culprit-if your clock is playing up it might be a good place to start

on your estimates Bob- yeah I agree
but a glovebox light250mA will flatten a battery overnight -8hrs enough to cause a no start issue so I would think 28mA would have a similar result after around three days or so-------?

willy
William Revit

Quick thoughts -

Standing water or ingress(?).

If it's one battery only, posts and connections clear of bracing bar or any other connection to car(?).

Any immobilisers or other non-standard electrical installations (PO wirings)?
Nigel Atkins

Stephen,
Had a similar problem. Replaced the radio/tape player/clock with a retro radio (no memory or clock), and now I can leave the car for months without draining the battery.
Might be worth just disconnecting the radio/clock to see if it makes a difference.
Patrick Callan

Stephen, what about a battery isolator? it won’t solve the problem but it could stop the issue. I’ve got one fitted in the heal board, I always remove the key when I leave the car as an extra theft deterrent. I also turn it off when it’s in the garage, this stops any battery drain, is useful when I’m working on the car and as I’m paranoid, it prevents electrical fires, which as my bedroom is over the garage gives me peace of mind.

Bob
R.A Davis

Fitting an immobiliser if wanted or required but you shouldn't need to change a good quality battery in good condition, connected correctly with all connections clean, secure and protected and fully and properly recharged after 2.5 years of normal use.

I'd first go with Bob's first suggestion of checking the charging of the battery.

The battery is one of the most oversold components on a car many times they can be revived to give much more useful life. As a general rule if the battery has had a slow discharge then a slow recharge is required. I go for low and slow recharge every time, these very quick and high rechargers could kill off a battery that would have been coaxed back on a low slow recharge.

Not using the car for months will cause more issues other than battery drain so as much as you can use the car all year round, regularly, on reasonable length journeys.
Nigel Atkins

Had just this issue with mine. Wasn't sure if it was the slightly more modern radio. Suspected the boot light. Did as Bob did and fitted a battery isolator with removable key. Useful for all reasons that Bob suggests, plus entertains passengers as part of a starting ritual: car in neutral; handbrake on; choke pulled if cold; power on; ignition on; fuel pump on - wait 'til ticking finishes; push starter button; if that has raised car from its slumber, check gauges and for an added flourish tap the odd one......
Peter Allen

That drain is a lot more than I would expect for a radio memory and especially a clock, even an alarm system should be less than that. The alternator should only be microamps.

As said disconnect each of those things in turn while measuring the current, and if it's still there with alternator, clock and radio (probably two feeds if it has a memory but doesn't operate with the key out) disconnected then it's something else. Disconnect the browns from the fusebox, ignition switch, lighting switch and hazard switch (if fitted) one by one. If still there you will have to disconnect the browns from the starter solenoid, and finally the battery cable from the solenoid. Be careful down by the solenoid.

When I stopped using my alarmed V8 every day I started getting battery problems and two lasted less than a year each. Fitted a cut-off switch - problem solved, but remember that it has to be one without a bypass fuse or you will be no better off. Fitting one in the earth cable is the simplest, but rather than reset the clock each time I used the car I chose to fit it in the 12v cable, and ran a separate fused supply back from the 12v battery connector just to that.

Excellent as a safety measure as well as security I fitted one to the roadster as well even though without clock, radio or alarm I'd not had battery problems. But on getting a ZS180 and not using that much in summer that also knackered batteries, so that got one as well, again with a bypass in place of the clock fuse.
paulh4

Paul
With your ZS ,I guess the body module would be Bosch, so what I'm about to thread drift on probably won't count - maybe -?
My neighbour bought a new Subaru WRX about a year ago and had issues with the battery going flat after about 2 weeks sitting, He complained about it several times, they fitted a new battery, still the same and he asked me to have a look at it
Put the meter on it and it had a continual drain of 0.5A for about 2 mins after locking and then a continual 0.1A after around 5 mins after locking as if the body module was drawing for the 5 mins and then switching 'almost' off-
I got onto a contact at Subaru and he looked into it for me and came back with , you have to put it in garage/storage mode if leaving for more than a week, that's interesting I thought so into the handbook and there hidden away is storage mode, initiated by double locking with the remote
So back on with the meter, locked the car 0.5A, locked it again and instant 0
Told the boss salesman at the local Subaru dealership where I happened to be contracting at the time and he said he'd never heard of it but they had had plenty of complaints so was happy to have a fix

willy
William Revit

Interesting, not come across that before. Absolutely loads of complaints by ZR/S/T owners found by Google, only looked through a few but none mention a 'storage mode'.

In my experience a new battery doesn't have a problem, but the older it is the more capacity it loses from the small alarm drain, and the shorter it can be left and still start. The Lucas manual says that once a battery has become discharged beyond a certain point the on-board charging system won't restore more than 50% of the capacity, you have to use a boost charger and that does work. Alternators can be limited to as low as 14.3v, whereas dynamos could charge at up to 15.5v, and some time ago I read that Mercedes were selecting voltage regulators for alternators at the upper end of the range because of owner complaints.
paulh4

More thread drift
On the Ford Rangers here, which are a popular choice for towing caravans etc, the alternator charging system output is controlled via battery sensing through the vehicle's ECU - they'll start up and charge at nearly 15V but if nothing electrical is used, within a minuite or two they're back to battery voltage-12.5v-the problem there is that if the caravan or vehicle has an aux. battery with a cutout, the cutouts usually work at around 13v so the aux. battery doesn't get charged-
You can get into the body module and turn battery sensing off which makes the charging system revert to like a normal alternator and run at 14v or so but usually most of these extra battery systems have been fitted aftermarket by those that don't know how the system works--The only ones that get set up correctly are those that come back through the dealership who have the ability to get into the module and switch it over
Nothing to do with MGs but interesting
William Revit

I'm aware of a mod to fit a silicon diode in series with the voltage regulator earth connection, which boosts the output by about half a volt.
paulh4

This thread was discussed between 01/07/2019 and 07/07/2019

MG MGB Technical index

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