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MG MGF Technical - Dying battery or abnormal current drain?

For a whole host of reasons, I've not been using my F terribly much these last 6 or so months - which may have something to do with the current problems I'm having - flat battery.

The battery I've got is a Varta unit, and is guaranteed for 3 years - it's only two years old, and I am finding that it's going flat within 24hours if left. If I disconnect the battery, then charge is retained (I've left the battery disconnected for a fortnight, and it still has 12.3V).

I've attempted to measure ignition-off power drain, but can't actually measure anything (?!)

So what's the chances that the battery is dying (it had gone flat completely a couple of times, but -recharged and seemed okay)or could it be that something is discharging the battery?
Rob Bell

Sounds like you now have a dying battery if its not holding the charge.
Is the alternator actually charging the battery when your driving ? I had a similar problem and found that the brushes had stuck and had to be replaced, after that all was fine (albeit a TVR), the trickle discharge from the cars alarm was enough to run the battery down and eventually even the battery had to be replaced.
Check at the alternator to see if its charging.
Hope this may help as I know how frustrating this becomes.
John Mac


A healthy battery will run a light etc all night without a problem. (except the headlights of course)
Maybe a boot light is staying on?

Get yourself a battery hyrometer and test the state of the battery. These units are cheap to buy and I've had mine for the last 20 years. It will tell you if the battery needs charging.

If you want to test the battery current drain during a no-use period, disconnect the +ve terminal and put an amp meter in series. It should be rated at least a few amps. It will show you how much current is flowing. Post the result later on.

Oh and during this time do not turn on anything in the car.



HA!!! the "Good DR" is Stuck!!!, perhaps a referal to " specailist" is called for!!. :o).

This used to happen on my bikes!. As part of the insurance conditions I had to have a Thatcham cat 1, this alarm would run a very low voltage through the bikes electrical systems all the time, the Battery would be flat in 2-3 weeks untill I put the "Optimate" charger on, then no more prods!!!.

Not sure if the F alarm does something similar though!!.

Anyway Rob!, whats it feel like to be a mere mortal again!!!. ;o)


some works before you claim battery warranty.
- check and replace the battery ground cable.
- check alternator connections, special the brown coloured cable



From bitter experience modern batteries do not like to be discharged, once they have been 'flattened' I have found you might just as well give up trying to charge them and get a new one.


PS before all of the electical wizz kids tell me I am wrong the above is MY EXPERIENCE only.
Ted Newman

Unfortunately, I think that you're right Ted - it was flattened after one or two periods of long lack of use, and I suspect that the battery is now as good as useless - but in the past batteries that are 'dead' tend not to retain any charge whatsoever, whereas this one is retaining a decent charge if not connected to the car...

Guys, thanks for your thoughts - already been through most of what you've all suggested - including putting my multimeter in series with the battery (cheers Branko :o)). Oddly, I'm not recording any current flow - so I guess I've got an instrument fault... :o(

John, Dieter, the alternator is almost certainly fine - if I drive the car after the battery has been discharged, the battery is then fully charged. Having said that, both of you are suggesting it, so am I missing something?

Mark - cheers mate ;o) ROFL - and yes, looks as though I need a trickle charger for the MG. Already have one for the Triumph. Perhaps a solar powered charger for the F would be a good investment? Any one got one that they'd recommend?

Anyway, I suspect that Ted's right, and have purchased another battery which I'll swap over sometime this week... I wonder what chance of claiming back on the original guarantee? ;o)
Rob Bell

had /having same problem local car sparky reackons its quite normal,metering current drain if memory serves me was approx 30 40mA, apparently although battery is able to supply aprox 80amp for startup current it only has app 40amp capacity at rest,was also told that to charge up fully would need a 400 mile trip,i think that is stretching it a little ,however i watched them load test my battery and it showed to be ok,they then constant current charged it overnight but after a week or two like others it had enough for a couple of revs but wouldnt start so now i carefully give it a charge an hour before departing , i dont disconnect it but work backwards from the battery and once connected then i'll turn the charger on,maybe ok but with all the electronics on board cant be too careful
b dix

Rob - On another Forum there was a long thread on Solar Power Chargers and the general concensus of opinion in the end was they were not worth having. If you have an adjacent power supply then a Trickle Charger for about 10 worked fine for me (a Draper).
There are a number of Solar Chargers on the market and if you use a Search Engine for "Solar Powered Chargers" you will get some info.

Thanks for that John - do you have the URL for that thread?

My Triumph sits inside the garage, so can benefit from a mains fed trickle charger. Unfortunately, the MG sits out on the drive, so this route is not so convenient - hence the thought of using a solar-powered device.

Will have to wire in a new power socket given that the standard cigar lighter is switched to the ignition. As I was going to do this anyway to power the TomTom satnav, it's not a big deal. Just need to find a suitable socket from somewhere...
Rob Bell

Many caravan owners use the solar power chargers BUT anything halfway decent is around the 150.00 mark
Ted Newman

>solar power chargers

Do they really have any option to work in the UK ?
.. I get my coat ...

>> Do they really have any option to work in the UK ?<<

Of Course!!!,.. we even know when!!,... as it turns out Summer is on a Wednesday this year!!.


Actually solar chargers work without direct sunlight - just as long as it is light and the cells are pointing in the general direction of the sun (or where we think it should be):-)
Ted Newman

Rob - Follow the following links: S Series/Search/type in Batteries or Solar power/all dates.
and you should get threads on Batteries and chargers.
John Mac

Take a look here.

If this works then there are a lot of good reasons to give it a try.
Ken Waring


<<<<Of Course!!!,.. we even know when!!,... as it turns out Summer is on a Wednesday this year!!.>>>>>>

Let me know which Wednesday I'll book my holiday.

Adrian Clifford

Rob - upon reflection I have just remembered the following.
Most of the thread above on Solar battery chargers may have been refering to, in particular, Car chargers as seen attached to the windscreen of the Paris/Dakar or Safari Rally type cars - I was anyway.
Awhile ago I had a Beach Chalet that ran the TV, about five internal lights and a power outlet all from a 24"x12" solar panel on the roof, this fed constantly two "heavy duty" Land Rover batteries, they didn't overcharge as there was a small panel containing a "trip" switch and 12 volt converter. The whole set up cost around 150.(Batteries were from Lucas)
I see no reason why this wouldn't work in keeping a car battery topped up once the method of wiring and overcharge protection was sorted and these packages were readily available through any good Camping Caravanning outlet.
Hope this option may help.
John Mac

Lead Acid batteries do not like being deep discharged. As you try to suck the last bit of energy from the battery, nasty things happen, if this happens often you'll probably notice that the plates inside are crinkled - not good.

Tip for measuring current from battery into car:

1. Connect the ammeters leads together
2. connect to the battery and load
3. separate the leads from the ammeter

The inrush current from a lead acid batttery to a car is often enough to blow the protection fuse in a typical multimeter, so taking these steps allows the inrush current to bypass this fuse, then you can separate and get the reading you want.

Of course fit a good fuse before doing this!

Kev - unfortunately, I think that I've probably blown the fuse on my ammeter (d'oh!!!). Thanks for the tip on avoiding repeating that feat in future :o)

John, Ken, thanks for the URLs, I'll check them all out :o)
Rob Bell

IIRC, Thatcham I alarm systems must consume <10mA from the battery. Besides that, there is very little that should be running on the car - the clock should be virtually zero, and likewise the various ECUs when the car ignition is off.

So you'll need your meter in the 200mA range when metering the load.

>So you'll need your meter in the 200mA range when metering the load.

No, better have 500mA and switch down to a lower range later.

The MGF will pull app. 200mA in the first minutes (up to 3/4 hours) after ignition off.

The alarm system takes 2 to 3mA later. After the ECU timer switched down the pink-brown cable to the charcoal canister.

Rob, (marc here, masquerading under a pseudonym for anti spam reasons)- you may not recall that I had a similar problem a few years ago. I was not using the car that frequently and often ended up with flat batteries. I went few quite a few so doubt if it was them. My MGR dealer couldnt find anything obviously draining the battery. If I recall there was some comment on this BB about the ECU not shutting down quickly enough and this could be draining the battery. Must say I didnt bother trying to work it out as the car was in my garage between events so I simply disconnected the battery. That did the trick! Still disconnect it now to no seeming ill effects.
m abbott

Marc, that is what I've been doing - disconnecting the battery. Bit of a pain to have to go out with a 8mm spanner every time I want to use the car though!
Rob Bell

Could you fit a motor sport type battery isolating switch? Quicker and easier than finding a spanner.
John Ponting

That actually is a very nice idea John - thanks :o)

The external one can be a bit ugly - and the internal one may not be all that great either - but I'll have a chat to Tech-speed (who've obviously got a lot of experience with this sort of thing) next time I am there.
Rob Bell

Try these people. they do an isolator switch that goes on the battery terminal


any chance of the name of the company or a link?
John Ponting


My excuse is that I'm in the US and suffering from jetlag

This thread was discussed between 26/06/2005 and 08/07/2005

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