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MG MGF Technical - Flat battery

Hi there

I have a P reg MGF 1.8i VVC. I don't use the car very often and it is left standing for sometimes long periods of time maybe 2 weeks. The problem is that when I go to use it after it has been left the battery is invariably flat even if it had been fully charged before. I wondered if the alarm uses so much power that this could be the cause, but I recently read that the alarm doesn't actually use a lot of power. I thought it might be something to do with when I fit the hard top but it happens just as much in the summer as the winter. It seems as though something is not switching off and power is being drained from the battery but I really don't know where to start to investigate this.

I bought a new battery a few months back but that is useless now because it got flattened so often. I'm getting desperate now as the inconvenience of this is making me consider getting rid of the car.

Anyone got any ideas orhad similar problem?
Eileen Barker


It's been reported several times on here that F's batteries seem to go flat after a couple of weeks. I don't think that there's anything wrong with your car. My car sometimes sits in the garage without moving for weeks on end and when this is the case I charge the battery at a "trickle" setting for 12 hours or so every couple of weeks. You can also buy battery chargers that constantly charge and discharge your battery to keep it in top condition. There was a thread a few months ago about this so have a look in the archives. I think they cost about 40.

David Clelland


I use an OPTIMATE charger/conditioner on my bike as this stands for a couple of weeks at a time - always starts on the button.

OPTIMATE do a charger rated for cars. They are left connected when ever the car is not being used and continually charge / discharge / check the battery state.

As David said, there are others on the market - I believe that Mike Satur is now listing one but haven't checked.



Try for the one that John is talking about and for another type - both round about the 40 mark.

David Clelland

And Moss sell one called CSI Airflow which I think also costs 40.

I have one and can confirm it keeps the battery on my F charged up over the winter months of non use.


Thanks to all

Finding lots of info in archives - lots of reading to do today - the ironing can wait!


Eileen Barker

I have an optimate for my MG and for my bike, it keeps the battery perfect - circa 40 make sure you buy the 12volt version and not the 6-12 volt one.

My son's car was laid up for 10 weeks whilst he was in Oz. On his return I had dug out the jump leads because of all the tales on this board.

However car fired first time and ran perfectly. Car is a 99 VVC with orginal battery.

Son has just gone back of Oz and car is again locked away until April when I hope it will start with the same vigour as last time!

Car is in a lock up garage with no battery conditioner.


Patrick Beet

Speaking from experience - be sure one of the reading lights under the rearview mirror has not been left on


Had the same problem, narrowed down to a short in the boot lock wiring, easy fix, if you know a sparky.
graeme arnold

I spend most of my time at 35000ft, when I come home after 6-7 days I usually find my battery flat in the mgf so much now that I'm considering to get rid of it. My other cars can be left standing for weeks but they never suffer from this problem. The wiring/drainage has been checked and given the ok. I replaced the battery thinking that's where the problem was but the same problem occurs with the new battery.



Halfords sell a "Draper" trickle charger for around 15 which can be left connected all the time. I've been using one all winter with no problems. Car starts OK even when not used for some time. Before that battery used to run down fairly quickly even at only 18 months old. HTH

You could always buy and fit a battery isolator switch for when the car is left for long periods. There would be no alarm of course - but who takes any notice of these?



What about one of those gadgets that plugs into the cigar lighter and had solar panels that sit on the dashboard. I have seen them in the Innovations catalogue. Don't know if they work though!
Billy Bob

Problem is I have to leave the f in the crew car park and there is no way I can use a trickle charger.
An isolater switch sounds a good idea but that means no alarm, resetting the clock and reprogrammimg the radio everytime I use it.
I still can't get my head around as to why it's going so flat so quick. The only thing using any power what so ever is the alarm system.Could this be the culprit and if so should I change it?

kind regards


>What about one of those gadgets that plugs into the cigar lighter
I think the F isolates the cigar lighter when the engine is off, so I doubt these would work (unless you stripped the cable and fed it to the battery directly.

Will Munns

>if so should I change it?
I think you mean could there, and if you do then the answer is no.
A solar powered tricle charger would be a great idea, but you would have to run wires for it (not rocket science)
Will Munns

Isolator switch available from Frosts (and no doubt others) has a bypass 16amp fuse that allows clock/radio, and more importantly ALARM to continue to function. Only if hot wiring is attempted, the fuse "blows" and immobilises everything until the connecting knob is re-inserted.Ony about 8-ish, I think
Blue Max

>Isolator switch available from Frosts (and no doubt others) has a bypass
>16amp fuse that allows clock/radio,

Which is great but...
1) it will not stop the drain problem at all
2) it cannot be used in motorsport as it will not act as a 'kill' switch
3) they all have the same key(?)

Errm why would you fit this?
Will Munns

If the battery is going flat after only 2 weeks of no use, then there IS a problem somewhere. MGFs do NOT normally flatten their batteries so quickly.

I KNOW that I can leave mine for 4-6 weeks, and it'll still fire up, first time.

I'd suggest to you that you have a current drain somewhere.

A couple of very good suggestions already regarding lamps that remain on - and these can be hidden in the front bonnet compartment or boot, so not immediately obvious (although this should be if the lamps are warm when you leave the car overnight and check them when you open the compartment in the morning). A short in the bootlid wiring is another good area to check, as the wires can become fragile through work hardening by the lid being repeatedly opened and closed.

Another area to check, that is not yet been mentioned, is the MEMS power-down signal to the purge valve. This should not remain live for more than 5 or so minutes, but it has been known for this signal to remain live to significantly longer.

Look at Dieter's web page ( ) where this signal is employed to provide power for the electric windows after ignition off, to give clues as to how to check whether this is the problem on your car.
Rob Bell

The moss catalogue has a few options for stopping batteries going flat.

Tom Randell

>An isolater switch sounds a good idea but that means no alarm
Do you have ABS? if not then another battery on the other side and a switch that alows you to switch between battery 1, battery 1&2 and battery 2 might be a good idea, i'm sue that boats have this kind of arangement (sort of required when you can't bump start it!)

Will Munns

Just a thought,
is it possible that the heated mirrors could not be turning off and drain the battery ? how can this be tested easily ?

Very unlightly, the windows would continue to operate
Will Munns

Yes most boats have 2 batts,the main idea of the switch is to always have starting power,
You should have a starter battery and leisure battery to provide power for lights radio etc.
Caravans have similar system with blocking diodes or split charge relays.

Seems like a lot of trouble to do all this work on an F.

One way to try to find drain is:

Put a multimeter across battery terminals,note the voltage then start pulling fuses if you find a change in voltage on one fused circuit then that is likely the problem.

The problem is then if that fuse covers more than 1 item to find the drain,but now it should be easier.
joe molloy

>Put a multimeter across battery terminals,note the voltage then
at the currents we are talking here I doubt that much voltage drop will be seen, so using an ammeter in series might be better. One note here, some circuits do pull a lot of current when re-energising (my old alerm used to lock all the doors) and this will blow the fuse in your ammeter (so only put the meter across when taking out fuses, not replaceing them*.

*obviously (or not) you will have to hold the ammeter accross the battery termainal and lead whilst they are still connected, then disconnect them, keeping the ammeter connected, failure to do so will cause the entire car to deenergise and then reenergize with the resultant fuse blowing
Will Munns

As well as the MG I run a Land Rover as well, this has two alarms on it and so downs the battery quickily esp in this type of weather.

Like Bryan of Gatwick, I to tend to leave it in secure carparks with no access to a power supply, a trip to Maplins solved it in that I got a solar powered charger that lives on the dashboard and is wired to the battery via a seperate fused lighter socket.

but that still does not sort out why its going flat, its:
the battery is stuffed
the alternator is not charging to capacity
you have a power drain

tony l

is it possible that the heated mirrors could not be turning off and drain the battery ? how can this be tested easily ?

Feel the glass to test if it stays warm?


cheers everyone!
I'll have to investigate further during the weekend with some of your thoughts.
I like the idea of a solar powered battery charger. Maybe this will be the solution.

kind regards,


Hi everyone

I've been reading everyone's contributions with great interest. I have bought a solar charger to give that a try but I believe I need to get it wired directly to the battery as I believe from comments here that the cigar ligher is isolated when ignition is off - there's one job for hubby. Also he is going to do the checks suggested to identify if there is a power drain. It seems there are still a lot of possible reasons why the battery is going flat. Perhaps I won't give up just yet cos I really do love driving the F.

I'll let you know if - no when - I get it sorted.

Thanks again to everyone.

E Barker

Some of the solar chargers I've seen have battery clips in addition to the lighter plugs...

Other alternative: there's an "always on" accessory connector on the fusebox (though I can't remember which one it is - its one of the 3 spades in the middle :-) )

The only time I've flattened the battery in mine was by leaving the door not fully closed when it was in the garage for a week - I suspect the interior lights were on... I'm surprised noone's ever measured the "normal" current that is drawn when the ignition is off and the alarm is set - that would tell us how long a battery OUGHT to last. Simple to do - disconnect battery, put ammeter in series, shut bonnet, set alarm, go and make tea to let it settle, and read it off...



I am currently experiencing exactly the same problems on my 96 VVC. I bought a new battery about 12 months ago and that has not improved things at all!

I am going to try the above suggestions ref. light being left on in the boot / bonnet and also the door mirrors.

If all that fails, I think it will be the option of purchasing a battery trickle charger.


Andy Beard

<<I'm surprised no one's ever measured the "normal" current that is drawn when the ignition is off and the alarm is set - that would tell us how long a battery OUGHT to last. >>

I measured my car today. Results were as follows:-

System at 12.5 volts. Car left overnight.
With alarm off and car unlocked drain was 0.032A
With alarm on drain was 0.067A

The MEMS relay holds in after switch off for approximately 9 mins (560 secs) but if the car is stone cold it switches off almost immediately.

So after switching off a warmed engine the drain was 0.13A for about 9 mins and then fell to 0.067A.

The results with the 'alarm on' may be slightly different for other cars because my volumetric alarm is home made (Jap imports have no VA fitted). I wouldn't have thought there will be much difference though.

Bruce Caldwell

Re the alarm drain - I seem to remember that the alarm switches 'off' some of its activities after so many hours/days from being set and gradually ends up with just the imobiliser being left active.

Ted Newman

Haven't heard of that Ted. Anyone else?

It was difficult to measure the current with the alarm on because you have to leave the bonnet open to disconnect the battery and put an ammeter in the line. Also you have to disconnect the bonnet switch while you do it.

Bruce Caldwell

I can confirm that on my TF the cigar lighter isn't isolated with the power off - my phone charger lights stays illuminated after I remove the keys...


Cigar lighter in my *F* is only active when the ignition is switched to 'ON'.

Ted Newman

Same as mine Ted - cigar lighter is 'dead' once ignition is switched off.

Darren, how long does the cigar lighter remain active after ignition off? It may be connected to the 'delayed power-off' circuit that allows windows to be shut after switching the ignition off. If you leave it a couple of minutes, it'll probably be switched off...
Rob Bell

try this: when you leave the car remove the second fuse in the under-bonnet fuse box (by second, I mean second from the left as you look from the front of the car). I'll almost guarantee that you'll then be able to leave the car for at least five weeks (from personal experience) with no battery drain.

If this IS the problem, then you need a new relay-pack: it's a pretty easy DIY fit, any MGF breakers should be able to find you one for v little money, just make sure you get the right type, as there seem to have been several different types. Can't remember part number I'm afraid.

What happens I think is that one of the relays in the pack locks on, and pulls about 1.5 amps power - enough to drain the battery in a couple of days.

This has happened twice to my car, so I don't know if it's an occasional fault with the relay-pack, or if I'm just unlucky.

Ed Clarke

This thread was discussed between 02/02/2003 and 14/02/2003

MG MGF Technical index

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