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MG MGF Technical - Discharging Battery +

Hi folks,

I’ve had an irritating problem with my F (96 Mpi) for some time. If used all the time she’s great, but if left more than 3 days or so, the battery discharges and she won’t start.

Just removed the battery and it’s still at 11.99V, but not enough to turn her over, so I suspect the starter could be a bit lazy, or would this be normal?
Also, how can I check where the current is draining, or are there known problems on the F? I was thinking of running a multimeter across the fuse terminals with ignition off to check for current, but am not sure if this would work.

Cheers, Russ

PS. I disconnected the boot light just in case, it’s not that simple :(
Russ68

This is a flat battery not a starter issue:
http://en.wikipedia.org/wiki/Automotive_battery#Terminal_voltage

Common issues like this are caused by:
a) boot light (you've checked!)
b) interior lights not going out (check the rear view mirror)
c) dead battery unable to keep charge
d) water or dirt between the terminals allowing battery to self discharge (look for salty rings)
e) engine ECU relay staying on (the ECU is kept alive for about 15 mins after the key is removed so it can run the fans etc. sometimes this relay can stick, or the ecu can send it a false signal and therefore it continues to draw current)

To test C/D (after A and B are isolated) try disconnecting the battery earth for a week and see if the car will start when you reconnect

To test E you must follow the following EXACTLY. This is made much easier by using a multimeter with clips rather than spiky probes.

1) Run the engine
2) Attach the red probe to the bodywork (if you don't have clips then clamp it under a bolt)
3) Attach the other end of the red probe to the 10A or 20A terminal on the multimeter
4) Switch off the engine
5) Wait for any fans to stop
WARNING TRICKY BIT
6) loosen the earth clamp from the battery BUT DO NOT REMOVE YET
7) Connect the black probe to the multimeter ground
8) hold the probe onto the battery negative terminal in a place which does not block the removal of the clamp
9) CAREFULLY remove the battery earth/black clamp WITHOUT letting the multimeter lead slip.
10) stand in the cold for 30 mins (failed test) or until the high number on the multimeter falls (passed test)
11) reconnect the battery earth
12) disconnect the red lead from the multimeter and then the car (if you don't disconnect it from the multimeter now you will inevitably attempt to use it to measure something else and either blow up the test circuit or the multimeter)
13) tighten the earth clamp back up.


When the multimeter is reading Amps, it pretends to be a straight piece of wire, so the purpose of this set of instructions is to keep the ECUs in the car thinking that the battery never goes away.

The reason why you need to be so careful is that when the car is first connected to the battery it initializes a load of things which all draw current - the multimeter will be designed to handle at most 10 or 20 Amps, it will not be able to handle the stratup current, but will be quite happy with the lower currents after that (assuming the headlights and fans are off).

Will Munns

A quick refresh says that the draw should be less than 10mA (perhaps 2 or 3) after the ECU is turned off, before that less than 500mA (perhaps around 200mA) but the wait time might be as much as 45 mins
Will Munns

Wow, thanks Will, that was comprehensive ;)

I'll give it a go...
Russ68

I had this issue, and found that an incorrectly wired stereo unit was the cause of the drain. I replaced it with a jvc unit, new battery was required by then anyway, and all has been fine since. Good luck, I needed an electrician with a multimeter to identify the faulty circuit.
Martin Costin

Are you sure the battery is OK?

I had a simlar problem. Battery seemed to charge OK, but would go flat within a few days.

New good quality battery fixed the problem.
Sam Murray

This thread was discussed between 27/10/2010 and 19/11/2010

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