MGF Mk2 Battery discharging overnight

The battery on my MGF Mk 2 occasionally discharges completely overnight, so the next morning the car will not start without a booster.  Does anyone know of the cause of this?  It is not consistant, just happens from time to time, last week it was fine for 6 days, then suddenly failed again on the 7th day,

I have heard that one of the relays associated with the ECU has been known to stick closed, but I have not found any further information, such as which relay, what circuit is it associated with, etc.

Its a tricky one because the fault is inconsistent.

As part of your fault finding I would fist reccommend a cheap digital voltage gauge and wiring it ideally to come on with the accessory position of the ignition switch (radio feed) .  There are several types available from Ebay and actually they dont look bad as a permanent fixture, about the same diameter as a cigarette lighter socket. 
That way you will be able to see if there has been a bit of a drop overnight but just not enough of a drop to prevent starting.

I would suggest to look at bare wires touching another bare wire at boot hinge and possibly getting your boot light to work when you actually do not expect it, and thus discharging you battery from time to time.

Notanumber wrote: Its a tricky one because the fault is inconsistent.

As part of your fault finding I would fist reccommend a cheap digital voltage gauge and wiring it ideally to come on with the accessory position of the ignition switch (radio feed) .  There are several types available from Ebay and actually they dont look bad as a permanent fixture, about the same diameter as a cigarette lighter socket. 
That way you will be able to see if there has been a bit of a drop overnight but just not enough of a drop to prevent starting.

You can get voltage meters the can just plug into the ciggy lighter socket, saving a bit of buggerising about.
But I agree with the idea of first checking for wiring insulation damage!

It’s not as much the voltage that requires monitoring, it’s the current.
There is a shut down sequence which resets all circuits down to a quiescent current of around 250ma (this was the value on my car a couple of days ago). Even inexpensive digital multimeter have a 10amp range, once this range is selected, it can be introduced in series with the battery & a useful indication of the current being drawn indicated.
Don’t try & start the car with this connected, it will melt in a most impressive fashion.
M

Airportable wrote: It’s not as much the voltage that requires monitoring, it’s the current.
There is a shut down sequence which resets all circuits down to a quiescent current of around 250ma (this was the value on my car a couple of days ago). Even inexpensive digital multimeter have a 10amp range, once this range is selected, it can be introduced in series with the battery & a useful indication of the current being drawn indicated.
Don’t try & start the car with this connected, it will melt in a most impressive fashion.
M

I have a simple in line ammeter that plugs into the fuse holder, this allows you to see the current leakage through individual circuits. Actually I have 2 one is 20 amps and the other is 60amps.
You do need to make sure everything on that circuit is turned off, then you use it to replace the fuse and you can see how much current draw is through that circuit. if every thing is off then the reading will indicate not only a leakage but also it's magnitude.

These devices you illustrate are a boon to anyone looking at problems. I was an early adopter & made adapters for use on the Land Rovers. Not long after I bought mine the price dropped so I bought a second, which I promptly drove over in a 101, having used it to resolve a problem which had dogged it since coming out of service. The removal of the internal battery is recommended if used infrequently.
M

As a quick fix, you could install a Richbrook Discarnect.  It takes a few minutes to install on the -ve battery post.  I use one on my 2004TF all the time.  It won't fix the fault but it will make it manageable. 

Ebay item number: 264539496409