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MG MGF Technical - flat battery

This might be a silly question, but my battery is completely dead. It started fine with jump leads, but after driving it for 40 minutes it lost all it's charge immediately, and wouldn't turn over. Could it be anything else but the battery ? (given that there were no warning lights lit for the 40 minutes I thought it might be charging)

>This might be a silly question,

never seen one, without mine !!
and silly answers.

Serious first try:
The problem is obviously the alternator and its components.

Did you ever realise (or, can you have a look wether it occurs), small engine oil or gearbox oil onto the alternator ?



Sounds like a cell in the battery has collapsed and broken the internal circuit of the battery. A battery that will not take any charge will also put considerable starin onto the alternator and so you should check out the cars system as soon as possible.

At this stage I think that the alternator is still generating since if there was nothing to start with the battery was obviously drained. If it started and then ran once the jump leads were removed then clearly the power to run the car was coming from the alternator.

I presume that whilst you say the engine wouldn't turn over after the 40 min run, there was some semblence of warning lights with ignition on, after the run and before you tried to crank the engine? If so then the battery is either not taking charge or not receiving it. The former is favorite, but before lashing out on a new battery do check that the cable terminals are a tight fit to the battery. Also check the terminal posts are clean (as are the inside contact faces of the cable connections to the battery) and then check that as far as you can be certain there is not cable damage.

Most motor shops have battery testers (not just the specific gravity testers) which place a load on the battery to see if it is still serviceable. It would also be an advantage to check the system is charging OK, but this needs a good battery to be connected to the car. This can be achieved by using a slave battery connected to the car via jump leads, but if this is on another car the engine and of this MUST be off.

A voltage check is what most hand held 'alternator testers' consist of. 12 to 12.5v for a static battery load with everything off, with 13.7 (+ or - 0.5v) for a static load with engine running. With headlamps (dip) on along with the heater fan the voltage should stay above 13v with the revs held at about 3000rpm for 1 minute. I add this simple check as most people can lay their hands on a DC voltmeter. Terminals should be attached across the battery terminals.

Roger Parker

"as most people can lay their hands on a DC voltmeter " . Yes, sure they do with Our Fluke lab meters. Fine as long as itīs restricted to voltage measurements !! Then they see the "Amp " setting on meter and try to find out " if thereīs any Ampīs left in the battery". And there is ALWAYS enough Ampīs left ........... <grin>,

Regards , Carl.


I have had a similar problem for the last two months (havn't had time to rectify it). The battery drains completely after about three days. It charges up OK using a battery charger, but even if I do not drive the car the battery will drain completely. I have had the battery and alternator checked out (both ok), disconnected the boot light, left the car with just the passive alarm on. All to no avail.


PS. I have to remember to charge the battery a couple of days before using the car. As it's the winter its only a mild pain in the ****.

did you measure the DC already ?
- disconnect the battery
- measure at first currency in 200mA range, then go down it more sensible ranges.
- the idle current sould be only abot 2 .. 5 mA, everything else signs any failure at your cars wiring or sensor.

(But do not forget to disconnect the bonnet-light bulb, before you measure)
Disconnecting the horns helps to protect your ears and from bad mood to your neighbours too while such test sequences ;-)

This thread was discussed between 21/11/1999 and 23/11/1999

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