Took my MX5 for a track day at Donington about 10 days ago, a round trip of about 400 miles including playtime. No problems of any kind. Next day I manouevered the car on the driveway to remove wheels and check brake wear, not used the car since. Battery is now completely flat - 1.3 volts - and will not charge. What can have happened? ( Battery is probably 4 years old ). Ideas?

One of the interior lights in the footwell left on?

A cell has collapsed?

Regulator pack in the alternator has failed and over charged the battery to the point of ruination.

Most likely cause is a parasitic drain an alternator diode perhaps. There is some voltage left so it will likely recharge, disconnect the battery recharge it fully and start doing normal checks.

Hi John, if battery won't accept charge then it's knackered anyway. Once new battery is installed I would check that it isn't charging over 14.5V & that there is no drain from the battery when turned off.

Cheers Russ

Did you disconnect the battery before trying to charge ? it as a bad diode pack can easily leak more current back to ground than most battery chargers can provide.

I had a similar problem on an MX 5 also after o long journey.

Did usual checks, recharged battery and send Alternator for overhaul
Good company checked it and reported on fault - No charge to me

I searched further and I am fairly sure it was a lack of or too little supply on the field terminal. the small one on the alternator ,l M6 if I remember.
I had disturbed a lot of stuff checking right through the system, so don't know exactly where the problem was but no recurrence in 12 months.

Thanks for the input chaps, quite a few things to check for. I am out of the country for a few weeks yet so cannot action anything. For the record neither my main charger or trickle charger would show any activity when attached to the battery ( in situ ). I am guessing that maybe there is a short somewhere and that the internal protection on the chargers is inhibiting them. ( I did not check battery resistance which would presumably have shown this )

You need to disconnect battery asap, then try a recharge, tho' if it's been connected with a drain all this time battery pretty much has to have had it anyway!

Once you have a charged battery on there you can start checking the other stuff, but alternator diodes causing drain down is easily isolated by disconnecting the alternator feed(s)

You need to disconnect battery asap, then try a recharge, tho' if it's been connected with a drain all this time battery pretty much has to have had it anyway!

Once you have a charged battery on there you can start checking the other stuff, but alternator diodes causing drain down is easily isolated by disconnecting the alternator feed(s)

Thanks Russ. I have a track day booked at Castle Coombe three days after we get back. I had planned to take the MX5 as it seemed to be inherently more reliable at this stage than my Cyclone which is still something of an unknown quantity. Looks like I will be taking the Cyclone instead even though the rear suspension is off at he moment for a spring change!