Flat battery after drive

I went for a drive to shop and on return removed shopping and left the Mx in the drive, expecting to take it out again. About 5 hours later, not having gone out again, went to park it and found the car totally dead. Battery voltage was 3.5 v, and every sign of life gone.

Put it on charge, and about an hour later, the battery voltage was 12v, so tried to start it but failed, but got a bit of a kick from the starter. Left it a bit longer and battery was at 12.5v and car started. So took it for a 5 km drive and everything was back to normal - wll sort of - car drove perfectly, but radio was reset, mpg was at zero, and fuel gauge was very low compared with the 50% it should be at. After drive, fuel was about right, mpg had got to 30 or so, when before it was at 47mpg.

Any idea why this would happen? I cannot think of anything other than I may have left the ignition on, but I do not think I did.

Do I need to do anything to get the ECU to recalibrate or does it do that itself? Will it just work everything out itself?

You need to say exactly which model for those in the know to be able help you.

1 Like

MX5 ND1 soft top Soul Red, 2017, 4.5k miles. Not done many miles in the last 12 months - maybe 700 miles in last 18 months.

1 Like

Manual pages - MX5 Manual | Mazda MX5 (Mk4) 2015+
This shows that the MPG will be reset when battery is disconnected.
Manual pages - MX5 Manual | Mazda MX5 (Mk4) 2015+
This shows what needs resetting after the battery is disconnected.

I would try charging the battery for at least 12 hours or if you have a modern conditioning charger leave it connected for a few days and see if the battery can be recovered or has been damaged beyond ‘repair’.

It does sound as though something was left on so do the above and we will keep our fingers crossed that everything returns to normal.
You could just check that the alternator is giving plenty of charge by popping your volt meter across the battery with the engine running at say 2500rpm. It should show a reading well above 12 volts.


Well, I checked tis morning and it read 12v. Put it on charge and it got to 13v. I wil leave it on charge again for a few more hours and see what it gets to.

Does anything else need resetting?

On our CX3, we had a flat battery due to the iStop ‘feature’, the engine was off, got out pressed the key fob ‘lock’ but didn’t check the flashers. Next morning the battery was too flat to start the car - wish I could completely disable the pointless iStop curse.

Luckily my MX5 is an NC and too old to have these stupid features!

I am not familiar with iStop ‘feature’ but that would account for it (perhaps).

I had the hood down, so would have stopped the engine and then closed the roof, and then closed the windows. Must have got out without pressing stop again, so car must have been left with the ignition on. Mystery solved (perhaps).

I must be more careful in future but hopefully no harm done. Is there a way to make the ND chirp when the car locks?

Hi S,
You have the ND1 so it does not have iStop so don’t be distracted by the post above.

You need to leave it on charge overnight. Any voltage readings during or within an hour of stopping charge can be misleading. Check the voltage after 3 hours to be confident of a ‘good’ reading.

The battery will run down if you do not use the car for a week so keep an eye on the battery state. A ‘smart charger’ could be worth investing in.

Yes you can make the car ‘chirp’. Section 9-10 in here MX-5_8EV5-EE-15E_Edition1.indb (mazda.media) tells you what can be personalised and how.

1 Like

Just one tip that may be helpful. When you stop and turn the engine off with the hood down DO NOT open the doors. The windows stay powered for a few minutes to facilitate final closing.

I have wrked that one out already. Normally, I stop the engine, close the roof, then close the windows, then press stop. I must have failed to press stop the final time, which left the igniton on. Stupid mistake, but no harm done.

Car now fully charged.so hopng to get out soon.

Hopefully, being discharged for a fairly short time won’t have damaged the battery too much

If you stop the engine and press stop again you go into accessory mode. Once you have pressed stop to stop the engine you do not need to press it again. This could be the root of the issue. Or may be not!

I think it is the keyless system that the brainless have trouble with. If I had to remove a key, then it would be shut down properly. I need to be careful in future.

Think your over thinking what you did wrong, just sounds like a flat battery and by the fact you took it only for a 5km run after starting is not enough.Measuring voltage is not a science if the battery is good or not you need to get the CCA measured and compare against original CCA.

1 Like

I put the battery on charge until the charging current was near zero which meant it was fully charged - well I think that is what that means. It was taking over 6 amps earlier.

Anyway, hopefully all is well now.

1 Like