Accidentally flattened the car battery the other week by leaving the door open. Totally dead, got a jump start and drove 6 hours home.

Since then the car has started fine but the start/stop system hasn’t worked since, giving “battery still charging” errors.

Do you think I’ve permanently damaged it by running it so flat? Don’t know what the voltage fell to but it wasn’t enough to unlock the doors.

New battery £80, trying to decide if it’s worth buying a smart charger and running it overnight first?

Maybe it needs to be run more or trickle charged to get it back up to strength.

Just running it flat shouldn’t cause an issue unless there are other issues. 6 hours is a long enough run though I’d have thought . Is the alternator charging properly?

I don’t know much about stop/start but it may be it demands a really big brief spike in power so if the batteries not quite topped up it doesn’t work..

Running it flat can cause issues .

It ups the internal resistance and it may never gain full capacity again.

A proper smart charger can rejuvunate them and much more chance than the vehicle alternator …but a decent smart charger will be about 60-80 quid and you may still need a battery.

Can you borrow a. Smart charger perhaps ,?

The CTEK chargers are very good.

How old is the car.  Lead Acids don’t like being ‘browned out’.

Check Euro car parts for battery prices, and don’t forget the permanent 30% or more discount codes.

Good news: thrashed it back along the motorway today for 2 hours at 2000rpm and just before I got home the start-stop system showed “Available”.  Which is good, because I just discovered that a replacement battery is actually £240, not £80, as it’s advanced glass mat technology.

On the bright side, it seems that provided you don’t let an AGM battery sit in a deeply discharged state it’s relatively undamaged.

Thanks for the advice, though.  Ordered a CTEK charger anyway since it seems a useful thing to own.