Charging car battery by leaving running idle

arcon5

Just wondering how long you guys would recommend leaving the engine running to charge a battery that has run flat? Bearing in mind I cant take it for a long hard drive to charge it as it's SORNed.

Need to jump it, then leave it running to build up enough charge to fire up when it's collected for an MOT next week

F1F93

Depends why it went flat in the first place. If the alternator is broken, then no amount of running will charge it.
Assuming the alternator is fine, I'm not sure how long but obviously revving it higher will charge it quicker. If you have a digital multimeter, you can check the voltage for yourself to know when it's done.

Although, you can buy a dedicated battery charger for under thirty quid. It might be worth considering, as an alternative to wasting fuel if this sort of thing happens a lot? Would also remove the need for jump starting (which is not recommended for modern cars, as it can damage the car's ECU).

facade

45minutes to an hour should do, unless it is totally flat, then you are into 2 to 3 hours at least.

Make sure everything is off like the heater fan. Just sit in the car and play the radio/read a book for an hour with your foot tickling the throttle so it runs at a fast idle, but modern alternators will charge at idle.

TrickyWicky

As facade says at idle you do need to let it run for a long while or you actually end up with a battery that has very little charge. Sometimes its best to rev the engine and leave it revving for a little while too. Although the alternator is limited to an output of 14V this does seem to make a difference.

EdGasket

You are much better off getting yourself a battery charger. Disconnect one of the car leads while charging so any charger spikes don't damage the car's electronics. I have a sorned car, the battery always goes flat after a few weeks even though I run it for 10 mins every fortnight.

forgotmyname

If the battery is totally flat the alternator may never charge it. The alternator needs some voltage for it to do its job.

How long has it been flat? Batteries left in a discharged state deteriorate. The plates lose their efficiency.

A decent 8Amp car charger for at least 24 hours will do a much better job.

Mankysteve

Need a proper drive to charge the battery using the alt best bet as others have said is to take the battery out and get a charger.

Alias_Omega

eBay - Used.. probably a few for less than £10 local to you.

forgotmyname

Maybe i should have stocked up when Argos used to sell them for £4 for a 6amp or £6 for an 8 amp.

Although basic they do drop the charge rate as the battery gets closer to fully charged.

Nodding_Donkey

Get a solar powered trickle charge.

Norman_Castle

Couldn't you jump start it when its picked up. If its a mechanic collecting it they may have a jump pack.