Charging battery after a jump start

A friend accidentally let the battery in his car run out by leaving the car lights on. He managed to get it going again with a jump start, but he wasn't sure how long he'd have to drive to ensure that the car would have enough battery to start again later.

He ended up driving around for quite some time just to try to ensure that he was safe - but how long is really necessary?

The question really is what the voltage was just before the jump start. A jump start is nothing more than get the engine running from another battery. If the engine runs again, the alternator/dynamo will charge the partly depleted battery again.

Nobody can tell how long you need to drive to get the battery fully loaded again, as this depends on the state of the battery (old battery might need longer charging time), weather conditions (warm weather + warm engine = faster charging time), the !very dynamic! throttle position (high revs = more engine turns = more dynamo turns = faster charging), type of dynamo (big one = faster charging), electricity running from the battery (lights - A/C - fans - radio,..) , and finally type of engine (diesel engine = less load on battery while the engine runs = quicker charging).

To make sure you get the battery all juiced up prior to shutting of the engine, you need to turn all electricity off - lights - A/C - fans - radio etc.. Then you could either leave the car running on the driveway, or take it for a spin.

My suggestion would be to get in the car and drive for at least about half an hour - again totally depending on the depletion level - , while taking the things above into account. If you come back from your joyride -please- don't test if the car starts again and shut it off immedeately afterwards. This temporary huge load on the battery might leave it with just too less juice for your next start. Instead, just leave the car as is and hope the next time it will start again when you need to use your car :)

• turning off a/c, lights, etc. is a handy tip. Thanks. Commented Apr 16, 2015 at 13:18
• Car alternators do not charge with very high current. If the battery was completely flat, something like a š hour drive may be needed to recharge it. Or you could always use a battery charger. Commented Apr 16, 2015 at 13:46
• Indeed. You could just drive it home and then connect a battery charger. Commented Apr 16, 2015 at 18:54
• If you have one... Commented Apr 16, 2015 at 19:31
• @Pentium100 if you started a six hour drive in Lithuania, how many countries could you drive through? :o Commented Jul 17, 2016 at 20:59