how long is it safe to leave a battery charger on a 12 volt battery at a setting of 10 amps ?

i would only use 10 amps for jump starting... watch the battery temperature and bubbling in the battery which would be the release of hydrogen and depletion of water in the electrolyte... that is not desirable and could create an explosive condition...

12 hours is ok.

The battery should charge and the current should drop.

Did you want to keep it connected longer than 12 hours? If yes, why?

If anything other than Jump Starting NEVER. High amperage on a discharged battery shortens the charge and discharge cycle. Always charge a discharged battery slowly @ 2 amps overnight then disconnect the charger and wait one hour for the surface voltage to dissipate then test the voltage which should be 12.6 volts if fully charged.

Like Don said, I only use 10 amps for short bursts - like jump starting. Otherwise I'll use the lower setting and let it go overnight or all day. My charger is funky and old but it is a big professional model and has a timer on it and will also not overcharge - i.e. it regulates. However I have had cheap, discount store chargers which were not nearly as good, would overcharge a battery and one even had so much AC leakage that it destroyed the computer in a Saab. That was an expensive fix.

I don't know a thing about your charger. Quality varies drastically in this type of equipment.

6 hours will charge the battery enough. I'd not go over 10 hours.

If you can add distilled water to the battery make sure to check the level in the battery before charging it.

Don't try to remove any fluid from the battery. Once water goes in it mixes right away with the acid.

the charger will turn off automatically

How large is this battery?

How low is the charge?

Most importantly, does the charger shut off when it reaches a full charge?

These are all unknown questions so no one can answer your question as asked.

I would only use 10 amps for jump starting. Watch the battery temperature and bubbling in the battery which would be the release of hydrogen and depletion of water in the electrolyte. That is not desirable and could create an explosive condition.

On most modern battery chargers -------- indefinitely, because they shut off after the battery is charged.