How long does it take to charge a car battery with a 10 amp charger?

About 11 hours for a 100 Ah lead-acid battery from empty, or about 5 hours 33 minutes from half full. At 90% charging efficiency, an empty battery needs 100 / 0.9 = 111.1 Ah. Divided by 10 amps, that is 11.1 hours, or 11 hours 7 minutes.

Smaller batteries charge faster, so use your battery's own amp-hour rating. The formula is amp-hours needed, divided by efficiency, divided by charger amps. The car battery charging time calculator does the math.

Treat the answer as a minimum. Battery University says the final 30% of a lead-acid charge comes during a slower topping stage that takes another 7 to 10 hours, and puts a full charge at 12 to 16 hours. The guide on how long to charge a car battery explains each stage.