How Long to Charge a Car Battery (12 V Car, RV and Boat Batteries)

To estimate how long a 12 volt battery takes to charge, divide the amp-hours you need to put back by the charger’s amps, then allow for losses. A 100 Ah lead-acid battery charged from empty to full on a 10 amp charger at 90% efficiency needs 100 / 0.9 = 111.1 Ah. At 10 amps that is 11.1 hours, or about 11 hours 7 minutes. From half full, it takes about 5 hours 33 minutes.

The car battery charging time calculator runs this for any battery and charger. The same math works for RV, boat and other deep-cycle batteries.

The car battery charging time formula

Amp-hours needed = Battery Ah x (End % - Start %)
Amp-hours in     = Amp-hours needed / Charge efficiency
Hours            = Amp-hours in / Charger amps

For the 100 Ah example, empty to full:

1. Amp-hours needed: 100 x 1.00 = 100 Ah.
2. Allow for losses: 100 / 0.9 = 111.1 Ah.
3. Divide by the charger: 111.1 / 10 = 11.1 hours.

From 50% to full, you need 50 Ah, or 55.6 Ah after losses. That takes 5.56 hours on a 10 amp charger, which is 5 hours 33 minutes.

Why 90%? Battery University puts the coulombic efficiency of lead acid at about 90%. Lithium-ion is above 99%, so a lithium battery loses far less.

Find your battery’s amp-hours

The amp-hour rating tells you how much charge the battery holds. It is printed on most deep-cycle, RV and marine batteries. If your label has no Ah figure, check the maker’s spec sheet.

You also need the starting state of charge. A battery that still starts the car is rarely empty. Charging from a guess of 50% gives a more realistic answer than assuming 0%.

Why the last 20-30% takes longer

The simple formula assumes the charger runs at full current the whole time. Lead-acid batteries don’t charge that way. According to Battery University’s lead acid charging guide, a lead-acid charge has three stages:

• Constant current charge, which applies the bulk of the charge. The battery reaches about 70% in 5 to 8 hours.
• Topping charge, sometimes called an absorption charge. The voltage holds steady and the current tapers. This fills the last 30% over another 7 to 10 hours.
• Float charge, which holds the battery at full.

The same source puts a full lead-acid charge at 12 to 16 hours, and up to 36 to 48 hours for large stationary batteries. So treat the formula’s 11 hours 7 minutes as the minimum. The topping stage adds time on top of it.

Battery University also says the topping charge matters. A lead-acid battery that is regularly denied a full charge will slowly lose capacity through sulfation.

What size charger to use

The usual range for a lead-acid battery is 0.1C to 0.3C, where C is the amp-hour rating. For a 100 Ah battery that means 10 to 30 amps. Battery University says makers recommend 0.3C. The C-rate entry explains the notation, and the battery charging current calculator works out the range for any battery.

A small maintenance charger is useful for keeping a parked car topped up, but it takes a long time to refill a flat battery. The formula still applies: halve the amps and you double the hours. The FAQ on how many amps to charge a 12 V battery covers sizing in more detail.

RV, boat and lithium batteries

Deep-cycle lead-acid batteries in RVs and boats follow the same three stages. A lithium replacement charges differently. It is more efficient, takes higher current and does not need a long topping stage. The guide to lithium vs lead-acid charging compares the two.

If you charge from a solar panel instead of a wall charger, the limit is panel watts and sunlight. The guide to solar battery charging uses the same 100 Ah battery.

Leaving a battery on the charger

A charger with a float stage can stay connected, because it drops to a lower voltage once the battery is full. Battery University says a fully saturated battery should not dwell at the topping voltage for more than 48 hours before dropping to float. It also notes that not all chargers have a float stage, so read the manual.

To see how long the charged battery will power your gear, try the battery runtime calculator. For a fast answer on one common setup, see how long to charge a car battery with a 10 amp charger.