Battery Charge Time Calculator

This battery charge time calculator estimates how long a battery takes to recharge from four everyday numbers: its capacity in amp hours, how much charge you need to put back, the charger current, and the charging efficiency. Amp hours describe how much charge a battery holds, so a 100 Ah battery can in theory deliver 100 amps for one hour or 10 amps for ten hours. To recharge it you have to push those amp hours back in, and because some energy is lost as heat, especially in lead acid batteries, the real time is longer than dividing capacity by current alone would suggest. You tell the calculator the capacity, the percentage of charge to replace, the charger output in amps, and an efficiency figure, and it returns the charge time in hours and minutes, the decimal hours, the amp hours being replaced, and the effective current once losses are taken off. It suits car, marine, caravan, solar and deep cycle batteries, and helps you plan an overnight charge or size a charger. Bear in mind chargers taper near the end, so the last stretch to full runs slower than a straight line estimate, and never charge a battery in a way its maker does not allow.

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Ah
%
A
%
9h 25m
estimated charge time
In hours9.41 h
Charge to replace80 Ah
Effective current8.5 A

Chargers taper near full, so real time to 100 percent runs a little longer. Use about 80 percent efficiency for lead acid and 90 to 95 percent for lithium. Estimate only.

How it works

The amp hours to replace are the battery capacity multiplied by the percentage of charge you need to put back. The effective current is the charger current multiplied by the efficiency, because some of the current goes to heat rather than stored charge. Charge time in hours is then the amp hours to replace divided by that effective current. The result is converted into hours and minutes. Higher efficiency and a bigger charger both shorten the time, while a larger battery or a deeper discharge lengthen it.

Worked example

You have a 100 Ah battery that is 80 percent empty, a 10 A charger, and 85 percent charging efficiency. The charge to replace is 100 times 80 percent, which is 80 Ah. The effective current is 10 times 0.85, which is 8.5 A. The charge time is 80 divided by 8.5, which is about 9.41 hours, or 9 hours and 25 minutes. Because the charger will taper as the battery fills, allow a little extra to reach a genuine full charge.

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Frequently asked questions

How do I calculate battery charge time?

Divide the amp hours you need to replace by the charger current, then divide by the charging efficiency. For a 100 Ah battery charged 80 percent at 10 A and 85 percent efficiency, that is 80 divided by (10 times 0.85), which is about 9.4 hours.

Why does the last part of charging take longer?

Most chargers taper the current as the battery fills, the absorption stage for lead acid or constant voltage for lithium, so the final 10 to 20 percent charges more slowly than a simple estimate suggests. Real time to full is usually a bit longer than the calculation.

What efficiency should I use?

Lead acid batteries are roughly 80 percent efficient when charging, so use about 80. Lithium batteries are more efficient, around 90 to 95 percent, so use a higher figure. Lower efficiency means more energy is lost as heat and charging takes longer.