How this calculator works
A battery stores charge measured in amp hours. If it is at 20% of its capacity, the charger has to put back the other 80%, which for a 5 Ah battery is 4 Ah. A charger feeding 2 A would need two hours to move 4 Ah into the cell if the process were perfect. It is not: part of the current turns into heat or drives side reactions, which is the charging efficiency, and 90% is a fair working figure for lithium-ion and lead acid. The arithmetic then divides the missing charge by the current and the efficiency. Real chargers also taper near full, so the last 20% takes longer than the average rate suggests, and this estimate is optimistic for the final stretch.
Formula
charge to add = capacity x (1 - state of charge). Charge time = charge to add / (charging current x efficiency). Solving the other way: capacity = current x time x efficiency / (1 - state of charge), and current = capacity x (1 - state of charge) / (efficiency x time).
Worked example
A 5 Ah battery at 20% needs 4 Ah. At 2 A and 90% efficiency, the time is 4 / (2 x 0.9) = 2.22 hours, about 2 hours 13 minutes. Charging a 5 Ah battery from empty in 1 hour at 90% efficiency needs 5 / 0.9 = 5.56 A.
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Frequently asked questions
Why is charge time longer than capacity divided by current?
Because the charger does not store everything it delivers. Dividing by the efficiency accounts for the losses, and real lithium chargers also reduce current in the constant voltage phase near full.
What efficiency should I use?
About 95% for lithium-ion, 85% to 90% for lead acid, and around 70% for nickel metal hydride, which drops more at high charge rates. These are typical values rather than specifications, so use the figure from your cell datasheet where you have one.
Does a bigger charger always charge faster?
Only up to the cell rate limit. Most cells should not be charged above 1C, which is one hour from empty, and exceeding it shortens life. A larger charger beyond the limit adds heat rather than charge.
Why does the last part of the charge go slowly?
Chargers switch from constant current to constant voltage near full. Current falls as the cell approaches its upper voltage, so the last few percent can take as long as the middle of the charge.