Whether you're sizing a charge controller or just wondering when your bank will be ready, charge time comes down to a simple relationship between capacity and current — with one real-world adjustment for the energy lost as heat while charging. This calculator handles both, and tells you the C-rate you're charging at so you can check it's safe for your battery.
The basic formula
At its simplest:
Charge time (hrs) = capacity (Ah) ÷ charging current (A)
A 100 Ah battery charged at 20 A takes about 100 ÷ 20 = 5 hours in theory. But charging isn't perfectly efficient — some energy is lost as heat — so the realistic time is a bit longer. Dividing by the charging efficiency (say 85%) gives about 5.9 hours. This calculator shows both the ideal and the realistic figure.
What is the C-rate?
The C-rate is the charging current expressed relative to capacity. Charging a 100 Ah battery at 100 A is 1C (about an hour), at 50 A is 0.5C (about two hours), and at 20 A is 0.2C (about five hours). It's a quick way to check whether your charging current is reasonable — and crucially, whether it's within what your battery can safely accept.
How fast is too fast?
Every battery has a maximum safe charge rate. Lead-acid batteries usually shouldn't be charged much above 0.2–0.3C, or they overheat, gas, and wear out faster. Lithium batteries typically accept 0.5C or more, but still have a limit. Charging faster than recommended shortens battery life, so it's worth staying within the manufacturer's figure even though it means waiting longer. The calculator flags the C-rate so you can see where you stand.
Charging from partly full
You rarely charge from completely empty. If your battery is already 50% full, you only need to replace the other 50%, which halves the time. The "charge needed (%)" input handles this — set it to how much you're actually topping up.
Related tools
To size the battery bank itself, use the battery size calculator, and to see how long a charged bank lasts on a load, the battery runtime calculator. To make sure your charge controller can deliver the current you're planning, check the charge controller calculator.
Frequently asked questions
How long does it take to charge a solar battery?
Divide capacity (Ah) by charging current (A) for the basic hours, then allow for losses. A 100 Ah battery at 20 A takes ~5 hours ideally, or ~5.5–6 hours realistically. Charging from partly full is proportionally faster.
What is the C-rate of a battery?
Charge/discharge current relative to capacity. 1C fills in ~1 hour, 0.5C in ~2, 0.2C in ~5. For a 100 Ah battery, 0.2C = 20 A. Lead-acid often maxes at 0.2–0.3C; lithium accepts more.
Can I charge a battery too fast?
Yes — charging above the recommended C-rate makes heat and shortens life (and for lead-acid causes gassing). Staying within the rated C-rate keeps the battery healthy.