Battery datasheets rarely quote a maximum current in plain amps — they quote a C-rate. That's because the safe current depends on the battery's size. The same 100 A is gentle for a big bank and punishing for a small one. The C-rate normalises current to capacity, and the conversion is simply current = capacity (Ah) × C-rate.
What the C-rate means
A C-rate of 1C means the battery charges or discharges its full capacity in one hour. So a 100 Ah battery at 1C moves 100 A. At 0.5C it moves 50 A and takes two hours; at 0.2C it moves 20 A and takes five hours. The nominal time is just 1 ÷ C-rate hours. Lower C-rate means gentler current and a longer, cooler life.
A worked example
Take a 200 Ah battery bank at 0.5C. The current is 200 × 0.5 = 100 A, and the nominal time is 1 ÷ 0.5 = 2 hours. That 100 A figure is what you check your charge controller and inverter against: the controller's charge output and the inverter's peak draw must both sit within what the bank can safely handle at its rated C-rate.
Safe C-rates by battery type
- Lead-acid (flooded / AGM / gel): like a slow charge, typically 0.1C to 0.2C. A 200 Ah lead-acid bank wants only about 20–40 A of charge current.
- Lithium (LiFePO4): tolerate much faster rates, commonly 0.5C and often up to 1C. The same 200 Ah lithium bank can accept 100–200 A if the BMS allows.
Push past the rated C-rate and the battery heats up, loses capacity faster, and can trip its protection. When in doubt, design gentler.
Related tools
Once you know the current, work out how long a charge takes with the battery charge time calculator, how long the bank will run a load with the battery runtime calculator, and convert between amp-hours and kilowatt-hours using the Ah to kWh converter. To size the charge source, see the charge controller calculator.
Frequently asked questions
What is a battery C-rate?
It's how fast a battery charges or discharges relative to its capacity. 1C = full capacity in one hour, 0.5C = two hours, 0.2C = five hours.
How do you calculate charge current from a C-rate?
Multiply capacity by C-rate: current (A) = capacity (Ah) × C-rate. A 200 Ah battery at 0.5C is 100 A, charging or discharging in about two hours.
What C-rate is safe for a solar battery?
Lead-acid prefers 0.1–0.2C; lithium (LiFePO4) commonly handles 0.5C up to 1C. Always follow the datasheet — a higher C-rate means more heat and shorter life.