Battery C-Rate Calculator (Charge Time, Discharge Runtime, Safe Current by Chemistry)
Convert between amps and C-rate for any battery, estimate charge time including the constant-voltage tail for lithium cells, discharge runtime at a given current or power with rate losses, and check the current against safe continuous limits for Li-ion, LiFePO₄, NiMH and lead-acid.
How it works
C-rate = current ÷ capacity (A ÷ Ah) · CC charge time = Ah to add × CC fraction ÷ (I × efficiency) + CV tail (Li-ion ≈ 30–60 min, LiFePO₄ shorter, lead-acid hours) · runtime = effective Ah ÷ I, effective Ah reduced at high rates (Peukert: lead-acid ≈ (C ÷ 0.05)^−0.2) · Wh = Ah × V
C-rate normalises current to capacity: 1C charges or drains the battery in an hour, 0.5C in two. It is the number that battery limits are written in, and it makes charge times and runtimes chemistry-comparable. Charging is not simply capacity ÷ current — lithium cells take the last 20% in a constant-voltage tail whose length barely depends on the charger, and lead-acid drags on for hours. Discharging fast costs capacity, dramatically for lead-acid (whose rating assumes a 20-hour drain) and only mildly for lithium. This calculator applies those rules and checks your currents against safe continuous rates for each chemistry.
Worked example
A 3,000 mAh Li-ion cell charged at 1.5 A is at 0.5C — comfortable. From 20% it needs 2.4 Ah: about 1 h 21 min of constant current for the first 80% plus roughly a 32-minute constant-voltage tail, about 1 h 53 min in total. Discharging at 600 mA (0.2C, 2.2 W) runs about 5 hours; the cell stores about 11.1 Wh.
Frequently asked questions
What does 1C mean?
A current equal to the capacity number: 1C for a 2,000 mAh cell is 2,000 mA, which would empty (or nominally fill) it in one hour. 2C is 4 A and half an hour; C/10 or 0.1C is 200 mA and ten hours.
Why does a phone charge to 80% fast and then slow down?
Lithium charging switches from constant current to constant voltage near 4.2 V per cell; the current then decays as the cell fills and the charger stops around 0.05C. That tail takes 30–60 minutes regardless of charger power, so fast charging mostly shortens the first 80%.
How fast can I discharge a battery safely?
Energy-type Li-ion cells 1–2C continuous, high-drain cells (marked 10C–30C or by amps) far more, LiFePO₄ 1–3C, NiMH about 1C, and lead-acid should stay near 0.2C or below for capacity and life. Check the datasheet's continuous and pulse ratings and keep the pack cool.
Related calculators
Electronics estimates use simplified models and typical component values. Check the datasheet limits (voltage, current, temperature, gate charge, ESR), prototype, and measure — especially for anything connected to mains, batteries or motors. Lithium batteries can catch fire if over-charged, over-discharged, shorted or charged cold; use a proper charger and BMS.