What the C-rate is
The C-rate measures how fast a battery is charged or discharged relative to its capacity. At 1C, the current equals the capacity in amp-hours, so a full battery empties in one hour. At 0.5C it takes two hours; at 2C, thirty minutes.
C-rate = current (A) ÷ capacity (Ah) and time (h) ≈ 1 ÷ C-rate
Example
A 5 Ah, 3.7 V lithium-ion pack drawing 2.5 A:
- C-rate: 2.5 ÷ 5 = 0.5C;
- ideal discharge time: 2 hours (120 minutes);
- stored energy: 5 × 3.7 = 18.5 Wh;
- power at that current: about 9.3 W.
Why real times are shorter
The ideal time assumes the full rated capacity is available at any rate. In practice:
- at high rates, internal resistance turns more energy into heat and the voltage reaches the cutoff sooner, so usable capacity drops (Peukert’s effect, strong in lead-acid batteries);
- capacity is lower in the cold;
- old batteries have lost part of their capacity.
Charging
Lithium batteries are charged in constant current, then constant voltage (CC-CV). The current tapers at the end, so a full charge takes roughly 20% longer than capacity ÷ current. Many cells are specified for 0.5C to 1C charging; fast-charge cells accept 2C or more, but only within the manufacturer’s limits and temperatures.
Typical ratings
- Consumer lithium-ion cells: charge 0.5–1C, discharge 1–2C continuous.
- Power-tool and drone cells: discharge 10C to 30C or more.
- LiFePO4: often 1C charge and 1–3C discharge.
- Lead-acid: best below 0.2C; capacity is usually quoted at the 20-hour rate (0.05C).
Tips
- Lower C-rates mean less heat and longer cycle life.
- For a pack in series, use the capacity of one string; parallel strings add capacity and lower the C-rate per cell.
- Always check the datasheet for maximum continuous and peak currents.