Knowing how long it takes to charge a battery is useful for everyday planning. The basic calculation is simple, but real-world factors make it more complex. This guide explains the core method and key adjustments for a realistic estimate.
The Basic Formula
For constant-current charging, use this formula:
Charging Time (hours) = Battery Capacity (Ah) / Charging Current (A)
- Battery Capacity (Amp-hours, Ah): The total charge the battery can store.
- Charging Current (Amps, A): The rate at which electricity flows into the battery.
Example: A 20Ah battery charged at 2A would take 20Ah / 2A = 10 hours in ideal conditions.
Important Real-World Adjustments
- Charging Efficiency: Energy is lost as heat. Efficiency is typically 85-95%. Adjust the formula:
Actual Time = (Capacity / Current) / Efficiency
For the 20Ah battery with 90% (0.9) efficiency: (20/2) / 0.9 ≈ 11.1 hours. - Starting Charge Level: You rarely charge from 0%. If you start at 20%, you only need 80% of the capacity:
Time = ((Capacity × % to charge) / Current) / Efficiency
For 20Ah at 20%, needing 16Ah: (16 / 2) / 0.9 ≈ 8.9 hours. - Two-Stage Charging (CC-CV): Modern lithium-ion batteries use:
Constant Current (CC): Fast charging at max current until ~80% full. The basic formula applies here.
Constant Voltage (CV): Current slowly drops to top off the last 20%. This stage adds significant extra time not shown in the simple calculation.
Using Power (Watts) Instead of Current
Many chargers list power (Watts, W). Here’s the method:
- Find battery energy in Watt-hours (Wh): Capacity (Wh) = Voltage (V) × Capacity (Ah).
- Basic Time (hours) = Battery Capacity (Wh) / Charger Power (W)
Example: A 60Wh laptop with a 30W charger: 60Wh / 30W = 2 hours. Apply efficiency: 2h / 0.9 ≈ 2.2 hours.
Practical Example: Smartphone
Battery: 5000 mAh (5.0 Ah) at 3.7V. Capacity = 3.7V × 5.0Ah = 18.5 Wh.
Charger: 20W.
Basic Estimate: 18.5Wh / 20W = 0.925h (~55 min).
Realistic Estimate: Factor in 90% efficiency and the slow CV stage: Total time ≈ 1.5 hours.
Quick Tips
Manufacturer claims often only refer to the fast CC stage (e.g., "50% in 30 minutes").
For a rough guess, see how much charge is added in the first 15-30 minutes, but remember the last 20% is slower.
Conclusion
The core rule is Time = Capacity / Rate. For a good estimate, you must know your battery's true capacity (in Ah or Wh) and your charger's real output (in A or W). Then, adjust for efficiency losses, starting charge level, and the slow final charging stage. Understanding these steps will give you a reliable prediction for charging any device.