How to Calculate Battery Life
What is Battery Life?
Battery life is calculated from capacity (mAh), discharge current (amps), and voltage. Watt-hours = mAh × Voltage ÷ 1000, and runtime = Watt-hours ÷ Power draw (Watts).
Formula
Runtime(hrs) = Capacity(mAh) ÷ 1000 ÷ Current(A) × Voltage(V) | Watt-hours = mAh × V ÷ 1000
- C
- Battery Capacity (mAh)
- I
- Discharge Current (A)
- Wh
- Energy Capacity (Wh)
Step-by-Step Guide
- 1Runtime (hours) = Capacity (mAh) ÷ 1000 ÷ Current (A) × Voltage (V)
- 2Watt-hours = mAh × V ÷ 1000
- 3Real runtime is 70–80% of theoretical (efficiency losses)
- 4Temperature, age, and load affect actual capacity
Worked Examples
Input
5000 mAh at 3.7V, 0.5A draw
Result
37 Wh, ≈10 hours theoretical runtime
Input
Phone battery: 4000 mAh at 3.85V, average 1W usage
Result
≈15.4 Wh → ≈15 hours
Frequently Asked Questions
Why is real runtime less than calculated?
Efficiency losses from internal resistance, voltage regulation, and management circuits typically consume 20–30% of theoretical capacity.
How does temperature affect battery life?
Lithium batteries perform best at room temperature. Cold temperatures reduce effective capacity; heat ages the battery faster.
What is battery cycle life?
A cycle is one full discharge and recharge. Most lithium batteries last 500–1000 cycles before capacity degrades to 80%.
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