Battery Life Calculator
Estimate battery runtime in hours from capacity in mAh and device draw in mA or watts.
Applies efficiency derating and shows runtime at different usage levels.
Battery life estimation predicts how long a battery will power a device before needing a recharge or replacement — a critical calculation for IoT devices, wearables, and portable electronics.
The core formula:
Battery Life (hours) = Battery Capacity (mAh) / Average Current Draw (mA)
For devices with multiple operating modes:
Average Current = (I_active × t_active + I_sleep × t_sleep) / (t_active + t_sleep)
Accounting for battery efficiency:
Realistic Battery Life = (Capacity × Efficiency Factor) / Average Current
Efficiency factor is typically 0.70–0.85 due to voltage sag, temperature, and age.
What each variable means:
- Capacity (mAh) — milliamp-hours; how much charge the battery stores (e.g., an iPhone has ~3,800 mAh)
- Average Current Draw (mA) — the average current consumed by the circuit over time
- I_active / I_sleep — current in active vs. low-power sleep states
- Efficiency Factor — real-world capacity is always less than rated due to temperature, discharge rate, and aging
Worked example: An IoT sensor with a 2,000 mAh battery. Active mode: 80 mA for 100 ms every 10 seconds. Sleep mode: 0.05 mA.
t_active fraction = 0.1 / 10 = 0.01 (1% of time active) Average current = (80 × 0.01) + (0.05 × 0.99) = 0.80 + 0.0495 = 0.85 mA Theoretical life = 2,000 / 0.85 = 2,353 hours = 98 days Realistic (80% efficiency) = 2,353 × 0.80 = 1,882 hours ≈ 78 days
Typical battery capacities:
- Coin cell CR2032: 220 mAh
- AA alkaline: 2,500 mAh
- 18650 Li-ion: 2,500–3,500 mAh
- Smartphone (large): 4,000–5,000 mAh
- Laptop: 50,000–100,000 mAh (50–100 Wh ÷ 3.7V)