Battery Life Estimation: Why Actual Life Differs from Rated Capacity
4 min read · Last updated: 2026-05-08
Battery capacity unit: mAh
Battery capacity is commonly expressed in mAh (milliampere-hours). 1 mAh means the battery can supply 1 mA for 1 hour. A 2,000 mAh battery can theoretically power a 200 mA device for 10 hours.
Basic battery life formula
t (hours) = C (mAh) / I (mA)
Example: 3,000 mAh battery, 150 mA average current draw
t = 3000 / 150 = 20 hours
This is the ideal case. Real-world battery life is shorter due to several factors.
Factors that affect real-world battery life
| Factor | Effect | Notes |
|---|---|---|
| Discharge current | ↓ Higher current = shorter life | Peukert effect: effective capacity decreases at high discharge rates |
| Temperature | ↓ Extreme cold or heat reduces capacity | Li-ion drops sharply below 0°C |
| Charge-discharge cycles | ↓ Capacity fades with age | Li-ion retains ~80% capacity after 500–1000 cycles |
| Depth of discharge | ↓ Deep discharge accelerates aging | Draining to 0% frequently shortens lifespan |
| Internal resistance | ↓ Increases with aging | Causes voltage sag under load, reducing usable run time |
The Peukert effect
Most noticeable in lead-acid batteries, the Peukert effect means that the faster you discharge a battery, the less total energy you get out of it. Lithium-ion batteries have a smaller but still present Peukert effect. Discharging at 100 mA might give 20 hours, while discharging at 500 mA ends faster than the simple 6-hour calculation predicts.
Practical battery life estimate
Realistic life ≈ (C × efficiency factor) / I
Typical efficiency factors: Li-ion 0.85–0.95, lead-acid 0.70–0.80
Example: 3,000 mAh Li-ion, 150 mA, efficiency 0.9:
t ≈ (3000 × 0.9) / 150 = 18 hours
Key takeaways
- Basic formula: t = C / I
- High discharge current, extreme temperatures, and aging all reduce real-world life.
- Keeping Li-ion batteries in the 20–80% charge range significantly extends cycle life.