Question 1

How do I calculate battery life?

Accepted Answer

Divide the battery capacity by the load current and multiply by an efficiency factor: t (hours) = capacity (mAh) / load (mA) × η. For example, a 2000 mAh battery driving 200 mA gives 2000 / 200 = 10 hours at η = 1; with a realistic η = 0.85 you get about 8.5 hours.

Question 2

What is the efficiency or derating factor η?

Accepted Answer

η (0 to 1) is a safety factor that shrinks the ideal runtime to allow for losses the simple formula ignores: the battery's usable capacity above the cutoff voltage, converter inefficiency, temperature and self-discharge. A common starting point is 0.85; use a lower value for high-drain or cold conditions.

Question 3

How do I estimate runtime from power instead of current?

Accepted Answer

First convert capacity to energy: E (Wh) = capacity (mAh) × voltage (V) / 1000. Then divide by the power draw and apply efficiency: t (hours) = E / load (W) × η. A 3000 mAh, 3.7 V pack holds 11.1 Wh, so a 1 W load runs about 11.1 hours at η = 1.

Question 4

Why is real battery life shorter than the calculation?

Accepted Answer

The formula assumes a constant current draw and full usable capacity. In practice the discharge curve, temperature, cutoff voltage, peak/pulsed current and self-discharge all reduce the delivered energy. Treat the result as an optimistic estimate and tune η downward to match measurements.

Question 5

What efficiency value should I use?

Accepted Answer

η = 0.85 is a reasonable default for a healthy battery and a moderate, steady load. Drop toward 0.6–0.75 for high-drain devices, cold temperatures, old cells or switching converters with poor efficiency. Set η = 1 only to see the theoretical upper bound.

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