Recipe

Estimate a battery's life, best case

Use when: a circuit runs from a battery labelled in mAh and you know, or can measure, its average current

Goal: an upper bound on running time in hours, and the battery's energy in Wh

Taught in Power, energy, and heat · a later section extends this recipe with more cases

1. 1

   Use a measured current where you can; otherwise add up the datasheet currents of everything that is on.

   Example

   An Uno R4 Minima running Blink: about 33 mA typical (Arduino datasheet), plus an LED at 6 mA, gives about 39 mA.

   Why: the battery only cares about the total charge drawn per hour, which is the average current.

2. 2

   t = \text{mAh} / \text{mA}, in hours.

   Example

   A 2000 mAh pack: 2000 / 39 \approx 51 hours.

   Why: mAh is milliamps times hours, so dividing by milliamps leaves hours.

3. 3

   \text{Wh} = \text{mAh} \times V / 1000.

   Example

   2000 mAh at 5 V is 10 Wh; 2000 mAh at 3.7 V is 7.4 Wh.

   Why: only energy, not charge, compares batteries of different voltages fairly.

4. 4

   Say "at most about 51 hours", never "51 hours".

   • the board runs from a converter between the battery and the board — expect noticeably less, since the converter wastes some energy.
   • the battery is cold, old or a cheap unbranded cell — expect less again.

   Why: the division assumes every milliamp-hour on the label reaches the circuit, which never happens.