Battery Life Pro

This is the more detailed battery-life calculator. It handles projects that wake up, work, and sleep, and it accounts for the voltage regulator, which changes how much current the battery has to supply.

The theory

Most battery projects alternate between a high current when working and a tiny current when asleep. The average load current is the weighted mix:

average = (active current × active time + sleep current × sleep time) ÷ (active time + sleep time)

The battery does not always supply that same current. It depends on the regulator:

• No regulator: the battery current equals the load current.
• Linear regulator: the battery current equals the load current plus the regulator's own quiescent current. The voltage drop does not reduce it, it just becomes heat.
• Switching regulator: it converts power efficiently, so battery current = load current × Vout ÷ (Vbat × efficiency). A buck converter (lowering the voltage) reduces the battery current because the same power is delivered at a higher voltage.

Run time is then capacity × usable fraction ÷ battery current.

Step by step

1. Open the Battery Life Pro tab.
2. In Battery capacity (mAh), type the label value, for example 2500, and in Usable capacity (%) leave 80.
3. In Active current (mA) and Active time per cycle (s), type what the project does while awake, for example 120 and 5.
4. In Sleep current (µA) and Sleep time per cycle (s), type the sleeping figures, for example 20 and 55.
5. In Regulator, choose None, Linear, or Switching.
6. For a switching regulator, fill in Battery voltage (V), Rail voltage (V), and Switching efficiency (%), for example 3.7, 3.3, and 90.
7. Optionally enter the Regulator quiescent current (µA).
8. Read Average current of the load, Average current from the battery, and Run time.

Worked example

A 2500 mAh battery, 120 mA for 5 s then 20 µA for 55 s, with a 90 % efficient 3.7 V to 3.3 V converter. The average load is 10.02 mA, the battery supplies 9.93 mA, and the run time is about 201 hours (8.4 days).

Things to watch

• Self-discharge, temperature, and radio current spikes are not modeled.
• Regulator efficiency drops at very light loads, so a sleeping project can waste more than the calculation shows. Choose regulators designed for low quiescent current.
• Plan with about 70 to 80 percent of the rated capacity.