BioDeviceHub

Device Battery Life Calculator

Estimate a device's battery run time from capacity, voltage and average load, size a battery for a required run time, or total the energy a mains device uses.

Formula

E (Wh)=Q (Ah)×U×ηusable×ηconvE\,(\mathrm{Wh}) = Q\,(\mathrm{Ah})\times U\times\eta_{\text{usable}}\times\eta_{\text{conv}}
t (h)=E (Wh)Pavg (W)t\,(\mathrm{h}) = \dfrac{E\,(\mathrm{Wh})}{P_{\text{avg}}\,(\mathrm{W})}
Qneeded=P tU ηusable ηconvQ_{\text{needed}} = \dfrac{P\,t}{U\,\eta_{\text{usable}}\,\eta_{\text{conv}}}
QQ
rated battery capacity, in mAh or Ah
ηusable\eta_{\text{usable}}
the share of the capacity that can be used, below 100% with age, cold and cut-off
ηconv\eta_{\text{conv}}
of the regulator between the battery and the circuit
PavgP_{\text{avg}}
average power the device draws

How it works

A battery stores energy, which is its capacity times its voltage. Not all of it can be used: a battery is not discharged to zero, its capacity falls with age, cold and high drain, and a converter between the battery and the circuit wastes some of what it delivers. Multiplying by those shares gives the usable energy, and dividing by the average load gives the run time.

The load has to be the average, including standby, display and radio, not the peak. For a device that has to keep running, such as a monitor or a pump, a margin on top of the calculated time is needed, and the manufacturer's stated run time under defined conditions is the benchmark.

Worked example

A 2,000 mAh, 3.7 V battery; load 250 mW; usable share 80%; converter efficiency 90%.

  1. Nominal energy = 2 Ah × 3.7 V = 7.40 Wh.
  2. Usable = 7.40 × 0.8 × 0.9 = 5.33 Wh.
  3. Run time = 5.33 Wh / 0.25 W = 21.3 h.

The device runs for about 21 hours.

These are the values the calculator opens with, so you can check its output against this example.

Assumptions

  • A constant average load.
  • The usable share and the efficiency are realistic for the battery chemistry and conditions.

Common mistakes

  • Using the peak load instead of the average.
  • Forgetting that capacity falls with age, cold and high drain.
  • Mixing mAh and Wh, which differ by the voltage.

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Related equipment

Service documentation, failure modes and parts for the instruments this calculation is used with.