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Full changelogEngine 1.18.3

Battery Runtime & Sizing

Power & Regulators

Runtime of a duty-cycled load, with capacity derated for cut-off and temperature, and self-discharge counted as drain.

Inputs

mAh
V
Li-ion to 3.0 V ≈ 90–95 %; alkaline to 1.0 V ≈ 80 %; coin cell under pulse load can be 50 %
%
100 % at +20 °C, ≈ 80 % at 0 °C, ≈ 60 % at −20 °C for Li-ion
%
A
25 mA
s
A
6 µA
s
%/month
%

Results

Runtime

Not judged: the runtime you need is a product requirement. This figure is only as good as the usable fraction, derating and sleep current entered.
241.2days
Average load current Iavg
214.3µA
Usable capacity

6.29 Wh of 2000 mAh rated
1700mAh
All results (3)
Duty cycle
0.833%
Drawn from cell (with η)

incl. 55.56 µA self-discharge
293.6µA
Energy per cycle

one wake + sleep interval
47.57mJ

Active share of the drain: 97 %; sleep share: 3 %. Reduce the larger one first.

C-rate at the active current: 0.01 C. Rated capacity is usually quoted at 0.2 C; higher rates give less.

Accuracy

Verified against

1 independent case. See the working.

This is a design aid. The engineer remains responsible for the design and for checking the standard itself.

Parameter sweep

Vary one input over a range and see the answer and verdict at each step, as a table and a curve.Pro

Worst-case corners

Put a tolerance on each input and get the worst-case band around the answer.Pro

Waveform

The principle

Runtime is usable charge divided by average current. The hard part is the usable charge: the rated mAh is measured under laboratory conditions your product will not see.

Duty-cycled average current
Iavg=Iact D+Isleep(1−D),D=tactTcycleI_{avg} = I_{act}\,D + I_{sleep}(1-D), \qquad D = \frac{t_{act}}{T_{cycle}}
Runtime
t=CusableIavg/η+Isdt = \frac{C_{usable}}{I_{avg}/\eta + I_{sd}}
Usable capacity
Cusable=Crated⋅kcutoff⋅ktempC_{usable} = C_{rated} \cdot k_{cutoff} \cdot k_{temp}
Energy
E=CAh Vnom[Wh]E = C_{Ah}\, V_{nom} \quad \t{[Wh]}
C-rate
C-rate=ICAhC\t{-rate} = \frac{I}{C_{Ah}}
1 C discharges the rated capacity in one hour.
  • CratedC_{rated}label capacity, measured at 0.2 C and +20 °C
  • kcutoffk_{cutoff}fraction still available above your system's minimum voltage
  • ktempk_{temp}temperature derating factor
  • IsdI_{sd}self-discharge expressed as an equivalent constant current
  • η\etaefficiency of the regulator between cell and load
More detail

Where runtime is lost

  • Cut-off voltage. The system stops at its brown-out threshold, not the cell's rated end voltage. A boost converter that runs down to 0.9 V gets far more from an alkaline cell than a 1.8 V LDO does.
  • Pulse current on coin cells. A CR2032 has 10–20 Ω of internal resistance, rising as it discharges. A 15 mA burst drops the terminal voltage by 200–300 mV.
  • Temperature. Li-ion loses about 20 % at 0 °C and 40 % at −20 °C. Charging below 0 °C damages the cell.
  • Ageing. Design to end-of-life capacity, typically 80 % after the rated cycle count.
  • Small leakages. Protection FET, fuel gauge, pull-ups and the battery-monitor divider all count on a microamp budget.
Measure. Record a full wake-sleep cycle with a wide-range current probe or a source-measure unit. Where it disagrees with this estimate, trust the measurement.
Related: System Power Budget for the load side; LDO Dissipation & Limits for the quiescent term.

Engine version ⁨1.18.3⁩