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Full changelogEngine 1.26.4

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
A
25 mA
s
A
6 µA
s
Capacity derating
Li-ion to 3.0 V ≈ 90–95 %, alkaline to 1.0 V ≈ 80 %, coin cell under pulses ≈ 50 %
%
100 % at +20 °C, ≈ 80 % at 0 °C, ≈ 60 % at −20 °C for Li-ion
%
Self-discharge and efficiency
%/month
%

Results

Runtime
241.2days
Average load current Iavg
214.3µA
Usable capacity
1700mAh
All results (3)
Duty cycle
0.833%
Drawn from cell (with η)
Energy per cycle

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 ⁨1.26.4⁩