Power calculator
Battery life
What counts is the average current: a short burst, then a long sleep.
Free. It runs in your browser: nothing to install.
The example, worked out
Given
- Capacity
- 2400 mAh
- Nominal voltage
- 3 V
- Current awake
- 50 mA
- Time awake per cycle
- 1 s
- Current asleep
- 10 µA
- Cycle
- 60 s
- Usable share
- 80 %
- Self-discharge
- 1 %/month
Working
- the average over one cycle
- capacity over current is time
Results
The formulas
- Iavg = (Ion × ton + Ioff × (T − ton)) / Tthe average over one cycle
- life = C × usable / (Iavg + Iself)capacity over current is time
The usual traps
- Forgetting the self-discharge: a frugal device can lose more to the cell itself.
- Awake more than half the time: sleeping harder gains little.
Questions
How long will my battery last?
Iavg = (Ion × ton + Ioff × (T − ton)) / T: the average over one cycle. life = C × usable / (Iavg + Iself): capacity over current is time.
What does it give for typical values?
91.3 days on 2400 mAh at 877 µA average. Battery life: 91.3 days; Average current: 877 µA.
What are the usual mistakes?
Forgetting the self-discharge: a frugal device can lose more to the cell itself. Awake more than half the time: sleeping harder gains little.
In the app
Your circuit, drawn live
Change a value: the schematic redraws with your numbers on the parts.
The part you can buy
Results rounded to standard values (E12, E24, E96) and power ratings.
The working, shown
Every step written out. Try it first by hand, then check.
Into your notebook
One click puts the circuit on a sheet of your project.
Open the calculatorOpens in the app on 1 December, with Circuit Notebook 1.0
Also in Power
- Linear regulator heat What a 7805 or an LDO burns, its efficiency, how hot it gets.
- Transformer Turns ratio, secondary voltage, currents, the load seen from the primary.
- Buck converter Duty cycle, inductor, capacitor, the peak currents.
- Boost converter Duty cycle, inductor, capacitor, and the current the input must give.
- Flyback converter Turns ratio, duty, primary inductance and peak currents from the reflected voltage.
- Heatsink & junction temperature From the air to the junction through the thermal resistances; the heatsink you need.
- Zener regulator The series resistor, the Zener's heat, the current it wastes.
- Bulk capacitor The capacitor after a rectifier for a given ripple, or for a hold-up time.