Battery life calculator
Runtime of a duty-cycled load from active and sleep current, wake interval, self-discharge and the capacity you can actually use, with the remaining charge plotted over time.
- Battery life, years
- 1.82 years
- Average load current
- 12.99 µA
- Average battery current
- 13.26 µA Load, regulator and self-discharge
- Self-discharge, as a current
- 267 nA
- Usable capacity
- 211.5 mAh
- Active duty cycle
- 0.2 %
How this is calculated
I_load = ( I_active · t_active + I_sleep · (T − t_active) ) / T I_bat = I_load + I_Q LDO I_bat = I_load · V_load / (η · V_bat) + I_Q switching I_sd = C_rated · (self-discharge %/month) / 730.5 h C_use = C_rated · (1 − end-of-life %) · (1 − temperature %) Life = C_use / ( I_bat + I_sd )
- Model
- Constant average current drawn from a fixed charge, as in AN1416 Equation 4. Self-discharge is treated as an equivalent constant current on the rated capacity.
- Derating
- Both deratings are your inputs. Read end-of-life loss off the cell discharge curve at your minimum operating voltage, and temperature loss off the datasheet at your coldest condition. No chemistry curve is assumed.
- Rated capacity
- Capacity is specified at one load and cutoff. The Energizer CR2032 figure of 235 mAh is to 2.0 V on 15 kΩ at 21 °C. Higher pulse loads and lower cutoffs change it; check the datasheet curves.
- Month
- 730.5 h, one twelfth of a 365.25 day year.
- Microchip AN1416, Low-Power Design GuideDS01416A, 2011, Figure 4 and Equation 4
- Energizer CR2032 product datasheetForm No. 2032NA0618
- TI SLVA118A, Linear Regulator Design Guide for LDOsRevised June 2008, Equations 13 and 16
Questions
Energizer rates its CR2032 at 235 mAh to 2.0 V on a 15 kΩ load, about 0.19 mA, which it gives as 1245 hours. At a 13 µA average, the default here, it lasts about 665 days once 10 % is set aside for end of life and 1 % per year of self-discharge is counted.
Multiply each state's current by its time, add them, and divide by the cycle length. 5 mA for 20 ms every 10 s with 3 µA asleep averages 13 µA, of which sleep is 3 µA, about a quarter. Over years asleep that quarter is real capacity, so measure it.
At low average currents it does. 1 % per year on 235 mAh is an equivalent drain of about 0.27 µA. Microchip AN1416 shows a CR2032 sensor lasting 8.3 years with it included.
Yes. An LDO passes the load current straight through and adds its quiescent current, so a 1 µA part adds 1 µA to every hour of sleep. A switcher scales the load by the voltage ratio and its efficiency at that load, which is often poor at microamps.