Battery Life Calculator
Answer
t (hours) = (capacity_mAh × derating) / load_mA. Capacity is usually printed on the cell; load is your average draw, not the peak. Derating accounts for cutoff voltage, self-discharge and temperature — use 0.8–0.9 for coin cells, closer to 1.0 for fresh Li-ion under light load.
Calculator
How it works
Firmware engineers usually know the average milliamp draw from a bench supply or a shunt log. Pack labels give milliamp-hours. Dividing the two gives a first-order run-time before you model radio duty cycles or temperature.
Enter capacity, load and an optional derating factor below 1. The C-rate line compares your load to a one-hour discharge. Treat anything above 1 C as a prompt to open the cell datasheet, not as a guarantee.
Parameters used on this page
| Parameter | Value | Source |
|---|---|---|
| Run-time | t = C / I | Constant-current approximation |
| C-rate | C-rate = I_load / C_1h | Compares load to 1-hour capacity |
| Derating | 0 < derating ≤ 1 | User estimate for unusable capacity |
Worked example
A 2000 mAh Li-ion at a steady 100 mA with derating 1.0 gives 20 h (about 0.83 days). The same pack at 500 mA is 4 h and 0.25 C — fine for many cells, but check the datasheet if you are near 1 C or above.
Pitfalls
- Using peak transmit current instead of the duty-cycled average. BLE bursts kill the simple formula if you do not average.
- Ignoring that the BMS may stop at 3.0 V while your circuit browns out earlier.
- Assuming mAh adds linearly when you parallel mismatched cells — use matched packs.
FAQ
How do I calculate battery life from mAh?
Divide capacity by average load in the same units — mAh divided by mA gives hours.
What is a safe C-rate?
Many consumer Li-ion cells are rated for 1 C continuous; the calculator warns above 1 C and fails above 2 C as a sanity flag.
Should I include sleep current?
Yes. Convert your duty cycle to an average — (I_active × t_active + I_sleep × t_sleep) / period.
Does this model Peukert for lead acid?
No. It is a linear estimate. For heavy loads on lead acid, expect less run-time than the formula shows.
Related
Sources
- Constant-current discharge approximation used in early power budgeting.