Battery Runtime and Pack Calculator
Estimate how long a battery will run a load that is active some of the time and asleep the rest, and work out the voltage, capacity and energy of a pack built from several cells.
How it works
Runtime is the usable capacity divided by the average current: hours = capacity × usable fraction ÷ Iavg. The average current of a device that wakes up for a fraction D of the time is Iactive × D + Isleep × (1 − D), and a converter between the battery and the load raises the battery current by 1 ÷ efficiency. Usable capacity is less than the rating (typically 80 percent) because of the cut-off voltage, temperature and aging. For a pack, cells in series add voltage (S × Vcell) and cells in parallel add capacity (P × Ah): the energy is the product, and the C-rate of the load is the current divided by the capacity. See Batteries and Power Supplies.
Worked example
A board that draws 40 mA when active and 10 µA asleep, awake 1 percent of the time, averages 40 × 0.01 + 0.010 × 0.99 = 0.41 mA. From a 1 000 mAh cell with 80 percent usable that is 800 ÷ 0.41 = 1 952 hours (81 days), against 20 hours if it never slept. A 100 mA load behind a 90 percent efficient converter draws 111 mA from a 2 000 mAh cell, lasting 1 600 ÷ 111 = 14.4 hours. Three 3.6 V, 2.5 Ah cells in series, two such strings in parallel (3S2P) make an 10.8 V, 5 Ah, 54 Wh pack; a 2 A load is 0.4 C.
Things to know
Capacity falls with load current, with low temperature and with age: a cell rated at 2 000 mAh at a slow discharge may give much less at a high one. Use the datasheet curve for your current.
Primary (non-rechargeable) cells such as alkaline and lithium thionyl chloride have a capacity that depends strongly on the drain; the 0.8 factor is a starting point.
The calculator uses a constant battery voltage. A voltage that sags as the cell empties changes the current drawn by a regulated load (the converter draws more as the voltage falls).
Measure the real average current with a meter or a current-profiling tool: sleep currents are often higher than the datasheet says because of leakage and always-on parts.
Common questions
What is a C-rate?
The current relative to the capacity: 1 C is the current that empties the cell in one hour (2 Ah cell, 2 A), 0.5 C in two hours, 2 C in half an hour. Cells have maximum charge and discharge C-rates.
Why do cells in parallel add capacity but not voltage?
Their voltages are tied together, so the voltage stays the same, while each cell supplies part of the current.
How do I balance a series pack?
Use a battery management system (BMS) that monitors each series group, balances the cells and protects against overcharge, over-discharge and overcurrent.
Sources
- Cell manufacturers’ datasheets (Panasonic, Samsung SDI, LG, Energizer) for the discharge curves and rated currents.
- IEC 61960, Secondary lithium cells and batteries for portable applications; IEC 62133 (safety).
Last reviewed: October 22, 2026