Nominal voltage, capacity, and energy accounting
Battery packs are usually specified by nominal voltage, amp-hours, and watt-hours. Series cells raise voltage (Ns × Vcell) while parallel cells raise capacity (Np × Ahcell). Energy follows Wh = Vpack × Ahpack, letting you compare different chemistries on the same basis. For example, a 13s4p pack using 3.7 V, 2.6 Ah cells is about 48.1 V and 10.4 Ah, which is roughly 500 Wh of nominal energy.
Derating factors that protect real-world performance
Rated energy is not the same as usable energy. Conversion losses, conservative depth-of-discharge, and reserve margin reduce what the load can actually draw. This calculator applies Whusable = Whpack × η × DoD × (1 − Reserve). With 92% efficiency, 80% DoD, and a 10% reserve, a 500 Wh pack becomes about 331 Wh usable. These factors also help match expected aging, temperature drop, and measurement uncertainty. Designers commonly use lower DoD for cycle life, plus a reserve to cover aging, cold, and uncertainty.
Runtime planning from load power and duty cycle
Runtime estimates convert energy into time: Runtime ≈ Whusable ÷ LoadW. If the load averages 250 W and usable energy is 331 Wh, expected runtime is about 1.32 hours. For a 2-hour requirement at the same load, usable energy must be about 500 Wh. When loads are pulsed, use an average power over the mission profile, and validate peaks separately using current and BMS limits.
Current capability, power headroom, and heating
High-power designs must consider current limits and thermal rise. If a cell supports 10 A continuous, a 4p group supports about 40 A, and a 48 V class pack could deliver roughly 1.9 kW nominally. Real limits depend on cell internal resistance, wiring gauge, connector ratings, and BMS continuous and surge current. Leave margin for ambient heat, enclosure insulation, and airflow, and avoid operating near maximum ratings for long periods.
Configuration checks for target voltage and growth margin
Target system voltage is often a range, so series count is usually rounded to the nearest practical value. Use the suggestion to pick Ns close to the goal, then increase Np until the nominal Wh meets the derated requirement. For expansion planning, add reserve for future capacity fade and consider parallel module architecture for serviceability. Always pair calculations with cell matching, balancing strategy, and appropriate protection for short circuit, overcharge, and overtemperature.