Battery capacity and energy
Enter a nominal voltage, steady current draw, and required operating time.
Battery capacity, charge, and energy explained
Battery capacity can describe either electrical charge or stored energy. Amp-hours measure charge: a battery rated at 10 Ah can ideally supply one amp for ten hours, two amps for five hours, or another equivalent current-time combination. Milliamp-hours are the same quantity at a smaller scale, with 1 Ah equal to 1,000 mAh. Phones and small electronics commonly use mAh, while larger lead-acid and lithium packs are often labeled in Ah.
Watt-hours account for voltage and therefore describe energy more directly. Multiply amp-hours by nominal battery voltage to obtain watt-hours. A 12-volt, 10-Ah battery stores a nominal 120 Wh. Multiplying watt-hours by 3,600 converts the value to joules because one watt is one joule per second and an hour contains 3,600 seconds. This calculator performs all four conversions from a steady current, required runtime, and voltage.
The mathematical result is an ideal minimum. Real batteries do not deliver every rated amp-hour under every condition. Capacity changes with discharge rate, temperature, age, chemistry, and cutoff voltage. Lead-acid batteries show a particularly strong Peukert effect at high current, while lithium cells usually retain more of their rating but still lose usable energy through internal resistance and protection limits. Power converters, wiring, and regulators also consume energy. Designers commonly add a reserve margin after calculating the ideal capacity.
Voltage should be the battery pack's nominal voltage, not necessarily its fully charged terminal voltage. A lithium-ion cell commonly carries a 3.6- or 3.7-volt nominal rating even though it may reach 4.2 volts when full. Cells in series add voltage while retaining the same Ah capacity. Cells in parallel retain voltage while adding Ah capacity. Watt-hours add in both arrangements when identical cells are combined, which makes Wh useful for comparing packs with different voltages.
Current must represent the average load over the intended period. Devices with changing duty cycles can be modeled by calculating a weighted average: multiply each operating current by the fraction of time spent in that state and add the products. Startup surges may not greatly affect average energy but can still require a battery and wiring system capable of supplying the peak safely.
Use the result for early sizing, comparison, and energy budgeting. Then check a candidate battery's datasheet at the expected discharge rate and temperature, apply conversion efficiency, preserve the desired state-of-charge reserve, and account for aging. Batteries can deliver hazardous current and require suitable protection, charging, fusing, thermal design, and chemistry-specific handling.
Battery capacity calculator FAQ
What is the difference between mAh and Wh?
Milliamp-hours measure electrical charge, while watt-hours measure energy and include voltage. Multiply mAh by voltage and divide by 1,000 to estimate Wh.
Does a 10 Ah battery always run a 2 A load for five hours?
That is the ideal arithmetic result, but actual runtime is usually lower. Discharge rate, temperature, battery age, cutoff voltage, and conversion losses reduce usable capacity.
Should I use nominal or fully charged voltage?
Use nominal voltage when estimating the labeled energy capacity of a pack. Use a detailed voltage curve and load model when accurate end-to-end runtime is critical.
How much reserve capacity should I add?
The appropriate reserve depends on chemistry, reliability needs, temperature, and expected aging. Many early designs add 20 to 30 percent, then verify the choice against manufacturer discharge data.
How do series and parallel cells affect capacity?
Series cells add voltage while keeping the same Ah rating. Parallel cells add Ah while keeping voltage the same, and both arrangements add total Wh when identical cells are used.