Battery Capacity Calculator

How to use this battery capacity calculator

What this calculator does

This calculator connects three standard battery quantities: Voltage, Battery capacity in ampere-hours, and Stored energy in watt-hours. It also relates C-rate, Discharge current, and Runtime to full capacity. The central identity is E = V × Q, where energy E is measured in watt-hours, voltage V in volts, and charge capacity Q in ampere-hours. The relationship is an ideal electrical conversion; it does not predict usable energy under every real load, because temperature, internal resistance, cutoff voltage, aging, and conversion losses can reduce delivered energy. The IEC definition of battery capacity under specified discharge conditions is useful context for interpreting a nameplate Ah figure.

When to use it

Use the calculator when you need to convert a battery label from Ah or mAh to Wh, compare packs that use different nominal voltages, estimate the current associated with a stated C-rate, or translate a C-rate into an idealized full-capacity time. It is also useful when checking whether a power-bank, drone, medical-device, or backup-battery specification is internally consistent.

How to calculate

1. The calculator opens with a ready-to-use demonstration: 12 V, 2.2 Ah, and 26.4 Wh, plus a 0.2 C example. The matching Excel workbook is available immediately.
2. For the main equation, edit any two of Voltage, Battery capacity, and Stored energy. The remaining field is marked Calculated and updates automatically. Use each adjacent unit selector to switch V/mV, Ah/mAh, or Wh/kWh; the number is converted so the physical quantity stays unchanged.
3. Open Other battery parameters when you want rate information. Edit one of C-rate, Discharge current, or Runtime to full capacity; the other two update from the current battery capacity. Current can be shown in A or mA, and runtime in h or min.
4. Read the fixed-unit Live results for a consistent comparison. Select Download Excel to export the current validated model. Select Reset to clear the demonstration and all data fields; after Reset, Excel export is disabled until a complete valid state is entered again.

Input guide

Voltage is a required positive decimal when it is one of the two active main inputs. Enter V or mV; 12 V is the demonstration value. Higher voltage increases Stored energy for the same Ah, or reduces the required Ah when Stored energy is held fixed. Do not confuse a device's USB/output voltage with a battery pack's nominal cell-stack voltage when converting a nameplate capacity. Voltage unit is the required V/mV selector and changes only the display scale, not the underlying voltage.

Battery capacity is charge capacity, entered as a positive decimal in Ah or mAh; 2.2 Ah is the demonstration. Higher Ah raises Stored energy at fixed voltage and raises Discharge current at a fixed C-rate. The common mistake is treating Ah and Wh as interchangeable without voltage. NIST's conversion table notes that 1 ampere-hour equals 3,600 coulombs. Battery capacity unit is the required Ah/mAh selector and converts between Ah and mAh.

Stored energy is a required positive decimal when used as a main input, in Wh or kWh; 26.4 Wh is the demonstration. Higher Wh requires proportionally more voltage or Ah depending on which other quantity is held fixed. Stored energy unit is the required Wh/kWh selector and converts Wh and kWh. C-rate is an optional positive rate such as 0.2 C; higher C-rate means higher current and a shorter idealized full-capacity time. The IEC describes charge rate in terms of rated capacity divided by a time base in its charge-rate terminology.

Discharge current is an optional positive current in A or mA; the demonstration result is 0.44 A. If you edit it, the calculator infers C-rate as I/Q and then runtime as 1/C. Discharge current unit is the required A/mA selector for that field and converts A and mA. Runtime to full capacity is an optional positive idealized time in hours or minutes; the demonstration result is 5 h. If you edit runtime, C-rate becomes 1/t and current follows from I = C × Q. Runtime unit is the required h/min selector for that field and converts h and min. Do not read this idealized time as a guaranteed device runtime under voltage cutoffs or varying loads.

Output guide

The header pills echo the current Voltage, Battery capacity, Stored energy, and C-rate for quick scanning. In Live results, Stored energy is the calculated or reconciled energy in Wh; Voltage is the canonical voltage in V; and Battery capacity is the canonical charge capacity in Ah. C-rate expresses the selected rate relative to full capacity, Discharge current is the corresponding current in A, and Runtime to full capacity is the reciprocal-C idealized time in hours. These are exact identities within this simplified model, not a recommendation about safe charging or discharging limits.

Worked example

With the startup values, E = 12 V × 2.2 Ah = 26.4 Wh. At 0.2 C, I = 0.2 × 2.2 Ah = 0.44 A. The idealized full-capacity time is t = 1 / 0.2 = 5 h, or 300 minutes. Those are the same values shown on first open and written into the startup Excel workbook.