Calculate battery capacity in mAh, Ah, and Wh, plus discharge energy, runtime, and C-rate for any battery pack.
| Battery Type | Voltage (V) | Typical Capacity Range |
|---|---|---|
| Li-ion (18650) | 3.6 – 3.7 | 1500 – 3500 mAh |
| LiPo (1S) | 3.7 | 500 – 5000 mAh |
| Lead-Acid (12V) | 12.0 | 1000 – 200000 mAh |
| NiMH (AA) | 1.2 | 600 – 3000 mAh |
| Alkaline (AA) | 1.5 | 1800 – 2800 mAh |
| LiFePO4 | 3.2 | 1000 – 6000 mAh |
Battery capacity tells you how much charge a battery can store and deliver, and it is the single most important number for predicting how long a device will run. The most reliable way to find the real capacity of a battery — as opposed to the optimistic figure printed on the label — is to discharge it at a known constant current and measure how long it takes to reach its cutoff voltage. This calculator turns that simple measurement into capacity (mAh and Ah), stored energy (Wh), and average voltage, and shows a representative discharge curve and test setup.
Capacity is the product of the discharge current and the discharge time. If you draw a steady current for a measured number of hours until the battery is empty, multiplying the two gives the charge the battery delivered. This is why capacity is expressed in amp-hours (Ah) or milliamp-hours (mAh) — it is literally current multiplied by time.
Capacity in mAh only describes charge, not the actual usable power, because it ignores voltage. Two batteries with the same mAh rating but different voltages store different amounts of energy. To compare batteries fairly, convert to watt-hours by multiplying the capacity in Ah by the average voltage. Watt-hours represent the true energy content and are the figure used for laptops, power banks, and electric vehicles.
As a battery discharges, its terminal voltage gradually falls. The discharge curve plotted above shows this behaviour: a relatively flat plateau through most of the discharge, followed by a sharp drop near the end. Testing must stop when the battery reaches its cutoff voltage — the minimum safe voltage — because discharging further can permanently damage the cell. For a typical Li-ion cell the cutoff is around 3.0V, while the full-charge voltage is about 4.2V.
Capacity (mAh): 500 mA × 4 h = 2000 mAh
Capacity (Ah): 2000 / 1000 = 2.000 Ah
Average voltage: 3.70 V (nominal Li-ion)
Energy: 3.70 V × 2.000 Ah = 7.40 Wh
Different chemistries have characteristic nominal voltages and capacity ranges. A Li-ion 18650 cell typically sits at 3.6–3.7V with 1500–3500 mAh, a single-cell LiPo at 3.7V can span 500–5000 mAh, a 12V lead-acid battery may store from 1 Ah to hundreds of Ah, and AA-sized NiMH cells usually hold 600–3000 mAh. The reference table above summarizes these typical values.
Select your battery type to load typical voltages, then enter the constant discharge current and the time it took to reach the cutoff voltage. The calculator instantly returns the capacity in mAh and Ah, the stored energy in watt-hours, and the average voltage, along with a discharge curve and a clear test-setup diagram. Adjust the values to model different test conditions and compare results.
Not always. Manufacturers usually rate capacity at a low, ideal discharge rate and fresh condition. Real measured capacity is often lower, especially at high currents, low temperatures, or after many charge cycles.
A C-rate expresses discharge current relative to capacity. 1C discharges the full capacity in one hour, 0.5C in two hours, and so on. Testing at a moderate C-rate gives the most representative capacity figure.
Watt-hours include voltage, so they represent the true energy a battery can deliver. This makes Wh the correct unit for comparing batteries of different voltages and for airline travel limits on lithium batteries.
Testing battery capacity by constant-current discharge is a simple, accurate way to know exactly how much energy a cell can deliver. By measuring current and time and applying Capacity = I × t, then converting to amp-hours and watt-hours, you can verify battery health, estimate runtime, and design reliable battery systems. Use the interactive calculator above to run the numbers for any battery and chemistry.