Battery Capacity Calculator
Enter your battery voltage and capacity to convert between milliamp-hours (mAh), amp-hours (Ah), and watt-hours (Wh), then find how long it will run a device, how fast it charges, and what each charge costs. Switch battery chemistry to adjust for real-world efficiency and safe discharge depth. Results update as you type.
What is battery capacity and why does it matter?
Battery capacity is the amount of electrical charge a battery can store, usually expressed in milliamp-hours (mAh) or amp-hours (Ah). Combined with voltage, it gives energy in watt-hours (Wh), which is the figure that actually tells you how long a battery will run a device. A 5000 mAh battery at 3.7 V holds 18.5 Wh, while a 2000 mAh battery at 11.1 V holds 22.2 Wh. That is why voltage matters: the higher-voltage pack stores more energy despite having fewer milliamp-hours. This calculator lets you convert between all three units and immediately see the practical results.
How to calculate battery runtime
Runtime depends on usable energy, not total energy. Most chemistries should not be fully discharged: Li-ion and LiPo cells degrade quickly if drained below about 20% state of charge, while lead-acid batteries suffer sulfation below 50%. The formula is: runtime (h) = usable Wh / load power (W), where load power = current (A) x voltage (V). For example, a 5000 mAh Li-ion cell at 3.7 V with an 80% usable fraction supplies 14.8 Wh. A device drawing 500 mA (0.5 A) at 3.7 V consumes 1.85 W, giving a runtime of about 8 hours. This calculator applies the correct reserve for the chemistry you select.
C-rate: what it is and why it matters
C-rate (often written as "nC") describes how fast a battery charges or discharges relative to its capacity. A 1C rate discharges the full capacity in one hour; 2C does it in 30 minutes; 0.5C takes two hours. Most consumer Li-ion cells have a continuous discharge rating of 1-2C, while high-drain cells (power tools, RC vehicles) may support 10C or more. Exceeding the rated C-rate causes excessive heat, accelerates degradation, and in extreme cases can cause swelling or fire. This calculator shows your C-rate so you can verify it is within the manufacturer specification.
Charge time and battery chemistry
Charge time is not simply capacity divided by charger current, because no charger delivers 100% of its energy to the cell. Li-ion chargers are around 99% efficient in the constant-current phase, but the constant-voltage top-off phase slows and the overall cycle (from 0% to 100%) can take longer than the simple formula suggests. Lead-acid batteries are typically 80-85% efficient. NiMH and NiCd cells lose more energy as heat during charging, dropping to 75-80%. This calculator uses per-chemistry efficiency values and accounts for the starting state of charge, so the result is closer to real-world times than a bare capacity/current division.
Typical battery capacities and voltages by application
| Application | Typical capacity | Nominal voltage | Chemistry |
|---|---|---|---|
| Coin cell (CR2032) | 225 mAh | 3 V | Lithium |
| AA alkaline | 2500 mAh | 1.5 V | Alkaline |
| Smartphone | 3000-5000 mAh | 3.7 V | Li-ion |
| Laptop | 4000-8000 mAh | 10.8-14.4 V | Li-ion |
| Power bank (portable) | 10000-30000 mAh | 3.7 V | Li-ion |
| Electric bicycle | 10-20 Ah | 36-52 V | Li-ion |
| 12V car battery | 40-100 Ah | 12 V | Lead-acid |
| Electric car (EV) | 40-100 kWh (nominal) | 400-800 V | Li-ion / LFP |
Common nominal values. Actual capacity varies by manufacturer, temperature, and age.
Frequently asked questions
How do I convert mAh to Wh?
Multiply the milliamp-hour rating by the battery voltage and divide by 1000. For example, a 5000 mAh battery at 3.7 V stores 5000 x 3.7 / 1000 = 18.5 Wh. The calculator does this automatically when you enter capacity and voltage.
Why does usable capacity differ from rated capacity?
Rated capacity assumes a full 0-100% discharge, but doing that repeatedly shortens battery life significantly. Li-ion cells are usually only discharged to about 20% state of charge in practice, leaving 80% of the rated capacity available. Lead-acid batteries are typically limited to 50% depth of discharge. This calculator deducts the recommended reserve automatically based on chemistry.
What is a good C-rate for everyday use?
For most Li-ion cells, a discharge C-rate below 0.5C is very easy on the battery and maximises total cycle count. A rate of 1C is normal for phones and laptops. Rates above 2C generate significant heat and should only be used if the cell is explicitly rated for high-drain applications. Staying below the rated C-rate is one of the best ways to extend battery life.
Does a higher mAh rating always mean longer battery life?
Only if the voltage is the same. A 10000 mAh pack at 3.7 V and a 5000 mAh pack at 7.4 V store the same 37 Wh. For comparing batteries in different devices or packs, always convert to watt-hours first.
How much does it cost to charge a battery?
Divide the battery energy in watt-hours by 1000 to get kilowatt-hours, then multiply by your electricity rate and divide by the charger efficiency. For a 5000 mAh Li-ion cell at 3.7 V and a rate of $0.16/kWh: 18.5 Wh / 1000 / 0.99 x 0.16 = about $0.003 per charge, or roughly $1 for every 300 charges.