What is Battery Capacity?
Battery capacity is a measure of the total amount of electrical energy a battery can store and deliver. It tells you how much charge a battery holds and, combined with the voltage, how much energy (in watt-hours) the battery contains. Understanding battery capacity is essential for selecting the right battery for your device, estimating runtime, and comparing batteries across different chemistries and form factors.
What Are Amp Hours (Ah) and Milliamp Hours (mAh)?
Amp hours (Ah) and milliamp hours (mAh) are units of electric charge that describe how much current a battery can supply over time. A battery rated at 1 Ah can deliver 1 ampere of current for one hour, or 2 amperes for 30 minutes, and so on.
- 1 Ah = 1000 mAh — milliamp hours are commonly used for small batteries like those in phones, earbuds, and wearables.
- Ah is standard for larger batteries such as car, deep-cycle, and EV batteries.
The capacity does not change with voltage — a 100 Ah battery at 12 V and a 100 Ah battery at 24 V hold different amounts of energy, even though both are "100 Ah."
What Are Watt Hours (Wh) and Kilowatt Hours (kWh)?
Watt hours (Wh) measure the actual energy stored in a battery, combining both voltage and capacity. This is the most useful unit for comparing batteries because it accounts for voltage differences.
- 1 kWh = 1000 Wh — kilowatt hours are used for large battery packs like EV batteries and home energy storage.
- Wh is practical for medium-sized batteries like laptop batteries, power banks, and power tools.
Battery Capacity Formula
The relationship between energy, voltage, and capacity is described by a simple formula:
Where:
- E — Energy in watt-hours (Wh)
- V — Voltage in volts (V)
- Q — Capacity in amp-hours (Ah)
This formula works in both directions. If you know the voltage and capacity, you can find the energy. If you know the voltage and energy, you can find the capacity.
How to Calculate Battery Capacity (Amp Hours)
To find battery capacity in amp hours when you know the energy and voltage:
Example: A 48 V electric bike battery stores 576 Wh of energy.
Capacity = 576 Wh ÷ 48 V = 12 Ah
How to Calculate Battery Energy (Watt Hours)
To find energy in watt-hours when you know the voltage and amp-hour rating:
Example: A 12 V car battery rated at 75 Ah.
Energy = 12 V × 75 Ah = 900 Wh = 0.9 kWh
Understanding C-Rate
The C-rate is a measure of how fast a battery is charged or discharged relative to its capacity. A 1C rate means the battery is fully discharged in 1 hour. A 2C rate means it is discharged in 30 minutes. A 0.5C rate (C/2) means it takes 2 hours.
C-Rate Formulas
| Parameter | Formula | Description |
|---|---|---|
| Discharge Current | I = C-rate × Q (Ah) |
Current in amperes during discharge |
| Runtime (hours) | t = 1 / C-rate |
Time until fully discharged |
| Runtime (minutes) | t = (1 / C-rate) × 60 |
Same as above, in minutes |
Example: A 100 Ah battery discharged at 0.5C rate.
- Discharge current = 0.5 × 100 = 50 A
- Runtime = 1 / 0.5 = 2 hours
Unit System: Metric vs American
Battery specifications vary by region and application:
| System | Capacity Unit | Energy Unit | Typical Use |
|---|---|---|---|
| Metric | Ah (amp-hours) | kWh (kilowatt-hours) | Car batteries, EV batteries, solar storage, industrial |
| American | mAh (milliamp-hours) | Wh (watt-hours) | Smartphones, laptops, AA/AAA batteries, power banks |
Frequently Asked Questions
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How many mAh is 1 Ah?
1 Ah = 1000 mAh. To convert amp-hours to milliamp-hours, multiply by 1000. To convert milliamp-hours to amp-hours, divide by 1000. For example, a 5000 mAh phone battery is equal to 5 Ah.
-
How do I calculate battery runtime?
Battery runtime depends on how much current your device draws. The basic formula is:
Runtime (hours) = Capacity (Ah) / Current draw (A)For example, a 10 Ah battery powering a device that draws 2 A will last: 10 / 2 = 5 hours.
If you know the C-rate instead, use:
Runtime (hours) = 1 / C-rate. -
What is the difference between Wh and Ah?
Amp-hours (Ah) measure electric charge — how much current can flow over time. Watt-hours (Wh) measure energy — the actual work the battery can do. Wh = V × Ah, so the same Ah rating at a higher voltage stores more energy. Always use Wh for comparing batteries at different voltages.
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How many Wh is a typical car battery?
A standard 12 V car starter battery is typically 40–80 Ah, which equals 480–960 Wh (0.48–0.96 kWh). However, starter batteries are not designed for deep discharge — they deliver large bursts of current to start the engine. Deep-cycle batteries used for solar or marine applications can be discharged much more deeply.
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What C-rate is safe for lithium batteries?
Standard lithium-ion cells are typically rated for 0.5C to 1C continuous discharge. High-drain lithium cells (like those in power tools or EVs) may support 2C–10C. Exceeding the rated C-rate can cause overheating, reduced cycle life, and in extreme cases, thermal runaway. Always check the datasheet for your specific battery.
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How do I convert mAh to Wh?
To convert milliamp-hours to watt-hours:
Wh = (mAh × V) / 1000.For example, a 3000 mAh battery at 3.7 V: Wh = (3000 × 3.7) / 1000 = 11.1 Wh.
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What is the nominal voltage of common battery types?
- Alkaline AA/AAA: 1.5 V
- NiMH AA/AAA: 1.2 V
- Lithium-ion cell: 3.6–3.7 V
- LiPo cell: 3.7 V (nominal), 4.2 V (full)
- Lead-acid cell: 2.0 V (6 cells = 12 V battery)
- LiFePO4 cell: 3.2 V
- Car battery (lead-acid): 12 V
- E-bike battery: 36 V or 48 V
- EV battery pack: 400 V or 800 V