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Home›Electronics & Physics›Battery Capacity Calculator

Battery Capacity Calculator

Convert Between Ah, mAh, Wh and Calculate Capacity

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Convert Units Ah from Current & Time

Capacity Value

Unit

Voltage (V) needed for Wh conversion

Battery Capacity Formulas

Ah = mAh / 1000

Wh = Ah × V = (mAh × V) / 1000

Ah = I(A) × t(h) (from discharge)

C-rate: I = C × Ah (1C = capacity current)

Battery capacity is the fundamental specification for energy storage. It determines how long a device can run and how much energy is available. The standard unit is Ah (amp-hour) or mAh (milliamp-hour). For comparing batteries across different voltages, use Wh (watt-hour) which represents actual energy content.

⚠ Battery capacity is not constant! Peukert effect means higher discharge current = less total capacity. A battery rated 100Ah at 20-hour rate may only deliver 70Ah at 1-hour rate. Always check the discharge rate used for the rating.

Understanding Battery Capacity

Battery capacity specifies the total charge a battery can deliver under specified conditions. A 100Ah battery can theoretically deliver 100A for 1 hour, 10A for 10 hours, or 1A for 100 hours. In practice, capacity is rated at a specific discharge time (e.g., 20 hours for deep cycle, 5 hours for lithium). Higher discharge rates reduce effective capacity due to internal losses.

Common Capacities

Phone: 2000-5000mAh. Power bank: 5000-30000mAh. Laptop: 3000-10000mAh. Car: 40-100Ah. Solar: 100-300Ah. EV: 50000-100000mAh.

C-Rate Examples

100Ah at 1C = 100A (1h). 100Ah at 0.5C = 50A (2h). 100Ah at 2C = 200A (0.5h). High C-rate = high power, reduced capacity.

Energy Comparison

Use Wh to compare: AA NiMH=2.5Wh, phone battery=15Wh, laptop=50Wh, power bank=100Wh, car battery=600Wh, Tesla=60000Wh.

Series & Parallel

Series: voltage adds, Ah stays same. Parallel: Ah adds, voltage same. 2× 12V 100Ah in series = 24V 100Ah. In parallel = 12V 200Ah.

Teaching Example: Convert 5000mAh power bank at 3.7V.
Ah = 5000/1000 = 5 Ah. Wh = 5 × 3.7 = 18.5 Wh.
To charge a phone (3000mAh @ 3.7V = 11.1Wh): 18.5/11.1 = 1.67 charges (assuming 85% efficiency = 1.4 charges).
At 1A draw (5W): runtime = 18.5/5 = 3.7 hours.

Applications

Consumer Electronics Solar Storage EV Range Calc UPS Design Medical Devices

Frequently Asked Questions

mAh vs Ah?▼

1Ah = 1000mAh. mAh for small batteries (phones, power banks). Ah for large batteries (cars, solar). Same measure, different scale.

How to find battery capacity?▼

Measure discharge: constant current I over time t. Ah = I×t. Use battery analyzer. Check datasheet for rated capacity at standard discharge rate.

What is Peukert effect?▼

Higher discharge current = less total capacity. Formula: Cp = I^k × t where k>1. Lead-acid k≈1.3. Li-ion k≈1.05 (negligible).

What does C/20 mean?▼

C/20 means the battery is rated at 20-hour discharge. A 100Ah C/20 battery delivers 5A for 20 hours. Useful capacity at higher rates is less.

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