Calculate battery capacity in mAh, Ah, and Wh based on current draw and runtime. Essential for battery selection, power budgeting, and energy storage planning.
2026-03-28T00:00:00Z
Battery voltage in volts
Current consumption in amps
Desired runtime in hours
4,000
mAh (milliamp-hours)
14.8
Wh (watt-hours)
💡 What this means: You need a battery rated for at least 4,000 mAh (or 4 Ah) at 3.7V to power a device drawing 0.5A for 8 hours. The total energy stored is 14.8 Wh.
Battery capacity is the amount of electric charge a battery can deliver at a specified voltage. It's commonly measured in amp-hours (Ah) or milliamp-hours (mAh) for smaller batteries, and watt-hours (Wh) when considering the voltage. These measurements tell you how long a battery can power a device at a given current draw.
Think of battery capacity like a fuel tank: mAh/Ah is the volume of fuel, while Wh is the total energy available (accounting for voltage). A 4000 mAh battery at 3.7V stores more energy (14.8 Wh) than a 4000 mAh battery at 1.5V (6 Wh), even though they have the same mAh rating.
Understanding battery capacity is essential for selecting the right battery for your application, estimating runtime, comparing different batteries, and planning power budgets for portable electronics, electric vehicles, solar systems, and backup power solutions.
Calculate the battery capacity needed to power a device drawing 0.5A at 3.7V for 8 hours:
mAh (milliamp-hours) measures electric charge, while Wh (watt-hours) measures energy. Wh = mAh × voltage ÷ 1000, so it accounts for voltage. A 4000 mAh battery at 3.7V (14.8 Wh) stores more energy than a 4000 mAh battery at 1.5V (6 Wh).
Theoretical runtime assumes constant current draw and 100% efficiency. Real-world factors reduce runtime: discharge rate efficiency (higher current = less capacity), temperature effects, battery age/condition, voltage drop under load, and self-discharge.
Yes, but use Wh instead of mAh. A 2000 mAh 7.4V battery (14.8 Wh) stores the same energy as a 4000 mAh 3.7V battery (14.8 Wh). When comparing batteries, Wh is the true measure of energy capacity.
C-rating indicates maximum safe discharge rate. 1C means you can discharge the entire capacity in 1 hour. A 4000 mAh battery at 2C can safely provide 8A (2 × 4A). Higher discharge rates typically reduce effective capacity.
Runtime (hours) = Battery Capacity (Ah) ÷ Current Draw (A). For example: 4 Ah battery ÷ 0.5A = 8 hours. Add 20-30% safety margin for real-world conditions and to avoid deep discharge which damages batteries.
Voltage determines the energy content. Two batteries with the same mAh but different voltages store different amounts of energy. Device voltage must match battery voltage (accounting for regulation/conversion), or you need a voltage converter.
Different chemistries have different characteristics: Li-ion (3.7V nominal, high energy density), Li-Po (3.7V, flexible shapes), NiMH (1.2V, affordable), Lead-acid (2V/cell, cheap but heavy). Chemistry affects voltage, weight, cost, and lifespan.
No! Li-ion/Li-Po batteries last longer with partial discharge cycles (20-80%). Deep discharge damages them and reduces lifespan. NiMH benefits from occasional full discharge to prevent memory effect. Lead-acid should never be deeply discharged.
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