Battery Life Calculator

About the battery life calculator

This tool estimates how long a battery will power a device by dividing the battery's usable capacity by the average current the device draws. Enter the capacity in milliamp-hours (mAh), the load in milliamps (mA), and pick a chemistry, and it returns runtime in hours and days, the usable capacity after derating, and the energy in watt-hours.

The headline number printed on a battery is its rated capacity, but you rarely get all of it. Real cells lose some capacity to internal resistance, voltage cutoffs, and the discharge rate. This calculator applies a chemistry-specific usable factor: Li-ion and LiPo deliver about 90 percent of rated capacity under moderate loads, NiMH around 85 percent, and alkaline closer to 75 percent because their voltage sags quickly under load. These are practical planning figures, not lab maximums, so they build a sensible margin into the estimate.

Two units cause most confusion. Milliamp-hours (mAh) measure charge and only compare fairly at the same voltage. Watt-hours (Wh = mAh x volts / 1000) measure energy and let you compare batteries of different voltages, which is why power banks and laptops are rated in Wh and why airlines cap carry-on batteries at 100 Wh. A 2000 mAh cell at 3.7 V holds 7.4 Wh; the same 2000 mAh at 1.5 V holds only 3 Wh.

How it works: the formula

Runtime is usable capacity divided by load. Usable capacity is the rated capacity scaled by the chemistry factor. Energy converts charge to watt-hours using the nominal cell voltage.

usable capacity (mAh) = rated capacity x usable factor
runtime (hours)       = usable capacity (mAh) / load (mA)
energy (Wh)           = rated capacity (mAh) x voltage (V) / 1000

usable factors used: Li-ion 0.90, LiPo 0.90, NiMH 0.85, alkaline 0.75

• Capacity and load must share the milliamp base. If your device draws 0.1 A, enter 100 mA. Mixing amps and milliamps throws the answer off by 1000x.
• The usable factor trims the rated capacity down to what you realistically extract before the device cuts off at its minimum voltage.
• Energy uses 3.7 V as the nominal lithium cell voltage in this tool. Multi-cell packs multiply that (a 3-cell pack is about 11.1 V).
• This is an average-load model. It does not simulate Peukert losses at high drain or capacity recovery during idle periods, both discussed below.

Worked example

Take the calculator's defaults: a 2000 mAh Li-ion cell powering a device that draws 100 mA on average.

1. Usable capacity: 2000 mAh x 0.90 = 1800 mAh.
2. Runtime: 1800 mAh / 100 mA = 18 hours.
3. Days: 18 / 24 = 0.75 days, so just under one full day per charge.
4. Energy: 2000 mAh x 3.7 V / 1000 = 7.4 Wh.
5. Margin: the 10 percent derate already shaved 2 hours off the naive 20-hour figure, which protects against optimistic planning.

Reference table: typical loads and runtimes

Estimated runtime from a 2000 mAh Li-ion cell (1800 mAh usable) at a few common average loads.

Common pitfalls

• Ignoring Peukert's law. At high discharge rates a battery delivers less than its rated capacity. A cell rated at a gentle 0.2C draw may give 20 to 40 percent less when pushed hard, especially alkaline and lead-acid. For heavy loads, lower the usable factor further.
• Forgetting standby and peak current. Most devices alternate between sleep and active bursts. Use a time-weighted average current, not the peak, or the estimate will be far too pessimistic.
• Confusing mAh with Wh. You cannot compare a 5000 mAh phone battery (3.7 V, 18.5 Wh) with a 5000 mAh power bank cell directly unless voltages match. Convert to watt-hours first.
• Assuming new-cell capacity forever. Lithium cells lose roughly 20 percent of capacity over a few hundred full cycles, and cold temperatures cut available capacity sharply. Plan around aged, cold-weather performance for critical uses.
• Skipping conversion efficiency. Boost and buck converters waste 5 to 15 percent. If your load runs through a regulator, real runtime is shorter than the raw capacity-over-load figure.

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Frequently asked questions