Battery Life Calculator - Runtime from Capacity & Current Draw

How to use this tool

Enter rated battery capacity, average current, regulator efficiency, and usable capacity. The 80% usable default leaves a reserve; efficiency models converter losses.
Optionally model active and sleep phases or enter nominal voltage for energy and C-rate estimates. The runtime appears in hours, minutes, and days.

Battery life formula

Runtime (h) = Rated capacity × Usable capacity fraction × Regulator efficiency ÷ Average current

Usable capacity and regulator efficiency describe different limits. Usable capacity is the fraction of the battery rating that you plan to discharge; for example, 80% leaves a reserve of 20%. Regulator efficiency accounts for energy lost while a converter or regulator supplies the device. The default 80% usable capacity and 85% efficiency therefore leave 1,360 mAh effective from a 2,000 mAh battery. If your capacity already includes a derating, set usable capacity to 100% to avoid applying it twice.

When sleep/wake modeling is enabled, average current is the time-weighted load: I_avg = (I_active × t_active + I_sleep × t_sleep) ÷ cycle period. Sleep current is entered in microamps and converted to milliamps. For example, a device drawing 100 mA for one minute and 100 µA for the remaining nine minutes averages about 10.09 mA. The same average current is used when sizing a battery for a target runtime.

Nominal voltage is optional and only adds context: rated energy in watt-hours is mAh ÷ 1,000 × nominal volts, while C-rate is the current used divided by rated capacity. These are estimates based on the pack's nominal voltage and rated capacity; voltage does not change the runtime calculation. Actual capacity can fall at high discharge rates and can also vary with temperature, battery age, chemistry, and test conditions.

Typical current draw reference

Arduino Uno (active): ~50 mA
ESP32 (Wi-Fi active): ~150–240 mA
Raspberry Pi Zero 2: ~300 mA
LED (5 mm, standard): ~20 mA
Small servo: ~100–300 mA

Battery chemistry comparison

Chemistry	Nominal voltage	Typical cycle life	Common uses
Li-ion (Lithium-ion)	3.6–3.7 V	~300–500 cycles	Smartphones, laptops, power banks
LiPo (Lithium polymer)	3.7 V	~300–500 cycles	Drones, RC vehicles, wearables (flexible form factor)
NiMH (Nickel-metal hydride)	1.2 V	~500–1000 cycles	AA/AAA rechargeable batteries, older electronics
Alkaline	1.5 V	Non-rechargeable	Remote controls, flashlights, low-drain devices
LiFePO4 (Lithium iron phosphate)	3.2 V	~2000+ cycles	Solar storage, e-bikes, high-safety applications

Self-discharge rates

Batteries lose charge even when not in use. Choosing the right chemistry for standby applications is important:

Li-ion / LiPo: ~2–3% per month - excellent for devices used weekly.
NiMH: ~10–20% per month - needs recharging before use after storage.
Alkaline: ~2% per year - long shelf life; suitable for emergency devices.
LiFePO4: ~1–3% per month - very low self-discharge; good for infrequent use.

mAh vs mWh: which matters more?

Capacity in mAh (milliamp-hours) is printed on most batteries and power banks, but it only makes sense when comparing batteries at the same voltage. A 3000 mAh Li-ion cell at 3.7 V holds 11.1 Wh of energy. An AA alkaline at 1.5 V with 2500 mAh holds only 3.75 Wh - less than a third as much energy, even though the mAh figures look similar.

mWh = mAh × Voltage - use watt-hours (Wh) to compare batteries of different chemistries or voltages fairly.

Frequently asked questions

Why does battery life differ from the calculated value?

This estimate uses a rated capacity, usable-capacity fraction, regulator efficiency, and average load current. Actual life can differ with temperature, battery age, chemistry, discharge rate, and changing loads; the efficiency field only represents converter or regulator losses.