Battery Life Calculator

Battery life estimation answers a question every portable-device designer and camper asks: how long will this charge last? Capacity printed on cells and packs — milliamp-hours (mAh) or amp-hours (Ah) — represents the total charge available under ideal discharge conditions. Dividing capacity by average load current yields runtime in hours, assuming voltage stays usable throughout the discharge curve.

The core formula is:

Runtime (hours) = Capacity (mAh) / Load current (mA)

Where Capacity is the battery's rated charge in milliamp-hours and Load current is the total average draw in milliamps. Convert Ah to mAh by multiplying by 1,000. For multiple parallel loads, sum their individual currents before dividing.

Real-world runtime falls short of the ideal number because of Peukert effects at high discharge rates, voltage sag near end-of-charge, temperature, and age-related capacity fade. Lithium-ion cells often deliver 80–90% of rated capacity at moderate C-rates; alkaline AA cells drop faster under heavy load. Treat calculator output as a planning estimate, then derate 15–30% for safety margin in product specs.

Worked example: A 3,000 mAh 18650 Li-ion cell powers an ESP32 module drawing 80 mA average during Wi-Fi transmit bursts and an OLED display at 20 mA. Total load = 100 mA. Runtime = 3000/100 = 30 hours. For a product claiming "up to 24 hours," that margin covers temperature and aging. A power bank labeled 10,000 mAh at 5 V actually stores 10,000 × 5 = 50,000 mWh; phone charging at 9 V/2 A (18 W) pulls more from the internal 3.7 V cells after conversion losses — expect roughly 60–70% end-to-end efficiency.

Enter each load component separately to see how subsystem changes affect battery life. Pair with the power calculator to convert known voltage and resistance into current when datasheets omit mA figures.