Battery Life Calculator

mAh (milliamp-hours) measures how much charge a battery holds — it tells you how many milliamps it can supply for one hour. Wh (watt-hours) is a more complete energy measure because it factors in voltage too: Wh = mAh × Voltage ÷ 1000. A 3000 mAh battery at 3.7V holds 11.1 Wh. When comparing batteries across different voltages, always use Wh.

Real-world battery life is typically 60–80% of the theoretical maximum. Several factors reduce efficiency: battery degradation over charge cycles (capacity drops ~20% after 500 cycles), power conversion losses in voltage regulators (typically 10–20%), background processes drawing extra current, temperature effects (cold weather can reduce capacity by 30–40%), and manufacturers rating capacity at ideal lab conditions.

Most lithium-ion batteries are rated for 300–500 full charge cycles before dropping to 80% of original capacity. Lithium polymer (LiPo) batteries used in phones and laptops typically handle 400–600 cycles. To maximise cycle life: avoid charging to 100% or discharging below 20%, keep the battery at room temperature, and use slow charging when possible.

The efficiency factor (default 85%) accounts for real-world losses — heat from voltage regulation, battery self-discharge, protection circuit overhead, and minor current spikes during operation. A brand-new, high-quality battery running at room temperature might achieve 90% efficiency. An old battery in cold conditions might only reach 60%. Adjust this slider based on your real-world experience with your specific battery.

The easiest method is a USB power meter (also called a USB tester or power bank tester) — plug it between your charger and device and it shows real-time current draw. Alternatively, check your device's official specs sheet for 'typical current consumption' or 'operating current'. For electronics projects, use a multimeter in series with the battery. Manufacturer specs often list average draw — peak draw can be 2–3× higher.