Estimate how long a battery will last under a given current draw.
Battery runtime is simply capacity divided by draw: Hours = mAh ÷ mA. A 3000mAh battery powering a device that draws 200mA continuously should theoretically last 15 hours.
Real-world runtime is usually somewhat shorter than this ideal number, since battery efficiency drops as it discharges and most devices don't draw a perfectly constant current throughout use.
This same math applies whether you're estimating a phone battery, a power bank, or a small electronics project — the relationship is universal to any device with a milliamp-hour-rated battery.
A 3,000mAh battery under a steady 200mA draw gives an estimated 15 hours of runtime.
Current draw usually isn't constant — screen brightness, radio use, and background processes on a phone, for example, all vary draw throughout use, and batteries also lose some capacity as they age.
mAh is capacity at a specific voltage; Wh (watt-hours) is a voltage-independent energy measure. To compare batteries at different voltages fairly, convert to Wh: Wh = mAh × V ÷ 1000.
Yes, significantly for lithium batteries — cold temperatures in particular can noticeably reduce usable capacity compared to room-temperature performance.