Batteries, rails & power budgets

A battery stores energy chemically and releases it as DC. Chemistry sets its personality: nominal voltage (a lithium-ion cell is about 3.7 V, an alkaline cell 1.5 V), whether it recharges, and how the voltage sags as it drains. A li-ion cell runs from roughly 4.2 V full down to 3.0 V empty. Capacity in milliamp-hours says how long it lasts; a 2000 mAh cell sustains about 100 mA for 20 hours.

On the board, power is organized into rails, where one shared supply voltage feeds a group of parts. A design might take 5 V from USB, regulate 3.3 V for the microcontroller and sensors, and boost 12 V for a motor. Each rail is generated once and distributed, and each has its own requirements for stability and cleanliness.

A power budget ties it together. List every part, its rail, and its worst-case current draw, then add up each rail. The totals size the regulators, predict battery life, and expose surprises, like a display backlight that dwarfs the rest of the design, while they’re still cheap to fix. Skip this step and you find out later why the regulator runs hot at its limit.