Battery
The battery starts the engine and runs the electrics while the engine is off. Once running, the vehicle charges its own battery through the DYNAMO (alternator).
The instrument that measures the battery's charging is the AMPERE METER.
If the fluid level in the battery falls, top it up with DISTILLED WATER - never ordinary water, which carries minerals that ruin the cells.
Keep the terminals clean and tight. Most "dead battery" faults are a loose or corroded terminal rather than a dead battery, and a battery that goes flat repeatedly is telling you the charging system is at fault.