The humble battery is essentially a huge power reserve, initially tasked with supplying necessary juice to the vehicle’s starting motor which cranks the engine to life with each push of the starter button. During this dance, the battery also shovels electricity to the car’s ignition system.
This helps to explain why a large displacement engine needs a bigger battery than your ride-on lawnmower – the size of rotating mass and electricity requirements are directly proportional. Small capacity batteries, ones which provide a smaller reservoir of electricity from which the car can draw, might be able to fire up a big engine in a pinch but will likely not do so reliably.
Despite doing all this work, the battery can’t exactly go on vacation after helping to start the engine. Working in concert with the electrical system is a device called an alternator. On the road, this part supplies electricity to a car’s various electric-powered accessories such as headlights and wipers. On occasion, the alternator cannot keep up with demand – consider a hot summer’s day in the middle of a family road trip when the air conditioning is running, the wipers are on, the stereo is playing, and all hands have their devices plugged into USB ports.
In extreme – but common – instances like these, the charging system is overworked and needs to call on the battery for an extra jolt of juice in order to keep everything running. If the vehicle has power-hoovering accessories added to it such as aftermarket lights and tools (an off-road truck would be a good example) the battery becomes an especially important part of keeping up with electrical demand.