The car battery is a heavy, rechargeable lead-acid or absorbent glass mat (AGM) unit that supplies the massive electrical current needed by the starter motor to crank the engine. Once the engine is running, the alternator takes over to power the vehicle and recharge the battery. It also acts as a voltage stabilizer for the electrical system and powers accessories when the engine is off.
Grab the jump box, this car battery is completely flat and won't even click the starter.
A healthy resting car battery should measure about 12.6 volts. If it drops below 12.0 volts, it is deeply discharged and may suffer permanent sulfation.
Related terms
Starter motor
A high-torque electric motor that engages the engine flywheel to crank the engine until it runs under its own power.
Accessory battery
A secondary 12-volt battery used to power electronics rather than start the engine.
Jump box
A portable battery pack used to start a vehicle with a dead battery.