No—not by itself. A conventional car’s 12-volt battery is replenished by the vehicle’s charging system while the engine runs. The alternator uses mechanical energy from the engine to produce electrical energy. That energy ultimately comes from fuel, so the battery is not creating free energy or powering a self-sustaining loop.
Does a car battery charge while the car is parked?
When the engine is off, the alternator is not being driven by the engine and is not replenishing the battery. Electrical equipment left operating can use stored energy instead. If a parked vehicle repeatedly has a flat battery, the battery, vehicle, and connected equipment may need diagnosis; do not assume it will recover simply by waiting.
Can an alternator recharge a drained battery?
The alternator charges the vehicle’s 12-volt system while the engine runs, but driving is not a dependable way to recover a deeply discharged or faulty battery. A battery that will not start the vehicle or repeatedly loses charge may need a compatible external charge and a battery and charging-system test. Follow the vehicle and battery maker’s instructions.
What about hybrid and electric vehicles?
Some hybrids and electric vehicles use regenerative braking. During braking, the vehicle can recover part of its kinetic energy as electrical energy and store it in a battery. This recovers some energy that would otherwise be lost mainly as heat, but it does not make driving energy-free: the vehicle still needs an energy source to move. Conventional hybrids use gasoline, while plug-in hybrids and battery-electric vehicles can also take energy from an external electrical supply.
Is regenerative braking the same as self-charging?
“Self-charging” is often used informally for a hybrid that can recover energy during driving without being plugged in. It does not mean the vehicle creates energy from nothing. Regenerative braking returns some energy from motion; it cannot replace all the energy used to move the vehicle or keep its battery charged indefinitely.
Related: Find out whether a car battery charges while idling, how long a battery lasts without an alternator, and how to tell whether the battery or alternator is at fault.
Sources: U.S. DOE AFDC: Electric Vehicles; U.S. DOE Clean Cities: Texas River Cities Plug-In Electric Vehicle Initiative Regional Plan; Ford: Battery Service and Testing Guidance
