The most effective way to charge a car's battery is to use a car battery charger, or to simply run the car so the vehicle can charge the battery while it runs. It is also possible to use another car to charge the battery with the right cables.
Under normal conditions it is rarely necessary to charge a car battery, as the vehicle's alternator usually produces sufficient electricity to recharge the battery as the car runs. However, drivers who take frequent, short trips may drain the battery so quickly that the alternator cannot replenish it. It is often sufficient to simply drive or run the car's engine while it is parked if the battery is drained but the car can still start. Turning off the car's stereo system, headlights and other electrical equipment usually speeds up the recharging process.
Battery chargers are devices that use main household power to recharge car batteries. Using a battery charger is usually only necessary if the car's battery is completely drained or the alternator cannot charge the battery itself. Chargers attach to the car's battery terminals using standard battery cables and can take up to a full day to completely recharge the battery depending on the specific battery and charger. Turn off the charger at the power source before unhooking it from the battery. Doing so helps avoid a potential explosion due to gases produced from the battery during the charging process.