A battery on a BMW 540i might need a charge for a variety of reasons. If the car has sat for a long time without being started, the battery will slowly discharge. Also, older batteries have trouble holding a charge. Extremely cold temperatures have a negative impact on batteries, and as the temperature drops, the battery will produce less power. Charging a battery is fairly simple, but the only way to safely do it is with a battery charger unit, available at most auto parts and retail stores.
Open the hood of the car and locate the battery on the right-hand side, under a plastic boot.
Lift up the plastic boot to expose the battery.
Place the battery charger on the ground, directly in front of the vehicle.
If the charger has a voltage switch, set it to 12 volts. If the device is only a 12-volt charger and does not have a switch, skip this step.
If the charger has a switch that sets the desired amperage (charge) rate, such as 6, 12 or 30 amps, set it to the desired rate: The higher the amp setting, the quicker the battery will charge. If the charger does not have an amperage setting, skip this step.
Connect the positive charger clip from the battery charger to the positive terminal on the battery.
Connect the negative charger clip from the battery charger to an unpainted part of the engine block or frame.
Plug the battery charger into the wall outlet.
Consult the battery charger's instruction manual to see the charging times for your particular charger.
Unplug the charger and then disconnect the charging clamps once the battery has finished charging for the amount of time specified in the manual.
Insert key into ignition and start the vehicle to check that the battery is properly charged.
