Can an Alternator Drain a Battery?

Recharging a car battery won't help if the car has a faulty alternator.

An automotive alternator is a device that converts power from the engine into electricity, which is used to charge the car's battery. In some cases, however, the alternator can actually drain the battery, causing significant problems.

Insufficient Alternator Voltage

An alternator that's not providing enough power can allow a battery to run down. According to Prestolite electric, an alternator producing less than 13.8 volts of power at idle is not producing enough current to charge the battery fully and needs to be replaced.

Bad Diode

A malfunctioning diode in the alternator can create a parasitic drain on a car's battery. Diodes should allow current to pass in only one direction, but a bad diode will keep the charging circuit open even when the engine in not running, allowing the battery to go dead. This often happens overnight.

Wiring Problem

Even when the alternator is functional, if the wiring between it and the battery is damaged, it won't keep the battery charged. A broken ground wire on the back of the alternator, a loose battery cable, or a faulty or dirty cable could prevent the alternator from providing an adequate charge.

References

• Edmunds: A Shocking Expose -- Your Car's Battery
• Prestolite Electric: On Vehicle Alternator Troubleshooting
• Auto Repair Help From The Honest Mechanic: My Car Has a New Battery But Something Keeps Draining The Battery Down