Battery warning light on: alternator and charging

The battery warning light (battery or amp symbol) indicates a problem with the charging system: usually the alternator is no longer charging (or not charging enough). Driving with this light on can leave you with a dead battery within just a few miles.

Common causes

Alternator failure: voltage regulator, brushes, diode or bearing. Serpentine belt slipping, worn or broken: the alternator pulley stops turning. Wiring or connectors corroded or damaged. Battery at the end of its life and no longer holding a charge (the light may come on at idle or under electrical load).

What to do

Avoid shutting the engine off unnecessarily: while stopped, lights and electronics still drain the battery. Get to a shop as soon as you can. Avoid long trips and unnecessary electrical loads (A/C, high beams if possible). Have the charging voltage (engine running) and the alternator (regulator, belt) tested.

Order of magnitude for costs

Alternator (remanufactured or new): $250–700 and more depending on vehicle. Serpentine belt: $80–250 (parts and labor). Voltage regulator only: $80–200. My Auto Check-up helps you prepare for your shop visit with a cost estimate.