Battery or alternator? How to tell why your car won't start

QUICK ANSWER

The battery starts the car; the alternator recharges the battery while the engine runs. If a new or freshly charged battery keeps going flat, or the battery light stays on while driving, suspect the alternator. A battery at rest should read about 12.6V or more; with the engine running the charging voltage should rise to roughly 13.5 to 14.5V.

Multimeter on a car battery showing 12.80 volts — A multimeter on the battery reads 12.80V: a fully charged battery at rest. If the car still keeps dying, the charging side needs checking.

Signs it is the alternator, not the battery

The battery or charging warning light stays on while you drive.
Lights dim and brighten with engine speed, or electrics act up while driving.
The car starts after a jump-start, then dies again within a day or so.
A whining or grinding noise from the front of the engine, where the drive belt turns the alternator.

The simple check

Engine off: battery voltage. About 12.6V or more is full; under about 12.4V it is low.
Engine running: the voltage should rise to roughly 13.5 to 14.5V. If it stays at battery level or falls, the alternator is not charging.

A worn alternator removed from a car — A worn alternator after removal. When it stops charging, even a brand-new battery runs flat.

A replacement alternator fitted in the engine bay, driven by the belt — The replacement alternator fitted, turned by the engine's drive belt.

Photos: an alternator job at SG Garage, used here to show the parts.

What we do when we come to you

Tell us your symptoms when you call or WhatsApp: it helps us bring the right help. If the battery is the problem, we fit a Delkor battery on the spot (onsite battery replacement). If it's not, the car may need a workshop: we can arrange a tow (from $80) or jump-start you ($50) so you can drive there.

Please note: If our technician comes to you and finds the problem is not battery-related, an onsite service fee of $50 to $80 applies. Final charges depend on location and service requirements.