Is it the battery or the alternator? How to tell without any tools

What each one does

The battery starts the car. The alternator, which a belt turns off the engine, runs the electrical system and recharges the battery while the engine is running. Once the engine is going, the car runs on the alternator, not the battery. That fact is the key to telling them apart.

It also explains why the two failures feel different. A bad battery tends to strand you before you leave, when you turn the key. A bad alternator tends to strand you after you leave, a few miles down the road, when the stored power finally runs out. When the car quit tells you a lot.

Clues that point to the battery

These usually mean the battery is the weak link:

• Slow cranking or clicking when you start, worse on cool mornings.
• After a jump the car runs normally, and it starts on its own again later in the day.
• The battery is more than three or four years old.
• The battery case is swollen, or the terminals are crusted over.

Clues that point to the alternator

These usually mean the charging side has failed:

• The battery light comes on while you are driving.
• Headlights and dash lights dim at idle and brighten when you rev the engine.
• A whine or grinding from the front of the engine that changes with engine speed.
• A smell of hot wiring or burning rubber.
• The car runs for a while and then dies on the road, and won't restart without a jump.

Two ways to test it

The first needs a jump start, from a friend or from us. Once the car is running, the jumper cables or jump pack come off. If the car keeps running, the alternator is doing its job, and the battery was the problem. If the engine stumbles and dies within a minute or so, nothing is charging, and the car was running on borrowed power.

Never disconnect the car's own battery to see if the engine keeps running. On modern cars that can damage the electronics.

The second way needs a multimeter, and it puts numbers on it. A healthy, rested battery reads about 12.6 volts across its posts. With the engine running, a working alternator pushes that to roughly 13.5 to 14.5 volts. If the reading stays near 12 while the engine runs, the alternator isn't charging. If it reads well below 12.6 with the engine off, the battery is low, but that alone doesn't say why.

What we can do

If it is the battery, we can jump it, at $49–$75 plus the $19 dispatch fee added on top, or replace it for $239–$255 on eligible vehicles and batteries, which covers the new battery and installing it. If it looks like the alternator, a new battery won't cure it, and that needs a repair shop. We don't repair alternators.

If you can't tell, say so when you request help and describe what happened and when. The driver can advise on site whether the battery looks like the problem.