Dead battery or bad alternator?

The battery starts the car and runs everything while the engine is off. The alternator takes over once the engine's running — it powers the car and recharges the battery as you drive (Interstate Batteries). The starter is the motor that turns the engine over when you turn the key or press the button. When any one of them fails, the car won't start — so the trick is reading the clues.

• A click — or rapid clicking — when you try to start it, or a slow, labored crank.
• Headlights that flicker within a minute of turning them on with the engine off (this check only works on regular flooded batteries, after the car has sat a few hours) (Interstate Batteries).
• Electronics glitching — a window that hesitates, a flickering cabin light, the radio forgetting its presets or your phone (Interstate).
• It still hesitates after a recharge or a long drive — a sign it can't hold power anymore (Interstate).
• It's worse on cold mornings, and the battery is three or more years old — Interstate says a battery is at its best for about three years. More in signs your battery is dying.

• The car dies while you're driving, or with jumper cables on, the electronics come on briefly and then fade out (Interstate Batteries).
• Headlights that flicker while you're idling with the engine running — a sign of a bad alternator and a weak battery (Interstate).
• A low reading with the engine running: Interstate says you want more than 13 volts, and lower can mean a bad alternator or a bad battery. Over 16 volts can damage the car (Interstate).
• The battery warning light comes on while you drive. It can point to either part, so get both the battery and the alternator tested.

Interstate Batteries lists the other usual suspects when a car won't start even with jumper cables: the starter if you hear clicking or grinding while the dash lights come on, the ignition switch if the electronics come on but there's no sound at all, or a fuse if the engine turns over and still won't fire. A battery can also be so dead it blocks the jump.

A $20 multimeter tells you a lot. These are Interstate Batteries' steps and numbers:

1. If you just drove, turn the headlights on for two minutes, then off — that burns off the surface charge that makes a reading look better than it is.
2. Set the meter to DC volts (the range above 15) and touch the red lead to the positive post and the black lead to the negative.
3. Compare the reading to the chart below. Then start the engine and check again — about 13–15 volts means the alternator is charging.

One catch: voltage only shows the charge right now, not the battery's health. A worn-out battery can read full right after charging and still fail to start the car a few hours later — only a load test shows that (Interstate).

We only do batteries. When we get to your car we test the old battery, and after the new one is in we test the charging system — a new battery won't fix a bad alternator, and if that's what we see, we'll tell you. If it's the battery, we swap it where the car is parked: book it, see what it costs, or check what to do on a cold morning.