Battery or alternator? Check before you buy a battery

The car will not start, so it must be the battery. Often it is. But a battery that keeps going dead can be the victim, not the cause, and a new battery will not fix a bad alternator for long.

Signs it is the battery

• It is dead after sitting overnight, then runs fine after a jump.
• Slow cranking on cold mornings.
• The battery is three to five years old or more.
• White or green corrosion on the terminals.

Signs it is the alternator

• The car dies while you are driving.
• The battery light comes on with the engine running.
• Headlights or dash lights dim and brighten.
• A new battery goes dead again within days.

Signs it is the starter

The lights and radio work fine but turning the key gives one click, or nothing. That points to the starter or its connection rather than the battery.

Why winter finds weak batteries

Cold weather cuts the power a battery can deliver while making the engine harder to turn over. A battery that was marginal in October often fails on the first cold Northern Virginia morning. Having it tested in the fall avoids the surprise.

Test all three together

At John's Auto Care we test the battery, alternator and starter together, so you replace only the part that failed. Some newer cars also need the new battery registered to the car's computer, which we handle.

We are at 8401 Terminal Rd, Unit G in Lorton, open Monday to Friday 8 to 5 and Saturday 9 to 3.

Battery and charging system testing at John's Auto Care.