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Close-up of an automotive alternator with serpentine belt and pulley, the charging system component diagnosed alongside the battery

Electrical

Is it the alternator or the battery? A 30-second decision tree

5 min read

About a third of the cars we see for a 'dead battery' actually have a charging-system problem. Replacing the battery without diagnosing the alternator means you are back in the parking lot in two months with the same problem. Here is how to tell the difference.

The 30-second decision tree

  1. 01Try to start the car. Does it crank slowly, click without cranking, or do nothing at all?
  2. 02If it cranks slowly: probably battery. Maybe alternator if the battery is recent.
  3. 03If it clicks but does not crank: weak battery, bad starter, or corroded terminal.
  4. 04If a jump start works and the car runs fine for hours: battery is the suspect.
  5. 05If a jump start works but the car dies again within a day: alternator (it is not recharging the battery).

Five alternator-fail symptoms

  • Battery warning light on the dash while driving
  • Headlights dim at idle and brighten when you rev the engine
  • Battery dies within hours of being jumped
  • Burning rubber smell from the engine bay (slipping serpentine belt)
  • Stereo cuts out, gauges flicker, or windows roll up slowly

Five battery-fail symptoms

  • Slow crank that has gotten worse over a few weeks
  • Battery is more than 3 years old in Texas (4 to 5 elsewhere)
  • White or blue corrosion on the battery terminals
  • Battery case is swollen or leaking
  • Several short trips in cold weather without a long drive in between

Why the parts-store free test misses things

A standalone battery test under load tells you if the battery still holds charge. It cannot tell you whether the alternator is properly recharging the battery while you drive. About a third of the time, a battery that fails the parts-store test is failing because the alternator quit weeks ago and the customer has been running the battery flat over and over.

Texas heat is the real killer

Sustained 100°F+ summers cook batteries from the inside. Three to five years is normal lifespan here, versus five to seven in cooler climates. If your battery is over 3 years old and you have a road trip coming up, replace it preventively. The roadside cost of a midnight tow is a lot more than the cost of a battery.

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