How to diagnose a car battery that runs down.

You start your car and discover the engine won’t crank because the battery is dead. Now what? You get somebody to give you a jump start, or you connect the battery to a charger and charge it up until it has enough juice to start your engine.

The next morning you go to start your car again, and the same thing happens. The battery has run down and the engine won’t crank. What is wrong?


    A car battery can run down for any of several reasons:

  • You accidentally left the lights on or some other accessory that pulls power from the battery even when the ignition key is off.
  • The battery is not being recharged while the vehicle is being driven (you have a charging problem)
  • There is a key off parasitic electrical drain on the battery because a relay is sticking, a module is not shutting down, or there is a shorted diode in the alternator.
  • Your battery is old and will not hold a charge anymore.
  • The battery needs to be replaced.

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The first things you want to check are the battery and the output of the charging system (alternator).
One way to do this is to turn on the headlights. If the headlights come on with normal brightness, the no-crank problem is probably not the battery, but a bad starter relay or solenoid, a poor wiring connection between the relay/solenoid and starter, or a bad starter.
If the headlights are dim or do not come on at all, you need to check battery voltage and charging output. For this, you will need a voltmeter. Select the 12 or 20 volt scale, and connect the volt meter red positive test lead to the battery positive (+) terminal, and the black negative test lead to the battery negative (-) terminal.

A fully charged battery should read over 12.6 volts. If the battery reads 12.45 volts or less, it is low (less than 75 percent charged) and needs to be recharged.

Battery Voltage and State of Charge:

12.66v . . . . . . . . . . 100%
12.45v . . . . . . . . . . 75%
12.24v . . . . . . . . . . 50%
12.06v . . . . . . . . . . 25%
11.89v . . . . . . . . . . 0%

(NOTE: these readings are at 80 degrees F. Battery voltage readings will drop with temperature roughly 0.01 volts for every 10 degrees F.)
(At 30 degrees F. a fully charged battery will measure about 12.588 volts, and at zero degrees F it will measure about 12.516 volts.)


After charging the battery or jump starting the car, connect the voltmeter to the battery the same as before and not the charging voltage. A charging system that is operating normally should produce about 13.8 to 14.3 or more volts at idle. If the charging voltage is less than 13.0 volts, the alternator is not putting out enough voltage and current to keep the battery charged. You should have the alternator tested (or bench tested at an auto parts store). If the current output is not up to specifications, replace the alternator.

After the battery has been recharged, the battery should also be tested to see if it is capable of holding a charge. This can be done with a hand-held electronic battery tester or a conventional load tester. The tester will tell you if the battery is good or bad. NOTE: Load testers require the battery to be fully charged for accurate test results. By comparison, most electronic testers will give reliable test results even if the battery is not fully charged. If the battery fails a load test, it needs to be replaced.

The average service life of a wet cell lead-acid car battery is only about four to five years, and can be as little as three years in really hot climates (like Arizona and Florida). AGM batteries are better and typically last 6 or more years. So if you have a conventional wet cell battery that is more than four or five years old, chances are it has reached the end of its useful service life and needs to be replaced if it is not accepting or holding a charge (and the charging system is working normally).



Source: Automedics

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