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Quick Battery Test Using Headlights

  #1  
Old 05-13-2018, 12:14 PM
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Default Quick Battery Test Using Headlights

I recently tried the quick battery test using the headlights with the engine off. My reading after a few minutes was 12.14 volts. The voltage dropped to 12.00 volts after adding the high beams. Those readings apparently do pass the test.

Does anyone know what readings might be seen with a fully charged, brand new battery? I wouldn't expect 12.6 volts but perhaps something modestly lower.

The battery is two years old and rated at 582 CCA.

A very knowledgeable individual told me that my readings indicate higher than normal internal resistance and possible sulfation. Thanks for any input.
 
  #2  
Old 05-13-2018, 08:30 PM
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What was the battery voltage prior to starting the test?

12.6 is fully charged after the engine is off and no electrical load for about 30 minutes.

Have you checked the battery voltage (alt test), engine running at 2000 RPM?
 
  #3  
Old 05-13-2018, 10:36 PM
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The battery typically reads 12.6-12.7 volts in the morning. The battery voltage after starting the engine measures 14.2 volts on a cool day and it then drifts down as the engine warms up.

The headlight test was done after a full charge on a battery maintainer. Specific gravity measures 1.290-1.295 which indicates 100% charge so the battery appears to be in very good condition.

However, the battery did run down last year from very infrequent use and parasitic drain. I wonder if grid corrosion has resulted in higher internal resistance. That could explain the 12.14 volt reading during the headlight test.

When I replace this battery, it will be very interesting to repeat the headlight test and measure the voltage with the new battery.

I'd be very interested in the voltage that others see when doing the headlight test.
 
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Old 05-14-2018, 12:22 PM
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What is infrequent usage?

For a good battery, a few minutes of headlight operation is nothing.

Car batteries don't like being run down to the point of struggling to start the engine or dead.

A battery will self discharge between 3 and 5% per month.

From your statement the battery appears to be weak. Weak batteries can do just fine for a time. Typically they fail when winter arrives with cold temps.

If you are leaving the car off for months, disconnect the battery. Some owners use a trickle charger.
 
  #5  
Old 05-15-2018, 01:54 PM
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I tried the headlight test again yesterday and the battery looked much better. The voltage drifted down to 12.20 volts after about two minutes and it then reversed and rose to 12.27 volts.

The 12.14 volt reading that I mentioned earlier was actually after having the headlights on for 25 minutes or longer.

The battery easily passes the headlight test described in this article. They suggest taking the measurement after about 5 minutes.

https://www.autobytel.com/car-owners...attery-117786/

The battery ran down last year when the car was only driven a few times per month. Parasitic current drain and short trips added to the problem. Note also that cars today are often lighter on charging to reduce alternator load and improve fuel economy. I now use a battery maintainer to keep the battery charged.
 
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