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My car still starts fine but when I turn off the car (just the motor . . the key is still in and "on") I noticed the headlights dim to almost off. It's and older car and the headlights don't turn off automatically.

The battery is over 4 years old and in Arizona "they say" batteries only last 2 or 3 years. Should I go and get a new battery or wait until the car won't start? Can I test the battery with a simple tester to see if it's still delivering 12 volts?

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On a good battery you should be able to measure about 13.8 volts. –  Peter K. Mar 16 '11 at 16:00
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@peter 13.8 volts would be a good voltage with the vehicle running and the alternator working correctly, not the battery itself. –  Larry Mar 16 '11 at 18:27
    
@Larry: The point I was trying to make is that it's not going to be 12, but will generally be higher. –  Peter K. Mar 16 '11 at 18:55
    
Also possibly relevant: mechanics.stackexchange.com/questions/372/… –  Iszi Mar 16 '11 at 20:44
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4 Answers

up vote 24 down vote accepted
  1. Preform open circuit voltage test. Vehicle off, battery disconnected, check the voltage with a DVOM (Digital Volt Ohm Meter)
    • 12.66 = 100% state of charge
    • 12.45 = 75% state of charge
    • 12.24 = 50% state of charge
    • 12.06 = 25% state of charge
    • 11.89 = 0% state of charge
    • 10.45 - 10.65 = bad cell, battery should be replaced
    If the battery is at or near 100% then go to step #5. If the battery is less than 100% then go to step #2
  2. If the battery is sealed (maintenance free) go to step #5 if not check the electrolyte level, if low add distilled water and go to step #4. If not go to step #3
  3. Hydrometer test, check the specific gravity of each cell, they should all be at 1.265 or more if the battery is in good condition. If all the cells are below 1.225 the go to step #4, the reading from the highest cell should be no more that 0.050 above the lowest cell, if it's more replace the battery you have a weak cell. Otherwise go to step #5
  4. Charge the battery. 3 minute charge test. Hook up the charger to the battery set the charger to 30 - 40 amps at 3 minutes check the voltage while charging if it's above 15.5 volts replace the battery it will not accept a charge, if below 15.5 volts reduce the setting on the charger to 2 - 10 amp range (less is better, fast charging reduces the overall life of the battery) and continue to charge the battery until the gauge on the battery charger show close to zero amps. Note this could take 8 or more hours depending on the state of charge and condition of the battery. Unhook the charger wait 2 hours for the surface charge to dissipate then go to step #1
  5. Load testing is done with a specialized tester, most of your local parts or battery stores will have one and provide free testing. First I will cover how to load test with a load tester and then a test you can do without a load tester.

    With a load tester load the battery to 1/2 the CCA (Cold Cranking Amps) for 15 seconds while watching the voltage it should not drop below 9.6 volts if it does replace the battery. Once the load is removed the battery voltage should recover to at least 12.24 volts within 5 minutes of the load test if not replace the battery.

    Testing without a load tester, this doesn't put as much of a load on the battery as the test above so if it passes this the battery could still be bad, if it fails however you can be sure the battery is bad. Disable the vehicle from cranking by disconnecting the coil wire or fuel pump relay. Turn on the bright headlights and crank the vehicle for 15 seconds (no longer than this and do not repeat this test within 3 minutes or damage to the starter can occur) with a DVOM on the battery see if the voltage drops below 9.6 volts or does not recover after the test as mentioned above. If either of those two fail then the battery is bad.

If the battery checks out the next test would be the charging system.

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@JoshCaswell The open circuit voltage chart I referenced showed no difference for temp, checking a few other sources show only a small temp correction for open circuit voltage. Here is a link to one article Specific gravity does change with temp, my gauge has a chart on it so I didn't think to mention it in the original post. The load testing voltage is also affected by temp -0.1 volts for every 10˚F below 70˚F down to around 30˚F, after that it's -0.2 volts until you get to 0˚F. –  Larry Dec 10 '13 at 16:30
    
That's interesting. I was just dealing with a dying battery this weekend. My main source was pacificpowerbatteries.com/aboutbatts/Car%20Battery%20FAQ/…;, whose chart apparently comes from a battery industry organization, BCI. It looks like I misunderstood the direction of the voltage change, though - your link makes it clearer (and is a nice straightforward read in general, BTW). I'm going to remove my earlier comment. –  Josh Caswell Dec 10 '13 at 19:06
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If you lack a battery tester, drive over to your local autoparts store and have them test it for free. The test is usually conclusive and will tell you if your battery is OK.

If you opt for a new battery, get an Optima red top or yellow top. These things last forever.

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I made the mistake of assuming that a Sears place could do this correctly on a battery that I bought from them. They couldn't. They incorrectly told me that the battery was good, and I was stranded again after that. I took it to my local garage (who also tested for free!) and they told me it was a goner. –  Peter K. Mar 16 '11 at 18:57
    
The exact thing you want them to tell you is the CCA of the battery. Not just "good/bad". –  Jay Bazuzi Mar 16 '11 at 23:40
    
What is "forever" in battery speak? I thought the 4-5 years I got with my current was "forever" in the heat of AZ since everyone selling batteries here insists you pay more for the bigger warranty battery since you are sure to get a new one free! The one I have now is a Kirkland/Costco brand made in Mexico. –  tooshel Mar 18 '11 at 20:42
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In a way, you have already performed the battery test by using the headlights. The headlights only draw around 100 watts / 8.3 amps. If the battery is having difficulty providing that amount of power, then the starter motor is certainly going to give you challenges in the near future because it draws hundreds of amps.

I would also check to make sure the alternator is working by checking the battery voltage when the car is running. It should be between 13.5 to 14V.

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Not necessarily, it could mean you have a bad ground or corrosion somewhere in your headlight circuit that's impeding the current flow. But in general, you're right to be concerned about the battery. I think @NoCarrier's got the answer -- have your local mechanic/parts store check it out. My batteries always seem to die without warning -- starts and runs fine one day, flatline the next.

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