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Battery Testing FAQ

Q: My batteries don’t seem to be holding a charge.  How can I determine if they need to be replaced?
Yes, it could be one or more bad batteries.  If they are more than 5 years old, then that’s the most likely issue.
There are a couple of ways to test batteries.  The best is with a capacitance tester (an expensive tool, and not likely to be available). A capacitance tester does not require that the batteries be fully charged to test.  All other tests require that the battery be fully charged before testing.  Only a professional will have the capacitance tester, so it’s unlikely you can find one, although a store that sells batteries might have one.
Next best is a load tester, basically coils of heating wire (like a toaster) enclosed in a metal box (with lots of holes for the heat to escape) that you attach to the battery with what looks like short jumper cables.  This thing loads the battery heavily and has a volt meter that (on a good battery) will drop from about 12.8 volts to 11.7 and then stay close to that for at least 15 seconds.   On every bad battery I have tested, the voltage falls quickly and steadily.  Most auto repair shops have one of these, and many discount auto parts stores will do this test for free.  You can do something similar with the starter motor of your engine.  With the “engine kill” cable pulled, or with the engine stop button pressed, crank the engine for 15 seconds or so.  Do this with only one battery hooked up.  A bad battery will run out of juice quickly.  Unfortunately, this is not very scientific, and I don’t know the amps your starter will draw, nor exactly what the “normal” battery voltage will be.  If you have several batteries and only one is bad, this test will dramatically identify the defective one.
The easiest way to test a battery is to measure the voltage “at rest”.  To do this test, fully charge the battery, then let it sit for 24 hours.  The measured voltage after 24 hours is the “at rest” voltage.  It should be 12.7 – 12.8 volts.  However, this test is not definitive.  You can have a battery that has very little capacity, and it could still pass this test.  This test will find a battery with a bad cell, which is sometimes indicated by single cell using more water that the other cells (in a flooded battery).
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