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Discussion Starter #1
I would like to test a spare battery to see if it's fully charged, can I do it with a regular digital multimeter? If so, do I check voltage? and what is the fully charged value? If it's about half charged what would the voltage be? Thanks.
 

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Doesn't Autozone check batteries for free anymore? The DVOM will check voltage but batteries need a load test to determine if they can handle a start/crank load.
 

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+1 for having it checked at autozone, but yes, set the multimeter to DC, if you need to pick a range pick the setting right above 10V. With the car off it should be 12V, running it should be higher, 13-14V.

But yeah, pull it and have autozone check it for you. Best way to determine if it's ok or not.
 

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An internet search will turn up several sites with the info. From http://en.wikipedia.org/wiki/Automotive_battery, 100% charge is approx. 12.60 volts, 75% is 12.35 volts, and 50% is 12.10 volts. As the article says, temperature can affect it, but those numbers are supposed to be good for 20°C (68¯F). They give a temp. correction formula of -0.022V/°C.

You'll get an artifically high reading right after charging due to "surface charge" (charge unevenly distributed giving falsely high reading). For accurate reading, let it sit for 1/2 hour after charge is stopped before taling the reading.
 

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Discussion Starter #5
Thanks for all the great info. Autozone or any parts store is a long distance away from me and how often can I go bother them, so I was trying to check it my self.
 
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