Battery Tester Lies?

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I think I'd agree with you, if the engine is cold and the car wasn't run for 12+ hours, there shouldn't be much surface charge and your voltage measurement should be accurate. It's not likely the voltage would rise after leaving the headlights on a while. Is the 12.2V you read from the cigarette lighter readout? I would double check with a DVM at the terminals just in case, big difference in charge state with just 2-3 tenths of a volt. Based on the dealer's measurements, 12.32V, 62F, I estimate the battery was at around 65%, a little less than the dealer's 75%. Like the others have stated, I would also suggest topping off the battery overnight and periodically check the resting voltage. If it keeps dropping, might be some parasitic drain (or ultimately just a dying battery!) and you'll get a better idea of whether you want to keep running the battery.
 
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Another note, if you're starting at 12.2V, then turning on the headlights for 10 minutes decreases the voltage down to 11.9V (I would again let this rest a while to let the voltage rebound), that would mean that you went from 50% SOC down to about 12% SOC. Assuming they're 55W each, that would calculate to using 1.52 Ah. With a capacity on the order of 40Ah, it should account for 3.8%, or roughly a .03V drop.
 
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Originally Posted By: Vikas
Does eliminating surface charge, raises the voltage?
No, it eliminates surface charge as a possible variable. Minimizing variables is what you do when you measure things, and especially it's what you do when you're troubleshooting. Since it's clear that nobody believes me here, I will tell you where I got the method I use for testing batteries: Bill Darden's Battery FAQ. Go here: http://batteryfaq.org/ -- Click on Car and Deep Cycle Battery Frequently Asked Questions (FAQ) 2014 -- Click on 4. HOW DO I TEST A BATTERY? -- Go from there. In Section 4 Darden also explains why it's important to remove surface charge (Item 4.3). I'll trust Darden before I'll trust anybody in this group.
 

Vikas

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I understand your rationale for eliminating possible variable. However, in this particular case, this variable, if applicable, would be working in the "wrong" direction and thus is not necessary. One has to understand the rationale behind a procedure if we are going to be scientific rather than doing something on the faith. I do agree with you that one would rather trust a well known authority than some forum participant but the better is to understand the principle behind a procedure before embarking on it. What you are suggesting may be completely valid e.g. may be turning on the headlight for 10 minute and then giving it a rest of 10 minute might actually increase the resting voltage (battery warming up?) but I do have hard time believing that. On the contrary, there are reports of turning on the headlight in frigid conditions just to warm up the battery apparently allows a car to start in case of marginal battery.
 
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Darden also says that letting the battery sit for several hours also allows the surface charge to dissipate (and is the recommended method). I would suggest NOT turning on the headlight for 10 minutes if the car is already cold. That would add another variable in the testing. Darden says to use ONE of the methods on the list. Not multiple. I would also trust Darden before anyone else in this group.
 
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Originally Posted By: EdwardC
I would also trust Darden before anyone else in this group.
I agree. Follow him, not me. Or you.
 
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I had a 2012 Tacoma that started failing in 6 months as soon as the cold snapped. 3 dealer tests, 1 AZ, 1 AA, all said fine. Paid to replace out of pocket, problem solved. Not a lot of faith in testers.
 
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