Batteries have internal connections that can become broken but still touch some of the time, and plates that can become loose and short out some of the time depending on the temperature or movement of the vehicle. These could allow a battery to load-test OK but still cause problems.
Pat
True - and would make the charger in the trunk of no value in those situations too. I started to make a similar comment as yours, but figured that the OP knows that and his comments were in the context of the particular problem he had. For example, he said "so I was never stranded", and *didn't* say "so I will never be stranded" (in his particular situation of that day). :)
I did use the charger once however in between my first and second NAPA visits when the car just refused to start (usually it would just really hesitate and then finally start up). It sent me on my merry way.
I don't think my problem was ever broken internal connections, I think that gal at NAPA just didn't know how to operate the battery tester (she was kinda cute though).
Part of the problem with battery testing is the circumstances involved. The battery should be fully recharged, then the load testing should involve removal of the surface charge either by applying a load or leaving tha battery sit for an extended period of time, perhaps overnight. Then the load should be applied and the voltage drop noted, then the recovery voltage noted. This needs to be repeated through several cycles. (I am assuming the use of a basic hand-held tester here - more sophisticated machines may have different methods, though I have read that some of their claims to superiority are overblown but that is another topic). Only a thorough test will produce an accurate result and, even then, it is not foolproof.
In many cases, dare I say most?, the tech just does a 10 second test and announces "It's good" or "It's shot"...in my opinion and experience the conclusions drawn are incorrect as often as they are correct.
Customer expectation may be a factor as well -- if I didn't think the battery had failed, I wouldn't have taken it to NAPA. They agree with me, sell me a new battery, and away I go.
Of course, I can't remember an instance in which this didn't solve the problem...
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