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Possible Duplicate:
Why we have 3 pins on some batteries?

I'm trying to build (DIY) open charger for mobile battery. I would like to know what is the usage the middle one?

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Is there anyway I could check the status of battery (i.e., charged to max)?

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merged by Kevin Vermeer Mar 7 '12 at 19:38

This question was merged with Why are there 3 pins on some batteries? because it is an exact duplicate of that question.