i have just obtained a 3gs and have a 3g to build i ordered a battery for my 3g my mrs also had a 3g so i got one for that too now the 3gs come along so i thought instead of putting my 3g battery in the 3g i put it in the 3gs then put it on charge powered up good then i unplugged it , dead!!! i then noticed the other battery i ordered looked different. 10 minutes resarch and this is my conclusion....... the 3gs is totally different in many ways so always put the intended parts in the phone as all the connectors may look the same but are just slightly different and easily broken. apple must love it.
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== Update ==
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if you look at both batterys they are blatently different as one has lots of tiny component on it and the other not so many and i found out the 3gs battery has got a circuit that stops it from overheating and overcharging.
i have just obtained a 3gs and have a 3g to build i ordered a battery for my 3g my mrs also had a 3g so i got one for that too now the 3gs come along so i thought instead of putting my 3g battery in the 3g i put it in the 3gs then put it on charge powered up good then i unplugged it , dead!!! i then noticed the other battery i ordered looked different. 10 minutes resarch and this is my conclusion....... the 3gs is totally different in many ways so always put the intended parts in the phone as all the connectors may look the same but are just slightly different and easily broken. apple must love it.