will backup batteries for 3g work with 3gs?

[Wtf0ver]

im traveling in a week and im trying to find a backup battery to work with my 3gs. i thought i read somewhere that a 3g battery will not work with a 3gs? not sure. if anyone can point me in the right direction of a battery that will work without having to remove my otterbox that would be great.

[scottjf8]

Cheap and clunky - http://www.amazon.com/Kensington-Battery-Extender-Charger-iPhone/dp/B0018DKODQ/ref=sr_1_2?ie=UTF8&s=electronics&qid=1255906855&sr=8-2

More expensive and awesome - http://www.amazon.com/mophieTM-Juice-PackTM-Air-Rechargeable/dp/B0028MJLAS/ref=sr_1_3?ie=UTF8&s=electronics&qid=1255910628&sr=1-3

[Wtf0ver]

Found one that works . The griffin tunejuice, $30 at best buy box says works with 3g, no mention of 3gs but it works. Takes 4 AAA batteries