Why would you constantly be buying Rechargeable batteries? The whole deal with those are they can be recharged and used again and again.
The PS3 pack is a 3.7v Li-Ion while the 360 pack is a 2.7v Ni-MH. The voltages of either of those packs wouldn't be the real issue though.
I'd consider the PS3 battery out as it's Li-Ion, and they need to be charged with a Li-Ion charger or nasty things can happen, so that would mean adding a charger into the controller as well. It would also have half the run time of a set of Rechargeable AA in that controller as it's not a big battery.
You'd really be reinventing the wheel using a 360 battery pack, as it's just 2 x AA Rechargeable batteries anyway, so there's a pile of cosmetic and mechanical work to get the packs to be used like on the 360 controllers.
With the 360 packs, you'd probably also want to have a quick charger to keep either 2 packs charged up while one was in the controller, or one charged while the other one was in the controller. Either way there, I don't see any real pay off versus getting a decent charger that can do 4 batteries at once, so you always have a fresh set on hand there and no work at all done to the controller. Don't get me wrong, hack away on her if you like, I just don't see the 360 packs being any kind of improvement over a normal 4 cell charger and then 6 decent rechargeable batteries so it always has a set ready to go. This post has been edited by RDC: Sep 12 2012, 05:55 AM