[DarkAngel]

I have a few concerns.
1) if you drop the gun, you trip or fall etc, you could yank out the wiring in your gun by accident, damaging the contacts and switch assembly in your gearbox.
2) your tethered to the gun
3) If your battery gets a short, you cannot ditch your battery/gun in the case that it catches fire or starts to combust. For example, if suddenly your battery starts to combust, you would have to rip off your gloves, rip off your jacket, all while panicing due to your body being on fire.
4) This is doubly so for lipo setups.

My suggestion is to stick with hot packs or something else. Ive seen enough batteries go up on the field to know that while the chances are fairly remote, the chances arnt THAT miniscule.

As for your P90 battery size issue, either do the JBatt mod, buttstock extender, or simply go lipo.