There are two basic battery sizes, Large and Mini. they are identified by their voltage, 9.6V and 8.4 V respectively. Most guns with full stocks come with a NiCd Large battery that goes in the stock, which is more than enough for a full day of gaming, if not more. Guns that fit batteries in the grips are often NiMH mini batteries, which might last you an entire day, but I often bring both just incase.
Just know if you buy a battery with a higher mAH, it also increases your ROF. It might just be easier to store a larger battery in your gun. Lots of weird shaped batteries are on the market to fit in weird places (eg, large type nunchuck-style batteries fit into collapsing m4 stocks)
Wiring a backpack battery is redundant, as the pack would offer wayyy too much power, and would limit your mobility as you would be teathered to your gun. That being said, ive heard of people building larger batteries for their guns, but it never works as the internals are not made to work at the torque or speed without modification.
Originally Posted by White_knight
your right,when it rains the burger joint across the street does have cats for 4.99$ but that doesnt mean that i can just go get all the grape jelly i want