A 9.6 or a 7.4 lipo at 15-20c will be your best bet. I am currently running a 2000 mah 7.4 tho I have it either in a mock peq or in a pistol taco mounted on the outside. Most of the batteries that will fit in the gas tube are not going to last as long as you need them to and you could potentially have that moment when you need your gun and your battery is dead. However if you choose to go the route of in the gas tube, any battery that will fit into a AK will work. Now a couple things that you need to upgrade right away is the piston and motor. The piston is from a bygone era of classic army where the second tooth is present and the AOE is..... well lets just say that I would be surprised if it lasted half a day of heavy fighting. And the motor is very lack luster as well. its magnets are weak and it is very inefficient. The gears are an 18:1 standard ratio so depending on the spring any motor should be able to delver 15-20rps I would also replace the gears but that's just me. Also note that the piston guide inside the gear box is very rough I would polish it. Here is what is going into mine. Matrix Godzilla motor, Angel custom piston piston head and cylinder, Siegetek 20.8:1 gears and a pommy m130 spring. Note that the RPS will be a little slower but remember that the box mag can only feed so many bbs at a time.