I'm a bit torn with my P90. The stock Echo 1 battery is a 9.6V mini, 1200 mAh Ni-MH pack built with 2.3A cells. It actually works great and I like it, although it's cold weather performance is horrible. Shooting indoors at home, I can easily get about 20 mags' worth of shots before having to recharge it. However, the 2 times this winter I tried to use it at the Mill, I got 3-4 mags out of it before it died. I know Ni-Cad are better in cold, but still, the stock battery shouldn't have died out THAT fast. The Elite Ni-MH battery in my MP5 lasted hours and didn't show the slightest sign of dying after using it most of the day in the cold.
So now I'm a bit torn between 3 options. First, there's the identical 9.6V mini I referred to, your suggestion of the J battery, which will deliver higher capacity and current, and finally, the Li-Po route... I've tried my P90 on a Li-Po and it was really nice. It didn't increase the rof that much higher than with the current battery (I expected more, but glad it didn't), but made the trigger extremely responsive...
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