Mini,

The manual recommends a Thunder Power 3 cell 2100mah lithium polymer (lipo) battery as an upgrade. It is a 11.1v battery as compared with the stock seven cell 8.4v nimh battery. It will give you a very noticeable increase in power but you will also need a charger that is lipo capable. Be sure to use the smaller prop that came with the stryker if you upgrade to 11.1v battery or it is possible you could fry your stock electronics. I used the larger 6 cell prop with the lipo battery and never burned anything up even though it says not to. I was looking for an excuse to get a brushless motor and ESC right away. For many other ideas on cool upgrades for your stryker here is a great thread to start with http://www.rcgroups.com/forums/showthread.php?t=389169. Probably anything you ever wanted to know about your stryker has been asked there.