An interesting tidbit from my book, Star Wars: Complete Cross-Sections:
The AAT's energy shells cannot be replenished by the droid crew onboard. Instead, the shells are reloaded when the tank returns to a landing ship or battleship, where mechanical facilities take the entire inside of the "foot" off the tank from below. A loaded replacement is then installed in its place.
Lego Armored Assault Tank:
Episode I Armored Assault Tank:
See Bricklink for parts information for today's build.