Int E0 Fn 0015  - Pcrobots V1.41 - "battery" - Determine How Much Battery  [g]

   AX = 0015h

Return: BX = current battery charge

Note:  the battery starts off with 1000 units of charge, and is constantly
     being charged by solar panels and constantly discharged by motion;
     the battery is charged at 4 units per turn and discharged at
     0.1*speed units per turn.