INT E0 - PCROBOTS v1.41 - "BATTERY" - DETERMINE HOW MUCH BATTERY POWER LEFT
	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.