in circuit with the operating elements 22 26 the microprocessor 10 thus controls which elements receive current from the battery 20 and for how long as will be described below this is used to measure the current discharged from the battery 20 and to provide an indication of the battery condition on a display 40 there are five distinct loads on the battery 20 when the x ray unit is turned on these include 34 idle 34 energy which is required by the electronics power supply 21 34 drive 34 energy which is required by the motor 22 to propel the unit 34 field light 34 energy which is required when the light 23 is turned on 34 prep 34 energy which is required by circuit 24 to prepare the x ray tube 25 and 34 x ray 34 energy delivered by the tube 25 during the prescribed exposure all of these loads except the drive motor 22 are relatively constant known values in the preferred embodiment the drive motor current is assumed to be constant at a level required to propel the unit at top speed on a level surface for these modes of operation the energy discharged from the battery 20 is as follows 34 conversion factor 34 is a measure of the x ray generator efficiency and is 13 in the preferred embodiment referring particularly to fig3 a approximately every 5 seconds the microprocessor 10 executes a program which updates the display 40 with a value indicating the remaining battery capacity in the preferred embodiment this is a percentage of full battery capacity and it is calculated as indicated at process block 50.