how can i compute the battery change level?

Thoughts are that you should be able to add a current sense resistor, and measure the current drawn.  Current * Voltage gives you the power drawn, and if rates are low, one should be able to determine the power drawn from the battery over time.  With some study, one can create a "table of charge" (you'll have to "learn" it), and thus determine where one is on the discharge (charge too) of a battery.  
 
Alan  KM6VV

"D" is a command specific to the SVS (Stereo Vision System) board. If you do not have and SVS board this will do nothing. If you do have an SVS there is a chip on the board that monitors the battery voltage and the "D" command retrieves the read from the chip.
 
T. Jump