Can see the picture ... but if you only want to see if a battery is in discharge or charge, on ASC712 is enought!
These current sensor supose to give about 512 on the analog inputs if no current.
If its over 512 is charging, if lower its discharging - EASY 
I have a AH-meter/logger for the last 84 days - and I simplyfied it buy using one current sensor.
The drawback: if you have charge but take out power too, you don't get seperat readings for out/inn.
http://forum.arduino.cc/index.php?topic=189314.msg1407671#msg1407671
See you want to take an MEGA - surely it takes even more current than my Arduino ..... my Arduino takes about 60-80 mA! So you will always have discharge (except if you have sun).