BatteryMonitor was an Ottawa Amateur Radio Club project I put togehter for the spring of 2011 and about 25 units were built.
Begin by looking at the 'Battery Monitor OARC_ClubProject_Candidate.pdf'document.  It will provide a quick overview of the unit.  If you're still interested, read the user manual and circuit description.
If you decide you might like to try building  one, at the tme of this writing I've still got some printed circuit boards available, but rgrettably I don't have any parts kits left.  If you're interested in the PCB send me an email, ve3czo at gmail.com.

PS don't substitue the sense resistors R1 and R2. Cheaper parts are available but you may end up coping with thermal electric problems when the sense resistors heat up.