The PIC will be a power supervisor in this application and we're going with two of these battery packs http://www.cell-con.com/pdf/D90494_DataSheet_BP.pdf.

What voltage does the SMBus use? If its 5 volts then that will be the PIC voltage. The PIC is going to monitor the battery time remaining and have some status LEDs and probably watch the temperature as well. If the batteries get too low then its going to disconnect the equipment. Nothing real fancy. I'm not sure what fuel monitor chip the batteries have but they do have 24 possible values to read from.