I documented the Raspberry Pi power system in a separate project because it is useful in its own right.  It provides push button startup, push button shutdown with essentially no battery current draw (1 microamp), ability to monitor voltage, current and power from the command line, and automatic shutdown when the battery voltage drops too low.  Here is the project: battery-power-system-for-raspberry-pi

pi@pi3:~ $ battery
battery voltage = 12.451 volts.  Run time = 0.3071861111111111 hours
battery current = 332.275 mA.  Capacity used = 98.555 mAh
CPU temp = 49.93 deg C.