20171215_042323_GMT U0C0, main battery voltage: 2836(9) 12.82(0.04) V
20171215_042359_GMT U0C0, main battery voltage: 2842(12) 12.85(0.05) V
20171215_042424_GMT U0C0, main battery voltage: 2838(3) 12.83(0.01) V
20171215_042447_GMT U0C0, main battery voltage: 2841(5) 12.84(0.02) V

Mark has:
12.59
12.59
12.59
12.58
Add 0.45V to Mark's readings to compensate for voltage drop across
the dioide he is using to measure. So his actual voltages:
13.04
13.04
13.04
13.03