FYI. Here's some real-time test data.....
Battery relatively new (6mo old) 60AH standard battery.
While Mac mini in sleep powered by P1900 in "Standby" mode (+18.5V output)
9:30AM - V(battery) = 12.46V
5:30PM - V(battery) = 12.32V
8 hours, .14V drop in V(battery)
8:30AM - V(battery)= 12.32V **NO CHANGE**
It appears that the battery had a small initial discharge and then settled out to 12.32V. The Mac resumed this morning with no problem.
This looks very encouraging.