add resister from +5V to top voltage divider R that is X10 to X100 the R-divider values to give 2,3 or say 5 point A/D reading with floating (no sensed voltage connected). The low current should not affect the sensed voltage reading. Could add a diode from +5 pointing to the new resister to stop any current from the 13V backing up to the 5V.
don
amgen