Detect the state On/Off a DC 12V single coil latch relay
It's not clear why you don't just monitor the coil voltage. It seems as though you do not trust the relay. 100 mV across a contact seems high.

CV/CC chargers will drive to ~14.4V in absence of load.

That seems to me to be your solution. Use a comparator set to 14 V to monitor the PSU voltage. If it's over 14 V then you've an open circuit. If it's less then you're connected.