when i disconect /connect the ups to the mains by closeing or opening th relay sometimes the arduino stop responding...now i see that its not the arduino that crashes/reset as all only the "serial part" i mean the arduino keep runing and doing what it should be..Only the seria/usb part get broken the tx led stays on and i lost comunication the pcside to arduino...i can only have connection back after taking the usb cable out and in again .... (strange)...
another strange issue, when i measure voltage divider when the bat is at ~26v gives 2,364v, and this happens when arduino have power...When i disconnect arduino from power i read on the voltage divider output something like 1.3v....why ? something to do with analog pin ??
"Looks like either the USB port on the Arduino or the USB port on the PC is locking up because of a transient." ??
Sounds like a snubber network is needed across the relay contacts - large inductive spikes could be causing the problem at switching times.