Using the relay output on the Phoenix Smart 50A (1+1) charger. Latest firmware installed.
I have set the relay up as ALARM, with a low trigger of 13.1V, and no delays.
Expected behavior:
The relay does not open until 15.4V, look at your setting to see the icon with the alarm bell crossed out. This is called hysteresis, you have a slightly different reset condition so if you are fluctuating at 13.1V your alarm does not keep going on and off.