Does a Quattro (or Multiplus) come to any harm if the mains input is connected before the battery?
Quite simply … don’t do it …
It is not advisable to put the system into operation without a finished DC connection because the control board “could” be damaged. Either use a power supply unit or wait …
Quattro and Multies does not support operation without battery. In the manual there are connection steps and battery connection is one of them, before grid connection