starmage

And just my luck - the noise stopped. All is now quiet - and that was before the installer was here
Looks like some weird combination of things (PV starting up (the sound started +-just after 6 and was basically gone when we checked now @ 9), Eskom online (LS ended @ 6), low load (+- 400w))
WIll keep my eye (or ear ) on it - installer had a check and all looks good (he had to add some surge protection device to the PV strings).
So yea this is just weird

Technically you can just use 1 MPPT -- BUT this is not recomended as you will loose efficiency. Also consider if your panels are not identical, and you connect them to the same MPPT, the panels will basically only produce as much power as the lowest panel.

Not exactly. The BMS will ask the inverter to please stop discharging at 10% (if you have some sort of communication channel), but you can take it below 10%. I know, because when we test these things, we actually hack out the comms part and we take it right down to zero to see when the battery really switches off (so that it can be documented for support purposes).
On a Victron system, the battery will indeed send DCL=0 (discharge current limit) at 10%, and this will cause it to stop discharging, or switch off if there is no grid. But in an emergency... you can use that last 10%.

As far as I am aware the BMS will shutdown at 10%SoC (state of charge) - hence 90% available for use. However the 6000 cycles is based on not going below 20%SoC.
So the Us3000 is nominally 3.5kw but useable is 3,2kw.And if you want the >6000 cycles, dont drain the batteries below 20%SoC. If you do drain it regulalrly so that the BMS shutsdown, the rated cycles drop to something like 4500