Victron: add grid-setpoint battery hold control (#31099)

[t0mas]

See issue #31099. The minimum-SOC based approach conflicts with Victron DESS. This adds an optional approach to adjust the battery setpoint by the current EVCC charging power so the battery ignores it.

[florian240483]

Is this only relevant for hold discharge mode or also the other modes? (Hold charge, force charge)

[t0mas]

We could use the same method for each of them. The tricky part is that a Victron system typically operates based on a grid setpoint unless you put it in very low level control mode. So the concept of hold charge/discharge is not that easy to implement. This method does it by increasing the grid setpoint by the amount of power being used for charging.

A force charge in Victron terms means setting the grid setpoint to for example 5 kW import, and then the battery charges with 5 kW minus the household usage.