question

@nick walton

Interesting post. As I see it there's no need..

All pv needs a 'box' to convert pv power to either DC or AC specs.

A Multiplus/Quattro can accept either. AC is first fed directly to loads, and any remaining converted to DC for battery charging and dc loads.

DC from pv can be used for battery charging, dc loads or converted to AC for ac loads.

You can have both AC and DC pv supplies in the same system, and running at the same time. The only thing the M/Q's can't do is pass ac > dc and dc > ac at the same time. But that's never necessary anyway.

https://www.victronenergy.com/live/drafts:off-grid_documents:choosing_dc_and_or_ac_solar

You mention "a small single-phase AC generator". That's never recommended for large M/Q's, they have to be sized to meet the minimum settings for the AC Input side, whereas pv-sourced AC usually goes to the AC Output side.

The AC Coupled design promoted as the new established "best practice" is the point of issue here.

This design only exists due to the fact that no battery inverter/charger is designed and manufactured to handle two separate DC connections and intelligently manage them - specifically, DC solar connection (input only) and DC battery connection (input/output).

If the battery inverter/charger was designed for two distinct DC connections it would make the AC Coupled design redundant. Why?

Because a dual DC battery inverter/charger would take the function of the "grid feed inverter's" conversion of solar DC to AC and subsequent distribution onto the household AC grid to feed household appliance load.

In the case of a grid-connected Quattro with dual DC connections we could have this scenario based on the Quattro's existing dual AC input and output functionality:
- AC1 input (utility grid connection)
- AC2 input (generator connection)
- AC3 output (to AC grid - using battery OR solar)
- AC4 output (to AC grid - using utility OR generator)
- DC1 input/output (battery)
- DC2 input (solar)

In this scenario the battery inverter/charger would decide how to distribute the DC2 (solar) input between DC1 (battery) and AC3 output to the household AC grid.

As interesting as this all is, that is not how it works anywhere, so can we keep the topic to be Victron related as it is presently drifting off theme and heading towards being closed. Thanks.

This is the question - WHY don't battery inverter/chargers have dual DC connections?

IF they did, there would be no need to complicate a battery based grid design with separate AC Coupled solar inverters and cabling.

Most people seem to use either ABB or Fronus PV inverters for the AC Coupled design. Victron could make this redundant by modifying their Quattro and Multiplus series.