I am running an off-grid installation that has PV connected in 2 places: directly to the battery via Victron MPPT controllers, and also AC coupled on AC out of the Multiplus 2.
I have an EV charger that I can instruct via MQTT how much power to use. I have Home Assistant, which means I can use any info available via MQTT from the Cerbo.
I want to come up with a way to make sure no PV is curtailed when the EV charger is connected. Meaning: I want to divert all surplus energy to the car.
For the AC coupled PV I believe I can simply use the AC frequency. My SMA PV inverter reduces output linearly between 51Hz and 52Hz. So essentially: if the Multi sets the frequency over 51Hz, I know that I can use more power to charge the EV.
How can I know that the PV connected via MPPT has been curtailed? How does the Cerbo communicate to the MPPT charger to lower output? Is there an MQTT topic that I can use? I have looked, but found nothing obvious.
Would it work to go on the battery current? Try to have a small, set current flow into the battery and regulate the EV charger based on that?
That is how solutions like evcc work when on-grid, where you use the grid current – if there is current exported, increase the EV current, and vice versa.
It’s complicated unfortunately. Knowing when DVCC decides to spill energy would be by far the best solution.
Take the scenario: on a really good sunny summer day the battery can be fully charged at 9:30 in the morning. At that time battery current will be minimal, oscillating between maybe 5A in and 5A out every few seconds. The same could happen earlier in the day, when the battery is still quite empty, but solar just happens to balance with loads. Or later in the afternoon, when the battery is full, but again solar is only just enough to cover the loads. Or anytime really, on a cloudy day when solar just balances with the loads.
Now somewhere in the Victron ecosystem, probably in the DVCC logic in the Cerbo, the system must know whether current is limited to prevent the battery from over-charging or whether there just isn’t more current available at the moment.
Or is everything really just regulated ‘naturally’ by specifying the target voltage for the battery, and each component (MPPT trackers and Multiplus) just does its own bit to get there?
Documenting my solution, in case someone else needs similar in future.
Unfortunately it turned into some rather complicated logic. But seems to work quite well now. Admittedly I’ve only tested it for a couple of days so far.
Increase power for EVSE when:
Multiplus frequency is over 51Hz
MPPT charger is in MPPT Mode 1 (absoption)
Battery charge current is above some threshold for some time (i.e. above 10A on average for 5 minutes)
Decrease power sent to EVSE when:
Battery is discharging for some time (i.e. current averages below 0 for 5 minutes)
Battery SOC is lower than a given threshold and battery charge current is low (i.e. SOC below 95% and charge current lower than 5A)
Both increases and decreases are in small steps, i.e. 50W at a time, and with some time in between to allow for system components to react. I.e. every 30 seconds.
Essentially: I don’t want to wildly change charge current (and even less so stop and start charging), I prefer to use the home battery as a buffer to even out small fluctuations. At the same time I want to ensure the home battery is reasonably full at sun-down.