My setup: Multiplus II 48/5000, with 2x MPPT 150/35 and AC-out coupled PV. 330Ah 48V LFP battery with CAN-Bus coupled BMS.
When the battery is full the charge voltage is changed from 55.2V (for bulk & absorption) to 53.6V (for float).
At that point the Victron ecosystem very quickly curtails all PV and dumps power from the battery until the new target voltage is reached.
Issue: when lots of AC loads are running that were mostly supplied by the AC coupled PV inverter, all of a sudden those loads need to be supplied by the Multiplus, which ramps up output.
In the attached screenshot this is illustrated. At that time AC loads were about 3kW, and very suddenly the Multiplus used 60A from the battery.
While that hasn’t happened yet, this same scenario can easily lead to a completely unnecessary overload.
Is there any way to slow down this dumping of power? Essentially: is there a way to slow down the frequency shift of the Multiplus that is used to curtail the AC-out coupled PV?
There is a very different situation to consider. When a big load is turned off and the battery is close to full, the frequency shift, and thus the curtailment of the AC-out coupled PV has to be as quick as possible to avoid over-charging the battery.
It’s only when the load is unchanged and the battery target voltage is lowered, there is no rush at all.
Yes, but the difference between float and absorb voltages probably only represents less then 1% SoC, and once at float voltage the mppt’s kick in again.
It’s not the reduction in available battery capacity that concerns me, it’s the fact that the Multiplus has to work very hard for no reason. The same drop could be stretched out over 1/2 hour instead of 5 minutes. In fact, that’s what is happening if there is very little load. Attached is today’s change from absorption to float.
An EV can very happily charge for hours at a time using 5kW, if it gets 2kW from the AC coupled PV and 3kW from the DC coupled PV via the Multiplus.
That still leaves enough spare DC current to slowly top up the battery. Then, at 100% SOC, the battery shifts to float voltage and all PV gets turned off in a rush and the Multiplus is forced to supply 5.5kW (5kW for the EV, 0.5kW for the remaining AC loads).
On a hot day that would very much be an instant overload.
I have an offgrid system. I first tried pv assistant, but that assistant did not work properly. It instantly increased frequency to 53Hz, effectively turning off the AC-out coupled PV inverter. Google search results suggested that might happen in a battery over-voltage scenario, but battery voltage was well below the maximum.
After double checking all settings and not finding a solution I tried ESS assistant and that has been working flawlessly with exactly the same settings for the inverter frequency response.
So ESS assistant. With nothing connected to AC-in. I don’t see a downside.
