Systems with Lithium batteries communicating to a GX device via CANbus with DVCC enabled.
In a ESS application there is a min SOC setting which stops the inverters discharging when batteries reach desired low level. Works well.
However in backup or Island-mode e.g. during a Grid power outage or indeed In any off-grid system application is there a setting to disable / shutdown / turn off the inverters from providing power to critical loads once a preset SOC level is reached?
To protect batteries from getting deeply discharged to the discharge floor.
E.g. when Pylontech SOC reaches 10% the Multiplus Inverters turn AC out off and auto turn back on once a reconnect SOC is reached. Could be a 10% hysteresis and output is re-enabled once the Solar MPPYs have partially recharged the battery bank.
This would be common settings in any other hybrid Inverter. Surely this has to be possible via the Cerbo GX?
This wouldnt be accurate or reliable. Same with Voltage based control, doesn’t work well with lithium batteries, factoring in voltage drops, different load scenarios etc. would be had to determine a preset static voltage level to set as a shutdown.
In some systems if both AC outputs aren’t needed have used AC-Out 2 for general loads, which can be controlled based on SOC due to the internal relay. Of course still have a small discharge load the inverters quiescent load once load disabled.
Another idea thinking of is to use the relay in the Cerbo, programmed based on SOC to control the inverters remote on/off switch via a 2 wire signal?
As SOC is a secondary value - calculated not measured- for a battery, it can inherently be in error due to integration of small errors in current measurement. This si particularly true at the low end, as SOC is syncronised to 100% at the high end. Any decision to turn off an inverter or load on a (Lithiuim ) battery should be based on the minimum cell voltage, or lacking this, the overall battery voltage.
With the cerbo aux relay working well based on SOC, instead of shutting down the Multiplus inverters I have decided to install a suitable modular contactor on AC-Out 1 distribution panel to disable any load circuits based on low SOC.
The benefit of this is the MPII’s continue to operate (idle or charge mode) and thus any external AC-coupled generation e.g. Fronius solar inverter also connected to AC-Out 1 continues to operate, conditions prevailing, and charges batteries alongside any MPPT Charger controllers.
The benefit of this is that AC generation can also charge the battery via the multiplus charger and thus get battery recharged and loads back operational quicker.
Downside is when there is no generation, the MPII inverters continue to consume small amounts of power (quiescent / no load) and the battery SOC will continue to fall until generation reconvenes. However with a suitable sized battery bank this consumption could be considered negligible.
I’m not convinced on Victrons recommended parameter setting for Pylontech below.
Having a Pre-alarm at normal operating voltage of 48V would be a nuisance and of no benefit.
Also the low voltage settings are too low in my opinion. No use having inverter shut down at 44V if the pylontech BMS discharge trips before this is reached (44.5V). Would be to late and system would be offline.
