The Discharge-Disabled SoC Threshold Parameter in the BMS
There is a parameter in the BMS called "Discharge-Disabled SoC Threshold" that can be found under the Configuration->CANbus page. It is enabled by default (with a default SoC threshold of 10%).
This function can be disabled if desired. However, this feature must be enabled for all AS4777 compliant MultPlus/MultiGrid-II based Victron Energy hardware used on the Australian grid.
This is to avoid the potential for a condition under which the system can otherwise wind up repeatedly starting up and shutting down (over and over).
Background: The Victron AS4777 Grid Relay Test
There is a specific (mandatory) hardware test performed by a Victron MultiPlus-II inverter/charger when operating in the AS4777 grid code, called the grid-disconnect relay test.
This test involves a short term disconnection of the incoming AC grid power to the inverter while the grid-disconnect (grid anti-islanding) relays inside the unit are tested.
During this test period, the MultiPlus-II based system has to support any 'downstream' customer load entirely on battery power alone (just as if the grid had failed). The site has to be supported on battery power alone for a period of up to two minutes, before the AC grid energy is again able to be used as required by the site.
At the start of the test period, the grid relays click repeatedly on and off and the MultiPlus enters "Inverter" mode (visible as such on the front panel LEDs), and the site is then supported on battery alone, before returning to the normal grid-interactive mode at the completion of the test period.
The MultiPlus-II must undertake this AS4777 grid-disconnect relay test after each reboot/restart of the inverter for any reason (including total loss of system power).
The unit starts the test when it believes there is enough energy in the battery system to support the site loads while the test is running.
If this test happens too early - before there is really enough energy in the battery to support the site loads - then the act of running this grid-relay test can result in the site power collapsing.
Because a collapse of all site power results in a subsequent MultiPlus-II reboot, the test has to be run again - creating the potential for an endless cycle of reboot-test-shutdown-reboot (and so on).
Ensuring the reliable completion of the relay test
The relay test starts when the battery appears to have energy available. By default, that decision is made based on battery voltage alone in a manner that can result in the relay test happening prematurely in ZCell installations (leading to the issue noted above).
To ensure that the relay test does not happen prematurely, the BMS enters a special operating mode any time it observes the ZCell system state of charge (SoC) is 0%. At this time the BMS starts sending the Victron system a discharge current limit (DCL) value of 0 (meaning: 'discharge not permitted').
As a consequence of this value being sent as zero, the AS4777 post-reboot relay test is deferred by the MultiPlus-II - regardless of battery voltage - until the first time that the SoC rises above the threshold configured into the BMS.
At that point (when the SoC has risen above the threshold value set), the BMS unlocks (permits) battery discharge, by sending DCL values above zero.
In response to this event, the MultiPlus immediately commences the relay test. It can complete the test safely because there is plenty of energy in the ZCell.
The system then continues to operate normally, with battery energy being able to be accessed as required by the energy system, all the way back down to zero.
When the SoC reaches exactly zero, the cycle noted above starts again.
AC Grid Failure during the initial recharge period after 0% SoC
Should there be a genuine AC grid failure during the period when the ZCell SoC is first rising from 0% to (default) 10%, and when the discharge limit is locked down to 0 by the BMS, then the inverter/charger will not attempt to support the site loads - and will instead power straight down in the case of AC grid failure as it is not permitted to access battery energy at that time.
Once the SoC has exceeded the configured threshold (default: 10%) for the first time, normal system operation resumes - and the site can be again be supported by battery energy if there is an AC grid failure.
Single ZCell Systems Cannot Provide 24x7 AC grid power backup
It is important to appreciate - as a system design issue - that a single-ZCell system cannot provide 24x7 AC grid failure protection (regardless of configured settings).
Each time a single-ZCell system undertakes a battery discharge-and-maintenance cycle, the site is not protected from AC power outages until the maintenance cycle completes and enough energy is subsequently charged back into the battery.
Where sites are set up to only charge the battery using excess solar energy (when solar power exists in excess of site loads), this subsequent recharge may not happen until substantially later (hours later, or even days later in some extreme cases - depending on solar weather vs site loads).
If 24x7 coverage is required for potential AC grid failures, regardless of weather, then the site must have the Victron ESS setting called "MinSoC" set to a value above zero. This results in grid energy being used (as needed) to keep a specified minimum amount of energy in the battery system in case the grid fails.
However, MinSoC can only be set to a value above 0 on a site with more than one ZCell installed.
That's because the automatic grid-recharge conflicts with the need for the ZCell to be completely discharged ahead of its periodic maintenance cycle.
Accordingly, multiple ZCells must be present if the site must have 24x7 'UPS style' protection against AC grid failure. A multiple ZCell site is managed by the BMS so that the maintenance cycles happen sequentially, not concurrently - and hence so there is always at least one ZCell that can be grid-charged as needed in order to support a nonzero value for "MinSoC" at all times.
A single ZCell site can still opportunistically protect against grid failure if there happens to be energy in the ZCell at the time - it is just that there will always be times when the ZCell is empty, as a normal consequence of the operation of the device - and when the site will not be capable of being protected against AC grid failures.