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I have a Multiplus, Cerbo and a custom BMS connected to 4 Tesla Model S modules that I'm currently bench testing. I wanted to validate that the Multiplus would still disable the output if the BMS for some reason failed to reduce the discharge current at the low voltage level. The battery voltage is currently displaying at 43.78V and the Multiplus has been configured to cut off at 43V, however it is already displaying an alarm and has cut off.
Can anyone shed any light on this please?
Check all the battery cable connections. There may be some voltage loss between the batteries and the multi. Do you have DVCC enabled? Is the "custom bms" communicating with the cerbo? If not, you will want to install a smart shunt or BMV so you can use DVCC so the multi has a more accurate battery measurement.
In addition to @derrick thomas answer...
Possibly also the voltage was that low with the systen load, and now with the load off it has 'recovered' a bit.
I do have DVCC enabled and the BMS is successfully sending data over the CAN bus to the Cerbo which is what's displaying on the UI. This is what I don't understand - the value showing on the Cerbo is higher than the value I configured into the Multiplus. Is there any way of reading out the low voltage cut-off on the Cerbo so I can validate it's configured correctly?
There's almost no load on the system so I don't think it's due to a high load voltage drop
@bjpirt It would be helpful to have a better understanding of your system. What size and length cables do you have between the batteries and the Multiplus? What is the reason for the cutoff set to 43v, noting the Tesla Battery modules are 22.2v nominal, not 24v nominal like normal 24v batteries and therefore the tesla modules can operates down to a lower voltage.
The high and voltages are not the final settings, they're just being set at the moment to test the BMS and make sure it's correctly setting the DVCC parameters to control the Victron. It's primarily to avoid having to actually fully charge and discharge the whole module to test the BMS behaviour at the configured limits. I wanted to set the Multiplus cutoff slightly beyond these so that it acts as a backup.
The setup is using 4xTesla modules in a 2x2 48V arrangement. I'm only testing with the smallest Victron Multiplus in the lab, so the max current is only 6A and the cables are more than capable of handling that - when it's fully installed on-site it will be using the 5kVA Multiplus and the cables will be sized accordingly. What puzzles me is that the voltages being displayed on the Cerbo don't seem to correlate to the voltages I've configured into the Multiplus.
The BMS is here in case you're interested: https://github.com/bjpirt/pyBMS - it runs on an ESP32 using low cost hardware modules.
I ended up making up my own 'BMS' with basic under / over voltage protection and interlocked with the Multiplus using a 2 wire interlock.
Are you using ESS?
If yes, the voltages set in ESS Assistant in Multiplus are active.
I'm not currently using ESS - there's no ESS Assistant configured in Multiplus. Should I be? I thought using DVCC was enough but am happy to take advice. The system is fully off-grid so I understood this meant it should not be used.
I asked because you did not mention if you are using ESS or not.
Why it matters in this case? Because when using ESS, the Dynamic cut-off voltages set in the ESS Assistant are the ones followed.
For an off-grid system, ESS should not be used (but technically you can, if you want; it will behave like an on-grid system during a grid failure).
Depending on what that BMS can do, one possibility is that it shuts down the inverter because the voltage of one cell drops too low (even if the battery voltage is still above the minimum limit you set).
