The system is under BMS control. The battery SOC is 100%, so the BMS has disabled charging. Excess DC-coupled PV energy is supposed to be fed into the grid; however, the MPPT charger is in limiting mode. Is this a bug?
What is the notification on the GX?
Do you have an input current limit?
Hello,
Thanks for answer.
You mean this current limit? It does not matter if I change it to 25, 32, etc.
There are several reasons the MPPT is limited and feed-in is not at full.
- When SOC is 100%, the current generated by MPPTs should be somehow kept under the limit the batteries can absorb in case the feed-in stops suddenly. I believe that is why the MPPT is in mode 1.
- Also, when feed-in, the voltage the inverter generates should be a tad more than the grid voltage in order for the feed-in to happen. If the grid voltage is high enough, also the feed-in is / should be limited. Hence the MPPT in mode 1.
Also, a combination of those two could happen. If the grid voltage increases outside your control, the feed-in is suddenly reduced and the excess DC could generate for a short period a slight higher DC voltage and as a consequence those warnings.
Today at the same situation (battery full, plenty of sun) MPPT charger is not limited and system feeds excess PV into grid. Is there a possibility to find reason why MPPT charger enters limiting mode?
Yes, but I hope you realize that such question is difficult to answer remotely, without knowing the full system status and configurationâŚ
Analyze with dbus-spy the electrical status of the system.
Meaning settings, currents, limits, states, etc.
Look there when MPPT op mode is switching between 1 and 2 and look side by side whatâs the variablesâ evolution, hence the reasonâŚ
Hi @kbb,
This is almost always caused when a 3rd party battery BMS sets the Charge Current Limit (CCL) to 0A when the battery is at 100% SOC.
This goes against the directions of the Victron BMS-Can specification.
The specification requires that when the battery wants the system to reduce the charge current, the BMS should reduce the Charge Voltage Limit (CVL) parameter instead. That has the effect of allowing the system to reduce the current flowing to the battery, but much more efficiently.
That also allows the Victron DVCC system to add an offset Voltage to the MPPT, which allows solar production to continue, and the MultiPlus to export the surplus.
This can usually be corrected with a firmware update to the battery BMS once the battery manufacturer understands the specification.
We clearly explain this exact situation in the BMS specification, and describe that this is the effect, so it usually occurs when a battery manufacturer has not read the specification, nor completed the testing protocol.
I would suggest you get in touch with your battery supplier. The Victron BMS-Can specification is available by request of a Victron Sales Manager for their region.
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Thank you so much for detailed explanation.
I contacted Basen Green (battery and bms supplier) with request for comment.
Hi @guystewart,
At the same time. SOC is 100% now, CCL is 0. Iâve disabled DVCC in Cerbo momentarily and enabled it back. MPPT charger became active and supplies excess into grid.
Hi @kbb,
To give some more back story as to why this happens.
When Victron was first developing our managed battery integrations years ago with DVCC and BMS-Can there were very few âmatureâ BMS systems on the market.
Two major brands were BYD and Pylontech. As these systems rolled out into the market there were lots of things we all learned. Some of the changes needed to be made by the battery manufacturer, and some were just faster for us to make in our Venus firmware.
As an example of one such feature, the CVL override in the DVCC menu that allows you to reduce the charge voltage limit target even lower than what the battery specifies when there is a cell imbalance, so you can slowly trickle charge the battery without triggering overcurrent warnings.
Another one of these issues was what youâre experiencing now with allowing MPPT export to grid.
To solve this problem immediately we wrote a special custom work around just for Pylontech batteries that bypasses that charge current limit setting and allows export.
This was a once off exception, and the permanent and sustainable fix was made to our BMS-Can battery specification so the issue could be resolved âproperlyâ by the battery BMS instead.
The issue today is that many BMS manufacturers âborrowedâ the behaviour of the Pylontech BMS rather than use the specification directly, but then arenât identified as Pylontech so they donât get the exception code.
It is not possible for Victron to make exceptions for all batteries, and the point of the specification is that we donât have to. The best learning from nearly a decade of managing lithium batteries is in there, and when properly implemented by the battery manufacturers it works great and doesnât require any extra effort from Victron except reading their test report and then adding them to the compatibility list.
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Yes, story is understandable and correct. People must follow guides and standards.
But question was why MPPT charger became active after toggling DVCC?
How long does it remain that way before it stops exporting?
As you can see from this screenshot, for about an hour, starting at ~16:00
And it does not stop export completely, it throttles output to some level, as also visible on the screenshot.
What Iâve observed from my systemâŚ
When you restart the system, up until itâs running again, it starts to consume from battery and battery voltage drops way below CVL.
Then, when starting to export at full power, little âenergyâ remains to top up the battery to CVL, so this âslow chargeâ like takes about an hour, depending on the battery size and PV capacity.
Then it remains / drops at about C/4 battery current, another fix for battery quirks, Guy has talked about.
But I may be wrong, I didnât studied line by line the DVCC, and if so, I stand correctlyâŚ