For the past three sunny days I am experiencing strange DESS behaviour. I am on dynamic tarif plan “Zonneplan” in the Netherlands.
Current behaviour:
During the cheap tariff window, DESS correctly initiates grid charging as per the dess schedule to fill the batteries. However, during this same period, solar PV output is being throttled down unnecessarily, even though the available solar power could contribute to the charging demand. This results in wasted solar energy and unnecessary grid consumption.
Expected behaviour:
When DESS activates grid charging during cheap tariff hours according to schedule, the system should use all available solar power first and supplement with grid power only for the remaining charging demand. Solar PV output should never be throttled while the system is simultaneously importing from the grid to charge the batteries.
The problem is 100% in dess because when I switch off DESS (during anomaly) and set the minimum SOC to 100%, loading continues and PV output is 100% utilized. (as should be)
There were NO clouds, grid voltage was 234V during anomaly
My system configuration is:
3x Multi RS Solar (v1.28)
1x Cerbo GX mk2 (v3.70)
6x Pylontech US5000
Welcome Bert. I am sure there are other owners with similar problems but either not knowledgeable on how to solve them, or too busy already managing their own systems complexities. Usually solving DESS behaviour boils down to getting very intimately knowledgeable about your own setup first, until you hit specific technical issues we might be able to help you with, without needing to study your system from scratch.
DESS settings look fine. Kinda conservative price amounts, but I guess thats on purpose. You might try to remove the negative offset, using the negative offset will cause the price to go down below 0 sooner (and export will be limited then). If you really want that huge price difference before trading starts just add the 4 cents to the battery costs.
It will only trade if (sellprice * efficiency) > (buyprice + battery costs)
But with ESS settings, I mean the settings on the GX device. Peak shaving, Export limiting, stuff like that.
Yes the big offset is on purpose and useally works well for me.
I already tried getting rid of the negative offset and it didn’t work. But to be fair, even with the offset the price would not have become negative anyways.
I will attach some screenshots regarding ESS but there is not much info there since most of the stuff is already in the Multi RS (not like the older Multiplus II firmware)
Like I wrote in the original post.
During charging (planned via DESS) the PV output is being throttled which is wrong IMO.
When I switch off DESS and just put in a higher minimum SOC, charging continues but now with maximum PV output (not throttled).
I am not aware of a switch or setting where you can give “priority” to grid charging over PV output, if so I am very happy to learn where it is.
The Multi RS has a max output of 100A. If for example DESS decides to charge at a sunny day with 13.8kW from the grid (thats rougly 95A per Multi RS) and my PV output is 3kW, thats another roughly 60A. One of the two has to be throttled. So I would assume (prices NOT negative) with DESS it decides to throttle the grid import and use max PV output, but it was not. And I think this is a wrong setting, either by my mistake or by a DESS bug.
When I checked a victron facebook group I saw at least 1 other user reporting the same behaviour.
I dare to say I am quite knowledgable with my own system setup, built it entirely myself.
I just wanted to check if other user were experiencing the same issue I noticed. If so, it might be worth reporting to Victron.
Also happy to learn if I made a mistake somewere with my setup.
It’s not so much about mistakes as it is about tailoring and tuning a system to a specific use-case. There are so many options that it is practically impossible to provide help on functional level question (Expected system to do A but system does B) without intimate knowledge of the several levels of detail below. A well behaving system is the end-result of many settings and compromises sometimes, there are no one-shot solutions.
That said, based an a handful of assumptions (such as your Multi RS having higher roundtrip efficiency than our Multiplus-II) I would offer the following starting points:
Try never to use the basic power and capacity settings to modify DESS behavior directly. It introduces systematic misalignments that tend to make it harder to get things right instead of the other way around.
For Lithium batteries, make sure all MP-II/Multi-RS/BMV-Smartshunt charge/discharge efficiency settings are set to 100% (maybe someday 99%). Especially the BMV is very good at coulomb counting (consumedAh) and lithium batteries have near 100% COULOMB efficiency. Which is not to say they are 100% efficient but that is counting energy (kWh) not electrons/lithium ions (Ah) and will therefore reflect energy losses due to internal resistance. Coulomb counting is not affected by internal resistance! Do not set any other compensation settings either, such as Peukert, meant for Lead-acid batteries, keep at 100%. All you need for DESS is accurate coulomb counting over the operating range, matched to actual energy capacity, again over that operating range.
Check (and set via CLI or Node-RED) you roundtrip (ENERGY: kWh) efficiency. For DESS I expect it to default to 90%. For a MP-II I would advice to start conservative on the lower end of 80% to 85% range and if that results in DESS missing trade opportunities first lower battery cost all the way down to 0ct. At a later time and based on measured (better) roundtrip efficiency values it always be tuned to actuals better. For a Multi RS that range would probably be 85% to 90%.
Make sure you measure your battery capacity at least once for the exact SoC operating range you intend to use the battery. Say you want to run 25% - 100%, it is utmost critical to measure (on the DC/battery side) the energy required to charge from 25% to 100% and to discharge from 100% to 25% and to make sure the set battery capacity accordingly to (1/0.75=) 1.33 * the average of the charge discharge energy for a 75% SoC change. The DESS scheduler (VRM Cloud) is quite well equipped to correct for energy losses based on the roundtrip efficiency, but only when the basic settings reflect actual conditions as close as possible.
Set the DESS AC (grid) power (import/export) limits as close as possible to what your system can maximally achieve under real life conditions. So the maximum AC feed-in power you can detect under no load no solar conditions, and the maximum AC power draw during maximum charging at a near full (95%) battery, again under no load no solar conditions.
Set the DESS Battery power limits as high as possible as supported by your batteries / BMS. Generally speaking (much) higher then the DESS Grid power limits.
All that just for starters, and then the real tuning starts
that exact situation happens, when the scheduler sends a 100% target soc while the battery reports a 0 CCL. Then, instructions send to the RS “colide” and the RS wrongly throttles solar rather than keep feeding in.
A fix is beeing worked on already.
(So, not a wrong setting or something you could easily workaround unfortunately)
