Design Advice for ESS without Solar

DIY
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Hi All,

I’m looking for a detailed guide or tutorial for setting up a MultiPlus-II GX 48/5000 (installation and configuration) for a home battery setup without solar on a single phase domestic property in the UK. I’ve seen some other similar and helpful posts, but non seemed to answer my questions. I’ve included a lot of my design and research below for anyone else considering this setup.

I want to make the most of the Intelligent Octopus Go; set the MPII to charge when the EV is charging, usually between 1130 and 0530, and discharge to feed the house during the day. There’s not real need for UPS, but that’s a nice to have. I’m not planning on exporting back to the grid.

I think I’m most of the way through designing the system, but there’s a few final bits I can’t find definitive answers for.

The basic system is going to be installed in a separate garage which has a single radial feed from the main house (and grid). I have a couple of Cat6 cables run too, one is for network / internet, the other is reserved for sense cables from the main grid in. This comes into a sub-main consumer unit in the garage. This has all been installed by an electrician who’s going to be finishing the end of this install and signing it off, he’s done the maths on cable diameters to meet regs.

I’m planning on going with a Fogstar 16.1kWh battery [1] which uses a Pace BMS (supports Victron Inverters via Victron Protocol or Pylontech Protocol[2]). Communication is over a VE.Can to CAN-bus BMS[3] Type B cable (Source: Fogstar Manual[4]). The DC side is pretty straight forward.

The Garage CU will connect to the MPII and to the EV Charger.

I haven’t found a definitive answer whether AC-IN on the MPII is bi-directional, there are a couple of comments[5] in various[6] places[7] that say it is, but the installation guide[8] seems to say that it can only be used as pass-through for grid.

As I want my entire domestic supply to be primarily served by the MPII (with grid backup in case of occasional >4000W load), do I need to connect AC-in and AC-out-1 or can AC-in provide the output from the battery?

I want to ensure that when the EV Charger is drawing load, the MPII is set to charge the batteries. Most of this is going to be done via scheduling and I know I could automate this through Home Assistant but I would ideally like something more tightly coupled. I haven’t been able to find an configuration guide on how to use a CT Clamp on the EVC radial to set the MPII to charge the batteries. Would this CT sensor be connected to one of the AUX sensor ports or the current sense port? Where can I find a configuration guide for this?

Does the rest of the design make sense? I can’t seem to find many guides or designs that don’t include solar and just rely on charging from grid at cheap rates.

Additional links as text because “An error occurred: Sorry, new users can only put 2 links in a post.”

  1. www.fogstar .co.uk/collections/server-rack-batteries/products/fogstar-energy-16kwh-48v-solar-battery
  2. www.fogstar .co.uk/pages/fogstar-energy-esr51-2-supported-inverters
  3. www.victronenergy .com/cables/ve-can-to-can-bus-bms
  4. cdn.shopify .com/s/files/1/1347/0997/files/FogstarEnergy_16.1kWh_Battery_compressed.pdf?v=1737370967
  5. communityarchive.victronenergy .com/questions/9317/multiplus-ii-ac-supply.html
  6. communityarchive.victronenergy .com/questions/239761/multiplus-ii-as-a-home-battery-for-load-shifting.html#answer-279144
  7. community.victronenergy .com/t/connection-question-on-the-multiplus-ii-in-a-ess-setup/15205/2
  8. www.victronenergy .com/media/pg/MultiPlus-II_GX/en/installation.html#UUID-b8310945-a1c7-2bfd-5f57-c494fa312015

Thanks!

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I can answer your question about the AC-IN being bidirectional. Yes it is. For The rest. Please read the guidelines. There you’ll find the text: “ Not really the place for System Design” and directs you to a Victron reseller for system design.

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If you want backup power when grid fails, then you need to connect everything you want to power from battery to ac out 1

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Hi, suggest you sign up on professional.victronenergy.com and do the free video training.
It will fill in a lot of gaps for you.
Also recommend using a capable supplier and not buying online, as the latter will provide little in the way of support.

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@purpleriver a single radial? What square are we talking as multi has potential to draw 50amps all by itself. As said by @ChiefSolar AC in is bidirectional.

But if you want to cover backup then the multi has to interrupt everything that needs to be backed up and handle it through ac1 out as @DuivertNL mentioned. Ie grid feed into the multi then multi feed to the board, as you want the multi in the garage the setup is similar to mine but I used 5 core 25m2 to reroute my main to the garage, into that CU and out of the multi via the rest of the 5core for whole house backup.

As for using the multi to look at the charger and start charging could possibly be done through NodRed but how no idea, it’s just easier through HA, plus the things you clamp with the Victron system it starts to want to cover the loads ie your charger will still draw power at idle so the Victron system could start charging, this stuff is better done at API level.

The video’s that @nickdb mentioned are worth there weight in gold and would give better insight in the configuration of the equipment and it’s free.

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You could trick the multi using ESS. If you put an energy meter on the house loads it will cover those. If you put a meter on the car charger and assign to PV invter and have it positive when its drawing power it will think there is excess solar and start charging. You will have to seperate the loads correctly. I have played around with something similar when I was setting a series connected ESS with 2 small ESS downstream of a larger 5000VA 48V system.

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Very dirty way, ie non of the VRM data would make sense for solar, if OP did get solar at a later date it’s going to be in the solar data and at idle the charger is probably going to use 4 to 6watts possibly more is the system now seeing this as now external energy in and start charging the batteries when intelligent dispatch signal has not been sent from Octopus to which will cause OP to be using peak rate.

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You mean its a very smart way. They do not want solar just to charge when the EV is running.

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Hi,

I’ve installed exact system like you described over a year ago, and it is fairly easy to do. The system is running flawlessly since day one. There is no UPS function needed so the Multi has just AC IN connected. With ESS installed and schedule charging time, you don’t need to use Home Assistant or anything else. The Victron system is perfectly capable to do what you need on its own.
The house is on British Gas EV tariff so the system is scheduled to charge batteries between 0:00 to 5:00am, the car is charging at the same time. During the on-peak hours ESS system is keeping grid load at around 50W. If you turn on load Multi will provide power for up to around 4.8KW… if you exceed it’s capacity let say you will turn 7KW load… Multi will provide its max (let say 4.5KW for easy calculation) and the rest 2.5KW will come from grid.
The only think that I’m not happy with is that the system with grid meter is quite slowly reacting to a sudden big load changes. If you turn 4KW load it will take about 15-20s before Multi will balance the grid load to 50W (so at start you will be pulling power from grid) and then when you turn it off it will export to grid for another 10-15s before it will balance grid load to 50W again. There is a way to improve this (by using CT clamp for example, but it is not always convenient/possible to do so).
Over all during on-peak hours the system is running 90% of the time from the batteries and saved customer just under £2000 in the electricity bill over the last 12 months, so well worth doing.

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I was curious. I had a look at mine, and it seems pretty quick. I have a pressure cooker running ATM and the rest of the house. I turned the oven and the air fryer on, that’s about 1500+3500W, and I have 2x 1200W ESS that will cut in as well. They were both full noise at 1200W. It did take about 15 seconds to stabilise the initial ramp to offset the load very quickly. I consume about 0.73kwh a day, looking at my bill.

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Octopus Intelligent is a dynamic profile yes it has its charging window but I assume OP also wants to utilise The other hours you get on intelligence not just a schedule, if OP wants the full benefits of the tariff, to do that you need to know when intelligent dispatch has been sent, you could monitor charger as @owenb79 said but that draws power continuously so will the batteries start charging?

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The Octopus Intelligent is a dynamic profile, and Victron won’t use it, but I think the charger will. You can spoof the Victron ESS to charge. One thing I did try was 2x AC out coupled ESS to charge when the 5000VA inverter was charging by simply using an energy meter on the input of the multi. When it was charging, that means there was excess solar on the grid tied inverters. As the AC output increased, the charger on the 5000VA decreased until it was all balanced. It was pretty cool to watch. You can do whatever you want with the Victron system. I push what the gear can and can’t do pretty far. It’s not in use, I now I have a large MPPT and dont use much excess solar to charge.

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Thanks for all the advice so far!

I’ve signed up for the professional site, watched a few webinars and read a few more of the guides.

AC-In is bi-directional unless the grid fails (which thankfully is extremely infrequently in South Wales), so I should be fine for 99% of the time on AC-in, I think I’ll wire AC-Out 1 as well unless there’s a good reason not to.

I still haven’t been able to track down a clear wiring and configuration guide for the Multiplus-II GX for an ESS system. Specifically the wiring of an CT sensors or energy meter.
Is a CT meter enough for monitoring grid load or do I need an ET112? If I need the ET112, can I combine the sense cables into one Cat6 cable for the EVC sense?
The Multiplus-II GX Manual’s Appendix seems to show that you’d connect the CT Sense cables to the External Current Sensor (K). If so, what can the Aux Analog inputs be used for?

I think I’m going to mostly rely on scheduling / home assistant for automations, but I’d prefer low-tech / more tightly coupled systems.

This is my current plan.

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Other questions from the thread:

Unfortunately, I can’t have solar as we have a small south facing roof area, as property is mid terrace with a gable front end. Garage roof is too weak to support solar. And we’re in a conservation area with an article 4 directive so would need full planning permission, which is a lot of faff for a very small usable solar area.

@Daza The single radial is a 10mm2 armoured. Qualified electrician as done the maths, details are in the diagram.

@Mysik thanks for the validation! My calculated savings are between £850 and £1200 per year. Which, with this solution is 4 - 6.5 years RoI. The last time I looked into it the RoI was >10yrs (that was a while ago). I’ve had a few quotes from managed installers and they come in at almost double the cost.

Later in the year (when PHEV arrives) I’ll go to Intelligent Octopus Go. IOG makes the scheduling a lot easier, but there’s always the temptation to use Agile and play with the half-hourly rates and Victron’s DESS system (which does support Octopus)

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Not clear in your schematic, but you can not connect ac in and out on the multiplus together!!

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If you use something like a Shelly EM, you can use Node-RED to check if the power is > a certain amount of watts and run an automation. I think changing the state to “keep batteries charged” will charge the battery. But everything will be connected to the grid. Nothing will be on the AC out of the multiplus unless you want to set up a UPS load. It should be easy enough to do.

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thanks @owenb79 and @DuivertNL, that makes sense. To confirm, for a UPS load I would need a separate radial back to the house from AC-Out. AC-In and AC-Out can’t run in parallel along the same radial.

I thought that sounded odd, but the wording in the manual confused me.

As nice as a full-house backup would be, I think I’ll drop the usage of the AC-Out as the house circuits are not setup in a way where the critical loads could be easily supported in the event of a power outage.

Out of curiosity, why can the Multiplus not continue to provide an output via AC-In in the event of a grid failure?

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If it would feed back on ac in during grid failure, you are going to feed everyone on grid, including electrocuting grid mechanics that are fixing the grid failure :blush:

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Thanks again for all the advice, I think I’ve answered all the questions I need to now and the latest planed design is below.

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Answers

Q: Is a CT Meter enough or do I need an ET112?
A: A Grid Meter is required for an ESS when there is any AC load between the grid connection point and the input of the MPII (Source: Grid Meter subtitle)

Q: How does the ET112 connect to the MPII GX.
A: It uses an RS485 to USB interface that must be less than 20cm. You can extend it using the twisted pair conductors in a Cat5/6 Cable. (Source). The other conductors in the Cat5/6 cable are unused so can be used for another purpose, in my case the required CT Clamp for the EVC

Q: Can I use the Multiplus-II as a UPS device.
A: Yes, but it requires a separate circuit that is not connected to the grid, so in my situation that would not be possible

Q: Is there an easy way to tightly couple the MPII to the EVC so that when the EVC is charging the MPII switches to charging mode.
A: No, there is no direct way to do this, but it is easy to achieve in software, either Home Assistant or Node-RED. Add a couple of Shelly EMs for extra inputs into Home Assistant.

Q: What is the External Voltage Sense for? Can I use it for reading data from a Grid Meter?
A: No, it is for compensating possible cable losses during charging (source). Note: There’s a typo in the manual here, it should be Terminal K not Terminal J

Q: What are the Aux inputs for?
A: I’m still a little uncertain on their full capabilities but they are commonly using with the PowerControl and PowerAssist functions

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@purpleriver

Have look into EVCC. It’s controlling your EV charger and seem to be integrating with Victron.