Backup power with an existing MultiPlus-II: a pragmatic solution for a heat pump and well?

Other languages German community
, ,
21

So, folks,

first of all, many thanks for all the helpful comments. I’ve learnt quite a lot again.

The situation for me is simply that I started out with an SMA system in 2019. If I had known then what I know today, I would probably have built everything around Victron from the outset. But there you go — hindsight is always 20/20.

The existing technology can’t really be operated off-grid. If I want to switch to Victron, I should really do it consistently throughout. So I worked out what a complete conversion would cost.

Basement (main system)

New purchases:

  • 3 × Victron MultiPlus-II 48/8000: approx. €3,300
  • 2 × Victron SmartSolar MPPT RS 450/200: approx. €3,000
    • would replace the SMA STP 8000 and STP 9000
  • Pylontech US5000, 24 kWh (5 modules): approx. €3,900
    • the SBS 5.0 with BXD HVL 7.7 would be taken out of service
  • DC busbars, fuses, cables, etc.: approx. €1,500
  • Könner & Söhnen KS 8100E ATSR Silent diesel generator: approx. €1,500
  • Generator cable set and AC connection to the Skylla and ATSR: approx. €300
  • Victron Skylla-i 48/100 charger: approx. €1,100
  • Socomec ATyS 4-pole 125 A mains changeover switch: approx. €650

Shed (approx. 50 m away)

New purchases:

  • 2 × Victron SmartSolar MPPT 150/60 VE.Can: approx. €1,000
    • would replace the SMA STP 5000
  • additional Cerbo GX: approx. €350

Existing components from the garage:

  • Victron MultiPlus-II 48/5000
  • Felicity battery with 15 kWh

Total cost

Materials only, excluding labour and modifications:

approx. €16,600

If I work out the cost of modernising the system with current SMA technology instead, I end up even higher. I’m also not sure whether I really want the new SMA technology. These days, there’s a lot of relabelled Chinese hardware behind it, and some of the devices are heavily dependent on the cloud and the manufacturer. Would that really be a good thing in the event of a global conflict?

In the end, I would be investing around €16,600 so that the system would actually do little more in everyday use than it does today. I would have more storage, genuine off-grid capability and a considerably more robust backup-power concept. But the question is, of course, whether it’s worth that price to me at the moment.

That would be a little bit of prepperism.

If Putin’s henchmen were actually to enter a NATO country one day, I’d probably think about it differently. Hopefully there would still be time to place orders and carry out the conversion. For the much more likely scenarios — a power cut lasting several hours or a couple of days as a result of grid faults or sabotage (eco-terrorists or expendable agents) — a much simpler approach seems more sensible to me.

That’s why I’d like to return to my original plan for the time being.

In an emergency, I don’t need to supply the whole house. Essentially, the groundwater pump and the heating system are all I need.

I’ve therefore decided to start by buying a manually started 6 kW inverter generator from Könner & Söhnen for around €700.

That would give me two options:

  • Run the heat pump directly.
  • Recharge the battery.

For charging the battery, I wouldn’t use the MultiPlus, but a separate 48 V charger. I’m quite interested in the server PSU approach. If anyone has practical experience with an Eltec Flatpack2, or knows of a neat, tried-and-tested solution, I’d be very interested to hear about it.

I could also envisage disconnecting six of the 18 PV modules on the garage roof from the SMA strings and putting them on a separate Victron MPPT 150/60. This would then operate on the same DC side as the MultiPlus, the Felicity battery and the charger.

That would give me a simple backup-power solution with a generator, battery charging and a small off-grid PV component for comparatively little money.
I’ll test whether the heat pump runs on the MP2 5000; I may need to limit it to 16 A or so, but that should be possible.

The system could then be expanded gradually: whenever an SMA device fails or needs to be replaced for some other reason, I would switch to Victron equipment.

We’ll see where the journey takes us.

22

Hello,

you don’t necessarily have to get rid of the SMA technology straight away; you can also use it in combination with the Victron system, which can control it via the mains frequency when operating in island mode. Of course, that only works as long as certain conditions are met: the PV inverter output must not be too high, and the battery must not be at 100% charge or just about to shut down due to overvoltage!

I would recommend a three-phase system with an MP2 5000 for the house, but then you have to make sure that you only use around 4 kW maximum per phase, and an MP2 8000 or, if sufficient, another 5000 for the heat pump. Since you already have a 5000, you could test this with the heat pump beforehand.

If you don’t want to pay attention to the total output per phase, you need at least an 8000 or 2 × 5000 per phase!

You can also distribute the heat pump across three phases. To do this, you either need three 240/120 V transformers (Victron also makes these) that can be switched accordingly, or a suitable three-phase transformer. The power distribution would then be in the region of 2:1:1 or somewhere in between, depending on the phase shift. With this setup, three MP2 5000s could supply up to 8 kW at 230 V. However, one Multi would already have reached its maximum output and would not be able to supply anything to the house!

I wouldn’t buy Pylontech batteries today either. At least the US2000C units have a self-consumption of around 150 mA per module, and with 14 modules that adds up to about 2.4 kWh per day, which I need just for the BMS!

There are other options that are considerably better and cheaper!

In any case, a battery of the required size will cost more than a Victron system without a charge controller.

I had to replace my 24 V battery in the garage, and the battery alone cost me around €5,000 for approximately 48 kWh. I then had to assemble it myself as well, since all the parts (enclosure, batteries, BMS, display) were delivered separately. The old lead-acid battery had considerably less capacity, but was also significantly more expensive (around €8,000–9,000 for 39 kWh, of which only about 50% was usable if you didn’t want to damage the battery and wanted it to last a long time).

I use JK BMS units with the batteries that aren’t connected to the system. However, I have since installed a Pi that collects the data from the JK BMS units in the garage. That’s 6 + 2 for 24 V and 2 for 36 V, all over Bluetooth. It takes almost two minutes to go through them once. Of course, it also works faster if you query them in parallel rather than sequentially. You can also use an ESP32 with ESPHome for this if you don’t happen to have a Pi available that isn’t being used for anything else.

Bye

23

Converting everything to Victron, i.e. connecting the PV directly to the battery, is nonsense too.
Leaving some of the PV system on AC is more efficient.

With the complete conversion you described, for example, I would leave the PV on the shed as it is and have it charge the Victron system via AC.

That would really be an important point. But even at just 16 A, the 5000 MP would already be almost fully loaded, leaving you with hardly any capacity on that phase for additional loads.
The fact that the heat pump is single-phase is really the biggest obstacle with your system here.

24

Have you ever thought about charging the battery from the car’s electrical system in an absolute emergency, e.g. at 2,000 W?

Surely there’ll be some car sitting around that could be used to turn the heating on every now and then via the indirect route of DC–AC, a Huawei PSU and the like, so that nothing freezes?

25

hello,

to stop the heating system from freezing, a pump might be enough; flowing water doesn’t freeze as quickly as stagnant water. And keeping the circuit above 0 degrees would still be a better option than using a heat pump for this. The question is whether he definitely needs a heating option or just frost protection.

A generator in the house with its exhaust vented outside already provides twice as much heat output as electrical output!

bye

bye

26

@Schinderhannes

With an MP2 5000, I would replace the SMA 9000 with an RS450/200, sell the BYD battery and replace it with another 15kWh Felicity battery.
Unfortunately, you can’t connect the SMAs to AC_out, but you have the generator as a backup to supply the 10kW from the RS450/200.

Then you can charge at 270A / ~15kW. With 30kWh, that would be a good 0.5C.

With 30kWh, you can also run your heat pump (max. 4.8kW) for a while and don’t have to keep the generator running 24/7.

During the transition period in March/April, you may not even need to run the generator at all.

Normally, when the grid is available, you can ideally charge your car at 11kW using the 8+5kW SMA units, while the RS450 charges the battery at 200A with 98% efficiency.

That’s the optimal combination.

27

I also have an SMA Sunny Island and a Victron MP2 running together. The Sunny Island is the master in island mode, which is implemented via an Envitec changeover box. I implemented the load distribution in Node-RED on the Cerbo. Two PV inverters, also from SMA, are single-phase and continue feeding into the island during the day. The third is three-phase and consequently remains switched off when the grid is unavailable. The Sunny Island regulates everything via frequency shifting, while the Victron is configured to accept a mains frequency of 40–60 Hz. I have already tested this, and it runs without any problems.

28

Hi I have an Eltek Flatpack 3K that I use to charge a Pylontech battery on the bench for servicing.

Originally, I was planning to use it connected to the Victron Lynx DC bus to add more amps than my MP2 5K could when charging the stack of Pylons from grid during nighttime off-peak. But to solve this I had a change of heart and replaced the 5K with an MP2 10K.

There is a small control board for the Eltek made in the UK by a really helpful hobbyist. It works very well for setting output voltage and current.

29

I’ll do something similar as well. As soon as either the STP8000 or the STP9000 fails, the PV string currently connected to the 9000 will be routed via an RS450/200 to the expanded Felicity battery. Then I’ll need three MP2 8000s or 10000s (depending on availability), and I’ll sell the MP2 5000, the SBS5.0 and the BYD battery. The two remaining SMA STPs will then be under the 1.0 rule.

30

Thanks for the info, I’ve just found a similar specialist (surfurf14) on eBay in Germany who does exactly that for me:

31

IMG_8902
IMG_8902750×1334 131 KB

The bottom connector looks much more secure, but my Eltek and interface board are the top version.

Take care when buying the Eltek!

32

If possible, stick with the MP2-5000 units. Apparently, the MP2-8000 system can’t easily be expanded. The 5000s can. You can run up to six 5000s in parallel across three phases = 18 units at just under 200 kW.

If I were you, I’d see whether using six 5000s might be a better option. If that isn’t enough, you can add another three 5000s.

33

I’d like to share my experience: I have two heat pumps (Panasonic L and K generations, 3 and 9 kW), one for each floor. I can start them without any problems on a Victron 6k5 in backup power mode.

The reason is that, in the event of a power cut, you can simply run the circulation pump at first and decide later whether to switch to heating mode.

I’d also recommend getting a 48 V DC backup generator. Taking an AC route makes no sense in your case, assuming you have to buy the generator anyway.

If you’d like more details, or want me to expand on my thoughts about your current setup, just let me know.