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timbo avatar image
timbo asked

How to expand/double up off-grid system

Hi all!

I have an AC coupled off-grid system: Victron Multiplus takes power from large prismatic cells lithium batteries and creates a micro-grid; grid-type inverter gets energy from solar panels and pump it into the micro-grid; Multi charges batteries from extra energy in the micro-grid. PV support assistant controls the balance of energy by the frequency shift. Everything works fine for more than 6 years already. I have enough energy most of the time.

Now I want to expand my system. The idea is to have another off-grid system with its own solar panels, inverter/s and batteries; this "new" system should be somehow connected to the "old" one.

The purpose of this exercise is to increase overall battery capacity and increase redundancy in the system. I do worry that if Multi dies, I don't have any redundancy - I would be without power.

What is a good design for this setup?

I can think only about connecting "new" system as an AC-grid input in "old" Multi. It should work. "Old" system will suck all energy from the "new" system to charge batteries but this is OK as the overall battery capacity in the global system will increase.

If "old" or even "new" Multi dies, I still have another system which can be quickly rewired as the main one.

Is there a better way to double up the system? Thank you.

Regards, Tim



offgrid
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shaneyake avatar image shaneyake commented ·
Can you list what you have already? Like Multiplus-II or one? What size system?

Do you have DC coupled PV or only AC? What is the size of PV inverter, etc.
What batteries do you have?
The more info the better we can help you.

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timbo avatar image timbo shaneyake commented ·

I tried to edit my question but it did not work - said "forbidden"

System as I have it now:

MultiPlus 48/5000/70-100

24 solar panels 250w each = 6kW

Schneider grid-tie inverter Conext RL - 5kW

16 CALB battery cells 400A 3.2V = 20kWh

Custom BMS between Multi and batteries to protect and balance cells.

This is an AC coupled system. Multi makes a grid. Conext pumps energy into a grid. "PV support" assistant on Multi controls power output by frequency shift. Multi charges batteries when there is an energy surplus.

Also, I have a portable diesel generator which I connect to Multi AC input if there is not enough sun. It does not happen often, so I manually connect a generator through 15A caravan-type plug when I need it.


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marekp avatar image marekp timbo commented ·
@timbo

There is no problem with adding more battery to your existing one.

If your current BMS is using "2 signal BMS" assistant you do not even have to replace it.

New battery will have to be the same chemistry and voltage and have same type BMS.

New battery will be connected in parallel to the old one and it's BMS signals in series with signals of the old battery's BMS.

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timbo avatar image timbo marekp commented ·
I understand it can be done. However, even if I get similar batteries, one pack will be new and another one is 6 years old. Capacity will be not the same. Getting identical BMS could be a challenge. Most components of my system are discontinued already.

Modern battery packs look to be better. 6 years ago there were not so many options. Full integration with Multi was available only on Victron batteries for a very high price. Now there are interesting options available. Suggested solution is reasonable. However, I think I can do better with more modern technologies.

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marekp avatar image marekp timbo commented ·
@timbo

6 year old LFP battery is still young.

You do not have to get similar batteries but the same chemistry. Those are available everywhere.

BMS also does not have to be identical but only same type (2 signal BMS).

It is difficult to make work system consisting of two independent MPs each with it's own battery feeding one another. There is always a risk of one battery pack charging the other. Pure waste of energy.

Simple works best, but it is your system, your money and ultimately you decision.

I wish you all the best.

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5 Answers
shaneyake avatar image
shaneyake answered ·

Thanks for the list of components.

My recommendation would be to expand your current system.
You add another 48V battery to your current system. I know you said your current bank is 6years old but that really isn't a problem. You could buy 16 more cells are parallel 1 old and 1 new cell.
Would be the cheapest option as your wouldn't need another bms.
You could also install another completely separate 48V bank as long as it is 16S LFP it will be fine.

I would add more solar via a RS 450/100, you can put another 6kw on it. This is the best way to have a system. Half AC and Half DC pv.

You could add a second multi in parallel if you want or split your loads in 2. Half on each inverter with transfer switch. In split mode you don't need the inverters to be the same.

As for AC coupling 2 Victron inverters. First I want to make it very clear that Victron does not support this config and with good reason. I have built lots of true microgrids with 3-12 inverters all AC coupled for installs across buildings where DC coupling was not possible.
It does work but it is really difficult to get working correctly and requires external Controls like a PLC or NodeRed to get working correctly. To synchronize load and evenly balance across batteries.
If you do what @knownot is suggesting the Batteries will have very uneven wear. They will also overload in a high transient load and you can't have a PV inverter coupled because you will break the 1:1 rule which cases stability problems. So in short, just avoid AC coupling Victron inverters if you can.


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knownot avatar image knownot commented ·
@shaneyake thanks for your detailed response (even though its not my question :). I am hijacking but its definitely on topic. For me the uneven battery wear is kind of the point (one battery pack is ni fe so I want to use cycle them preferentially because of their potential high cycle count). In timbo's case he may want to preserve either his new or old batteries. Im confused why you would suggest paralleling old and new cells under the same BMS - I understood that was a battery 101 no no. What you said in your last 5 sentences confuses me some also. I know what the words mean but I don't know of a 1:1 rule or exactly why this shouldn't be done. Any chance u could elaborate? Or point me to a resource? Thanks in advance.
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shaneyake avatar image shaneyake knownot commented ·

That makes sense.

timbo is using LFP cells that are 6 years old, I am assuming 2200 cycles max at this point with only a 5kw inverter. So the pack as had a pretty gentle life. I don't expect the current cells to have degraded much. If you parallel the packs or the cells it won't really make a difference. You just want the cell groups to be as similar as possible else you will have to balance. It would be really bad to do like 8 new cells and 8 old cells in series as you would limited to the performance of the weakest cell but with cells in parallel the cell groups will have the same performance even tho they are made up of old and new cells.

At least this is my opinion and I have a pack mixed like this but I only do like 0.1C discharge/charge on it.


So a Victron inverter that is grid-tie won't ramp up faster than 400W per second.

So if you turn on a 4kw load this is what it will look like.

0s : 4000W on main, 0W on second inverter

1s : 3600W on main, 400W on second inverter

2s : 3200W on main, 800W on second inverter

3s : 2800W on main, 1200W on second inverter

4s : 2400W on main, 1600W on second inverter

5s : 2000W on main, 2000W on second inverter

6s : 1600W on main, 2400W on second inverter

7s : 1200W on main, 2800W on second inverter

8s : 800W on main, 3200W on second inverter

9s : 400W on main, 3600W on second inverter

10s : 0W on main, 4000W on second inverter

Because of this cascading inverters does not increase your peak power only your continuous. So you can cause the main inverter to overload with a high peak.

Parallel Inverters don't have this limitation because the Master directly controlling the slaves, so that all inverters are actually at the same duty cycle.


The reverse is also true. So when a big load is turned off the secondary inverter will actually send power to the main. IE. -4000W, -3600W, etc.

This plus a PV inverter would cause the main to overload. Especially with big loads like 6-8kw. The 1:1 rule is that the inverter power on the output of a Victron inverter can not be greater than the inverters rating. You can read more about it here.

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shaneyake avatar image shaneyake shaneyake commented ·

https://www.victronenergy.com/live/ac_coupling:start

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knownot avatar image knownot shaneyake commented ·

@shaneyake thanks for your time, and link, I'm working on getting my head around it.

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timbo avatar image timbo commented ·
Thank you, @shaneyake. A lot of food for thoughts.

What is "split mode" you mentioned? Is it a phase split?

If I go to parallel - is modern model of Multi considered as "the same" for 6 year old Multi?


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shaneyake avatar image shaneyake timbo commented ·
Split is where you split up your loads into 2 groups. Then you put half on each inverter.


What is the serial number of your multi?

You should be able to get the same Multi.

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timbo avatar image timbo shaneyake commented ·
I'm away now, can not check the label. Device ID is 2624.

After some thinking, I'm moving towards an idea to double up existing battery cells, double solar panels (put them on DC) and buy new more powerful Multi or Quatro. Then I keep "old" Multi as a spare one for a redundancy. That system should be simple enough but ticks all my boxes.

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marekp avatar image
marekp answered ·

@timbo

If we assume that your current MP is big enough for your needs.

Than my suggestions are:

- If you want more battery so make it bigger.

- If you want more PV but your AC-coupled inverter is at max for your MP, install a DC-coupled MPPT with aditional PV panels. Anyway it is a good idea to have DC-coupled PV in off-grid for the situation when battery gets flat at night and system is down. There is nothing to recharge battery (only gas generator). DC-coupled MPPT will do it and once battery is charged system starts up and AC-coupled inverter can now start producing. Also charging is more efficient with DC charger because there are no losses on DC-AC-DC conversion.

- If you want be safe when your MP dies just buy a spare and keep it handy and programmed for quick replacement. You can hang it on the wall and ready to be connected with a couple of three way switches. :)

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timbo avatar image timbo commented ·
My batteries are prismatic large cells with custom BMS. It is not viable trying to get the same type of batteries now. Modern battery packs with build-in BMS and "native" communication to Victron are better. At the same time, "old" batteries are still good and should keep going for many years to come. So, it is not that simple to make my batteries bigger.

Spare Multi is a good idea. I had it too. However, if I need to connect a different battery pack, then it should be working - not just hanging on the wall.

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marekp avatar image marekp timbo commented ·
Is your current battery using CAN communication with GX or "2 signal BMS" assitant?
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timbo avatar image timbo marekp commented ·
2 signal BMS assistant - just close/open signal from BMS when batteries are empty or completely full. As I understand, both conditions never happen. Multi goes into float when batteries are full and I never allow batteries to go below 20% capacity.
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marekp avatar image marekp timbo commented ·
@timbo

Then there no problem in adding second bank of batteries to your existing one.

It just have o be the same chemistry, same voltage and same type BMS.

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knownot avatar image
knownot answered ·

Marekp makes a lot of good points. I however have a system somewhat similar to what you're proposing. I essentially feed one multiplus to another via the generator/shore input. It works - the end multi syncs with the supply multi just as it would a genny. You can program in some functionality as to how they interact also - I'm still figuring that out with my system.

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marekp avatar image marekp commented ·

@knownot

In may answer I assumed that safety in @timbo system is most important.

This is why I suggested to keep that spare MP unused, because if something catastrophic happens (ea. lighting strike) only used MP gets fried.

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timbo avatar image timbo marekp commented ·
There are two sides of the coin: (1) more batteries; (2) redundancy/safety. I want them both at the same time.

Extra consideration is cost effectiveness. It is possible to throw away existing system and build a new one with larger batteries and redundancy, however this is not good money-wise.

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timbo avatar image timbo commented ·
I have "a feeling" that it should work. It sounds logical.

However, there might be something I am missing. For example, how would frequency shift work if there are two solar arrays and two Multis?

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knownot avatar image
knownot answered ·

@MarekP has a good point about keeping your spare inverter safe but if your second battery bank is different to your first I think a second victron inverter is an easy way to go. I got an easy solar for my second and a good deal on some pylontechs. Wiring is simple, as you would expect. The receiving inverter takes a few seconds to sync the incoming supply and then you have available the sum of the two inverters. I do have a question about using the aux input of one inverter to monitor the battery voltage of another (connected) invert - from an equipment safety perspective, but the system works as is without additional complexity.

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timbo avatar image timbo commented ·
Thank you. It sounds promising. However, I have some concerns.

What about the following scenario: I have "new" system connected as shore/AC input to the "old" system. Then "old" Multi does not use "old" batteries but passes through energy from the "new" Multi and even uses that energy to charge "old" batteries. This process continues until "new" batteries completely die and stop "grid" connection from "new" Multi. At that moment, "old" Multi will pick up connection and continue power supply from "old" batteries. Is it correct assumption?

Continue from above. If "old" batteries are full but "old" solar inverter keeps pumping energy into the micro-grid - would the energy go back through "old" Multi AC input into "new" Multi grid? Which Multi will do a frequency shift to reduce energy generation on solar inverters?




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knownot avatar image
knownot answered ·

@timbo - first part - yes, but you can moderate that by reducing input current limit on the old system and prioritizing solar by how you set your charge voltages in your charge controller vs in your inverter/charger. Second part - No it won't back feed - that would have to be a grid connect inverter I believe. I understand the old multi will sync to the new before it accepts the current.

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