Multiple MPPTs+Batteries to 1 Inverter

[Mindpeter]

Hello, I am beginner in PV designing and I hope this is the first PV Systems design that I am doing of many more to come!! Therefore I would like to start with the right foot!!

I design a PV system for a office building of 5kW solar generation. I am using the following "module":

-6 Panels with and output of 24Vdc
1 -24Vdc MPPT
- 2 batteries in series ( 12Vdc each)

So I was wondering if it is possible to connect 2/3 Modules, so basically 3 MPPTs to each 3 groups of batteries and then to 1 Inverter? (as long is bellow the Power of the inverter)
Is some way of synchronizing needed, or the MPPTs+batteries will work independently and the system works smoothly?

Thank you in advance for your feedback!

[inetdog]

Hello, I am beginner in PV designing and I hope this is the first PV Systems design that I am doing of many more to come!! Therefore I would like to start with the right foot!!

I design a PV system for a office building of 5kW solar generation. I am using the following "module":

-6 Panels with and output of 24Vdc
1 -24Vdc MPPT
- 2 batteries in series ( 12Vdc each)

So I was wondering if it is possible to connect 2/3 Modules, so basically 3 MPPTs to each 3 groups of batteries and then to 1 Inverter? (as long is bellow the Power of the inverter)
Is some way of synchronizing needed, or the MPPTs+batteries will work independently and the system works smoothly?

Thank you in advance for your feedback!

Once you connect the three battery banks to the inverter you have connected them in parallel, They are now one big battery bank charged by three separate CCs.
There is nothing inherently wrong with that except that paralleled lead acid batteries should be identical model, age and history and even then keeping the parallel strings of batteries matched and equally dividing the charge and discharge current is very difficult.
Given a large battery bank (made up of size 4V batteries in series with each 4V battery being three times the AH rating of your planned 12V batteries), it need not cause problems to connect three separate CCs to that big battery bank. The CCs should have the same voltage settings to get the most out of all three sets of panels.

Running one set of panels to more than one CC will not work well with either MPPT or PWM type CCs.

[SunEagle]

Sounds a little scary to me.

A 5kW off grid system consisting of multiple solar arrays, 24 volt battery groups & chargers connected to one inverter.

This system has a lot of potential unequal resistance "wire paths" which will be running high amps and can be dangerous.

[Mindpeter]

My plan was to put each CC (MPPT)+ 2 batteries in parallel with 1 Inverter as you said. All the "modules" and components will have identical characteristics.
I did a sort of a wiring/block diagram, could you have a look? I dont know if I fully understod your 4V battery example (3x 4V batteries is it better then 12V batteries) !

When you say is very difficult,do you mean there is the need of a calibration on the MPPT. As I said I am still a beginner so I am still looking at this forum searching for good info regarding this issues.

[ButchDeal]

why not go 48V, use a single larger MPPT CC, and one string of batteries.
wires would be smaller, parts would be less, everything simpler and more efficient.

[Sunking]

Hello, I am beginner in PV designing and I hope this is the first PV Systems design that I am doing of many more to come!!

No you are not even close to being a Beginner. You are building a Fire Trap trapped inside a DIY 12 volt box. If you had done your homework you would know to use 48 volt battery using 2, 4, or 6 volt batteries of the appropriate capacity, and a single Charge Controller. Instead you are building a Fire Trap that if it does not catch fire will soon fail and have to be replaced by someone who knows what they are doing.

[Mike90250]

why not go 48V, use a single larger MPPT CC, and one string of batteries.
wires would be smaller, parts would be less, everything simpler and more efficient.

Not fair, he said it before I could. 5Kw screams 48V.

[Mindpeter]

I was trying to improve the efficiency of the PV production. The panels that I have to use must be 12V, so I was thinking to put only 2 of them in series. If I go for 48V then I need 4 panels in series and that decreases a lot ( I guess) the efficiency of the PV production due to partial shading of the array?!

Also, I wanted something modular, so it could be 5kW or 2.5kW.
So my premises when I started the design of this system is the follow requirements:

- Something modular
- 140W, 12Vdc Panels

Could you also explain the difference of having 2 series of 12V or 4 series of 6 V, 6 of 4V.....etc Is a matter of price/mAh?

A 5kW off grid system consisting of multiple solar arrays, 24 volt battery groups & chargers connected to one inverter.

So having battery groups + CC connected in parallel to 1 inverter is never a good policy and not a standard procedure? (too risky)

[Sunking]

I was trying to improve the efficiency of the PV production. The panels that I have to use must be 12V, )

Complete nonsense, there are no 12 volt panels and no reason they have to be 12 volts. You can use as high as 600 volts which is the best.

Hint you cannot charge a 24 volt battery with 24 volts. Sorry you are not even a Beginner.

FWIW those 140 wat panels are not 12-Volts. They are 18 volts so they can charge a 12 volt battery using a PWM Controller. They are called Battery Panels because of their Low Voltage and Low Power. They cost 2 to 6 times more than higher voltage Grid Tied panels. In short you are making every possible mistake you can make.

[Mike90250]

.....4 panels in series and that decreases a lot ( I guess) the efficiency of the PV production due to partial shading of the array?....

Where is the shade coming from ? Panels should not be installed casting shade on each other. Are there trees or other buildings ? I've got several arrays with 3 & 5 panels in series, no shade anywhere.

[Mindpeter]

I know they are not 12V (36 Cells, 17 Vmpp, Impp=8.17) I thought it was a standard name, 12V as they sell it. My electrical calculations were based in those values, Vmpp, Impp, Voc, Icc for protection and predicted production of the PV module.

I initially looked for micro inverters for each module, but what I read is that there are no micro inverters for 36 Cells Modules.

The shading that I am talking about is due to clouds, or cloudy days, and also I guess, beginning and ending of sunlight. Isn't that having a huge impact on the efficiency of a PV array, especially if you use a MPPT controller? I read that you can also use a bypass diodes to overcome that issue. Is the investment worthy?
Based in NASA website I am expecting a 4.94 kWh/m2 per day on site, so there relative few cloudy days per year.

Cos what I saw more is parallels of panels, connected to a CC, increasing considerably then the cable size. I would prefer series of panels, but I am afraid of an high reduction on the PV array efficiency.

Regarding the batteries, I do not have indeed enough knowledge as you said. Let me investigate more in this forum, I saw that you have interesting posts, thanks again Sunking for the warning! That was the reason I posted here and I was looking forward to your feedback.

[SunEagle]

I was trying to improve the efficiency of the PV production. The panels that I have to use must be 12V, so I was thinking to put only 2 of them in series. If I go for 48V then I need 4 panels in series and that decreases a lot ( I guess) the efficiency of the PV production due to partial shading of the array?!

Also, I wanted something modular, so it could be 5kW or 2.5kW.
So my premises when I started the design of this system is the follow requirements:

- Something modular
- 140W, 12Vdc Panels

Could you also explain the difference of having 2 series of 12V or 4 series of 6 V, 6 of 4V.....etc Is a matter of price/mAh?




So having battery groups + CC connected in parallel to 1 inverter is never a good policy and not a standard procedure? (too risky)

An off grid system the size of 5kW should be using high wattage solar panels (>200 watt) and charge controllers for a 48volt battery system. The battery system should be comprised of low voltage (2v or 4v) high Ah batteries wired in series as a single string going to a single 48v inverter.

Building a system using small wattage panels, CC's, low voltage battery systems wired in parallel and inverters has a high chance of wiring issues as well as producing high currents. There is a good chance of equipment failure or worse someone getting hurt.

[jflorey2]

I initially looked for micro inverters for each module, but what I read is that there are no micro inverters for 36 Cells Modules.

So connect two in series. Will be tough to get a system signed off with old mismatched panels though.

The shading that I am talking about is due to clouds, or cloudy days, and also I guess, beginning and ending of sunlight. Isn't that having a huge impact on the efficiency of a PV array, especially if you use a MPPT controller?

You're talking about two completely separate things.

One is shading, which is what happens when (for example) there's a tree nearby and the shadow falls on one panel first. In that case, when there is significant shading like that, you need to treat them like two separate panels (or two separate arrays) with a separate inverter on each set of panels. This is where microinverters do well.

The second is sun-hours. Yes, the sun goes up and down, but since the sun is very far away from your panels, they affect both of them the same way. If you have no shading issues (other than bad weather and night) you do not have to treat panels individually; series strings work fine in such applications.

I read that you can also use a bypass diodes to overcome that issue. Is the investment worthy?

Almost all panels have bypass diodes built in.

cos what I saw more is parallels of panels, connected to a CC, increasing considerably then the cable size. I would prefer series of panels, but I am afraid of an high reduction on the PV array efficiency.

If you don't have shading issues then there's no reason to not do a series string with an MPPT converter.

[SunEagle]

Oh. Something else I forgot to mention concerning your wiring diagram. Most charge controllers do not have load terminals with high amp ratings. You would be taking a chance wiring your inverter to them.

It would be better to wire the inverter to the battery terminals and fuse the wire accordingly.

[Sunking]

Mindpeter you need to stop right now. You have no clue what you are doing and are doing everything as wrong as you possible can including spending 2 to 4 times more than you have to using battery panels wired in parallel. Do you like pissing away your money on ignorance? You must be a pro at loosing money and screwing yourself.