Large charge controllers connected to large battery banks

schmek

Hi,

I was wondering about some connection basics when connecting large charge controllers to a battery pack. I've considered 600 volt MPPT charge controllers, with a capacity of handling strings of around 5 kW. The PV array total is about 100 kW, so this would include lots of charge controllers. I've read on the specification sheet that the charge controllers only operate with 48 Volt batteries. How is this connected when the battery pack is larger, say for example 100 volts? Do you couple the charge controllers in series to increase the voltage? 

Kind regards,
Sindre

littleharbor2

There is one large capacity high system voltage charge controller being sold on ebay. I know nothing of it's quality. If you enter,  " High Voltage PWM Solar Charge Controller,Battery Regulator 200A for PV System",  you will find it. Apparently they have a model which accepts a max input of 48,000 watts.

mcgivor

Charge controllers have nominal operating voltages, 12, 24, 36,48 and 60 volts, generally speaking, they can change only the voltages within its design specifications. Series connecting in order to achieve a higher charge voltage is not possible, to achieve this a controller designed for a higher voltage would be needed, do they exist?  perhaps, but generally users of battery connected systems stay within the common nominal range due to availability of ballance of system  components. Here is a controller with battery charge voltage from 48-240VDC  http://systellar.co.in/mppt-high-voltage-solar-charge-controller/ as an example. 

schmek

Okay, thanks for clarifying! This means that for a large battery pack consisting of several modules, the batteries must be connected in parallel if charge controllers are used? (With strings of max 48/60 volts)

Sindre

schmek

And what about solar string inverters for AC coupled systems. Can these be connected to operate with a high voltage battery bank?

mcgivor

In AC coupling, a means to charge the battery would be required, as well as an inverter capable of using the higher battery voltage, the output of which must be compatible with the string inverters to synchronize their output. Again there are probably systems available for commercial applications, but these would probably be very specialized.
A pdf of the charge controller linked above attached.

Estragon

Midnite Classics do up to 72v nominal banks, but only up to 250v array Voc.

The likes of ABB would probably have engineered solutions which if getting into higher voltage systems you'll probably want anyway.

mike95490

When you get over a 10kw system, it's beyond a small scale, homeowner setup.   You really need to have a professional engineer that is familiar with the technology, do the system layout.  I'm speaking of 3 phase, 480V to accomplish a 100kw system, along with many dangers that the layman does not know of one of which is Arc Flash:

 

schmek

Okay, thanks. Going back to the charge controller. Just to be clear, when using charge controller designed to operate with a battery of 48 volt, there is no way of connecting the charge controllers to a battery bank with a voltage higher than 48 volt? 
In other words, if you wanna use a battery bank with a high voltage, DC coupling is not a good option?

Sindre

BB.

In theory, you can make (for example) a 96 volt battery bank that is made of two 48 volt battery strings. And put one MPPT charge controller on the "low string" and place a second MPPT controller on the "high string".

It can work OK... However, you end up with with an electrical issue. In general, the negative battery connection is designed to be "system ground". The low string charger is OK. The high string charger has its "system ground" at +48 VDC.  And its "high side" at 96 volts. That means that both + and - connections are "hot" when compared to system/safety ground.

You now have to worry about the various external system connections (communications links, any digital on/off ports) interfacing between other controllers (you could end up with damaged ports and/or short circuits when connecting the "high string" to "low string" controller with their standard communication interfaces). And for safety, you would need to run breaker/fuses on the "high string" charge controller with fuses/breakers/2 pole breakers.

A standard "safety/earth" grounded controller--All Negative (Return) leads are at "ground potential" so no fuses/breakers are required. For the "high string" controller, the negative/return/signal ground leads, as well as the solar array return leads, are now at +48 VDC, and need to be fused/breakered for safety.

You could even end up with the metal chassis of the "high string" controller being at +48 volts--And could cause shock/electrocution/fire if shorted to local "earth/safety" ground.

With high voltage DC Battery Banks (>>48 volts), you also have, what I think is a major safety issue, where you need to monitor per cell per group of 6-12 volt cells. For example, with a 12 volt battery bank, if you have a failed cell (shorted), your 12 volt bank becomes 10 volts nominal voltage. Your system went from working to not working. With a 96 volt battery bank and a shorted cell, your bank would still work OK (nominal 96 volt bank "working" range from 92 to 100 VDC). It is very easy for a "failed" cell to "hide" in a high voltage bank (similar issue with open cell--A failed open cells starts to arc because of >>48 volts (bank voltage) across open cell).

We pretty much deal with "cabin" and "house" sized solar power systems (I certainly do not have any insight to what is out there for large solar/battery power systems)... There are probably other larger industrial sized system that could work with you--But you should start a company level discussion (with hopefully local suppliers) to see what they may have available for large commercial systems.

High voltage/High current DC systems are, in some ways, more complex and dangerous that high voltage/high current AC systems (which are scary enough). For example, a big problem with even smaller solar power system (10 kWatt or less) has been Arc Faults. DC current just starts and sustains arcs so much "better" than AC systems and need to be designed accordingly (which is not easy either):

https://www.youtube.com/watch?v=Zez2r1RPpWY

-Bill

schmek

Okay. But I can't really see how DC coupling such a large system is even relevant. From SMA's website, they only operate with inverters for such a large system. Does anyone know if DC coupling large off grid systems (150 kW+ PV and 130 kWh battery bank) have been done? From what you're saying, it sounds problematic to couple charge controllers to a large battery bank.

Dave Angelini

I was hoping you would talk with the large systems engineer at Schneider?

 I have seen some videos of some really large systems they have done. US military has some nice info on large power systems and I know a guy who takes care of them so it is out there.

The XW+ tops out at 102KW and with all the devices it would take an solar/accountant to calculate the return on investment. In the end the cost of just running a fossil fuel gen based battery may be the way to go. What is the expected life of a fish farm on the ocean? are the kinds of things you will need to know for an investor or even a system designer. What are others doing? Why so much power? Can it be reduced to an occasional large load generator and then a battery based smaller system.

Being Green can also mean you have to have alot of Green

Estragon

I think what's being said is this isn't likely something you want to attempt in the same way we do the typical systems here. When you get into higher voltage systems, especially DC, you should be talking to the likes of ABB or Siemens.

Mid-high voltage DC systems are certainly done, but not with the consumer level products we use. For example, most of my power at my city home comes via HVDC in the 100's of kilovolts which is stepped down and inverted to AC for local distribution.

Dave Angelini

Not what I am saying Estragon, I am saying this.

Estragon

Dave, I hadn't seen your post whe,n I posted... I would certainly include Schneider with the likes of Siemens. My point was you need engineering involved in planning.

schmek

Yes Dave, I have contacted the engineer, but I'm currently waiting for the reply. 

Dave Angelini

No Problem! Might as well add Haliburton also.

That is who my friend works for. They take stuff from both of our lists and make them all work together, most of the time

schmek

Okay, thanks! Have any of you reviewed Leonics? Found out they design all kinds of off grid systems, even with large charge controllers. Here is a same scale system I had in mind, DC coupled: http://www.leonics.com/system/solar_photovoltaic/customized/200kwp_en.php

Dave Angelini

It is your project and so no, I have no idea who they are and that would be enough for me to not go farther with them. Large projects need a bankable partner and a supply chain that has stood the test of time in my opinion.