Is it possible to but multiple charge controllers in series?

[gnhb]

Consider this layout.

8x Sharp panels @ 130W each wired in series, giving a nameplate voltage of ~140V DCc.

3x 48V DC Charge controllers wired in series with the PV array voltage across them.

12x 12V batteries, wired in series, with a charge controller charging sets of 4.

What do you think? Would it work? Would it damage the batteries?

Thanks,
GNB

[BB.]

No, you will not be able to place 8 solar panels across three solar chargers. It will not work because it would damage the solar controllers (over voltage) and, because each solar charge controller has variable input current/voltage control, they will not provide a "balanced load" to the solar panels where each shares 1/3 of the load.

Cut it back to 2 series strings of 4 solar panels, and connect those two strings in parallel into one 48 VDC controller--and you will be able to charge a 48 volt battery bank...

But I am not sure that is what you want. Do you want to charge a higher voltage battery bank? Or do you have a long distance between the solar panels and the load and trying how to efficiently configure the system (from the question in the other thread)?

There are lots of ways to solve various problems--but in general it is much easier to define your need/problem, then analyze the solution (or solutions) to address your needs. Rather than trying to offer a solution without any idea about what you are trying to obtain as the end result.

-Bill

PS: GNB, I am guessing you are in Thailand or SE Asia--so your available selection of components may be a bit more limited/costly--but you probably don't have the building/electrical permits/inspections that are required here.

In may change the answers a bit (or limit what you can easily do).

It is very possible to build a "one off" system that does what you need (safely)--but getting a knowledgeable local electrical engineer / electrician would probably be helpful too with your project.

PPS: you mentioned that you need 12x12=144 volts of batteries that need to be charged... There are certainly ways of doing this either directly (3x48 volt solar RE panels, each connected to their one 4 panel string--or charging a standard 48 VDC bank, then using an up converter or off-the shelf units to charge your 144 VDC string).

But, in the end, knowing the details of your power usage (what is the 144 VDC for--solar to power an off-the-shelf UPS system? emergency power, or long term solar power, ???) would be a big help

[niel]

hmmm, the only thing i can think that would take 144v might be an ev or electric vehicle. is that what you have in mind for this?

[BB.]

Larger computer UPS's also can have higher battery bus voltages too...

Here is an Ebay "Help" page with an inverter that uses 240 VDC as its battery bank voltage.

-Bill

[gnhb]

Thanks for the replies to my questions. I'm setting up an off grid system for a remote site and I've been studying the solar tech world for some time now.

This question was really just a question I was curious about. The voltages in the example were just for the purpose of the example.

I read on a thread on another forum that has since been deleted that a particular individual was running a 100V DC bus/battery voltage which he then wired into the rest of his house and could run computers and many non-motor driven devices. He could then use a smaller inverter for the motor and AC heating loads -- hair dryer anyone? :)

This got me thinking about PV systems from end to end, and about switching power supplies and inverters, and energy conversion in general.

I'll continue in the other thread.

Thanks,
GNB

[BB.]

You can run some AC devices with DC voltages--but not all. Between finding / selecting the appropriate devices (universal motors like many electric drills with brushes--but variable speed won't function, heating elements, filament lights, PFC type computer/switching power supplies) -- and having to use appropriate switches/breakers/fuses rated for the high DC voltage, and playing the games with either configuring multiple DC chargers (and being careful to isolate grounds and protecting against "ground now hot" shorts), and such... And the fact the DC voltage will between ~105 and 155 volts (discharged to fully charged + equalizing)...

Vs just adding an inverter and 10-15% more solar panels + battery capacity--it just does not seem to be worth the issues raised.

Use one inverter to take all of the battery issues and change it into stable 120/230/240 50/60 Hz AC. Then use standard off-the shelf components/systems wherever needed to save costs and make things easier.

Using heating devices (hair driers, hot plates, etc.) with solar electric power is usually a very costly use of energy (typically $1-$2 per kWhr after purchase price and maintenace/replacement costs are taken into account)...

Using solar thermal devices (hot air, hot water, solar ovens, etc.) are usually a better bang for the buck (air drier--may be worth it for piece in the family).

-Bill

[Brock]

I have had this idea kicking around in my head for a couple of years now. So sort of jumping back to the original question what about this?

Say you have two banks of solar panels at 90v at 50 amps each. Now from those two banks you really only run three wires down to the controllers.

Wire 1 – solar panel bank one positive
Wire 2 - solar panel bank one negative and solar panel bank two positive
Wire 3 - solar panel bank two negative

Now between 1 and 3 you have 180vdc, and 90vdc between 1 and 2 and 90vdc between 2 and 3. Then you wire them to the controllers like this

Wire 1 - 1st mx-60 positive side
Wire 2 - 1st mx-60 negative side
Wire 2 - 2nd mx-60 positive side
Wire 3 - 2nd mx-60 negative side

These two inverters in turn feed a 48v battery bank.

As either of them sweeps or whatever wire two would pick up the load. But in wouldn't most of the time wire 1 and wire 3 be carrying the majority of the load? And wouldn't this reduce line loss since it would be basically running 180vdc? Or did I totally miss something?

[BB.]

Brock,

What you have described is the standard 120/240 VAC North American Split Phase (or center tapped transformer) utility main connections for homes but in a DC form--which I am sure you know.

The problem (that I see) is related to grounding. Most devices assume negative ground. And the the typical MPPT controller output is not isolated (transformer coupled) from the control and panel wiring.

So--now, if you happened to need a +48 and -48 volt system--with common earth ground--It should work. However, I would check out any special conditions (from the solar charge controller mfg.) on positive grounding.

Also--it is possible that you will have variable voltage drop on the center leg--I am not sure how sensitive the controllers would be if they "see" some MPPT sweep related voltage drop on the common leg--if that would confuse them, or if the wire gauge is large enough, the drop is small with respect to the MPPT sweep voltage changes.

Probably is possible--but would it be worth the trouble to have a +/- dual power system... Or would it be better to pool the panels, controllers, and batteries into one system--then if there is "unbalanced" energy usage (inverter A uses more power than inverter B)--then you would be able to "share" the common panel/controller/battery pool between the A and B loads... Whereas separate loads and charging systems would not.

-Bill

[Brock]

Yes and yes The whole 120/240 center tap is what started me thinking about this in the first place.

I hadn't thought of the grounding on the negative side passing through the charge controller, which would be an issue floating ground or even not grounding the panels.

And yes I could see the voltage drop on the center leg during a sweep could confuse the sweep process potentially getting it off a couple of points.

I was thinking the advantage would be that you could run with a smaller wire size, but you would have to size each as if it were on it's own anyway because the other side might not be pulling a load. I suppose you could size it for say a 4% loss on one side, but then if both were running flat out you would actually only have a 2% loss total? And you would be basically be eliminating 1 of the four wires by doubling up on the center wire.

Or someone could just make a charge controller that would take 400 vdc in and 48 vdc out Even with the loss being more on the conversion I would think the reduced loss and expense in wiring would off set the overall loss.

[BB.]

If I recall correctly, "Someone" here a month or two ago intimated that there would be a new controller this summer that would move the voltage limit up from the 140/150 VDC that is common today for MPPT charge controllers.

Waiting and seeing.

-Bill