Two controllers on the same array to the same battery pack?

[CountryFit]

Howdy, noob here.  I did search but couldn't find the answers to my question so I am asking... Sorry if this has been asked.

I have 2750w panels on my motorhome.  They are on two arrays - a 750w and a 2000w.  The issue is on this 2000w.

It is connected to a Midnite Classic 150 charging 1200ah lithium battery pack.  By this time of year, April, I have already seen the output on the controller hitting the ceiling of 96A.  I understand I have excessive solar power and in need of a 120-150A controller but none exists out there.

Another possible solution is splitting up the array.  To do so I would need another set of wires from combiner box on the roof to the controller in battery bay.  Doing that is quite a PITA.  I consider that to be the last resort.

Now the question - Is there a way to share the same input current from the panels to charge the same battery with 2 controllers (one master another slave)?  I googled but there have not been many people touched the topic.  One or two said no, but no convincing reasoning about why.  So, I am asking experts here - is it doable?

If not, what other options are?   Really appreciate your info!!

[ClassicCrazy]

What voltage is your battery pack ?
Is it possible to reconfigure  your battery pack to a higher voltage ?  You would increase the amount of watts that the Classic could handle that way and if you needed to drop the voltage for devices you can get a DC to DC converter.

The other option if you were thinking of a second controller is to get another Classic and you can use that in Follow me mode with the other Classic.  That does work something like master and slave but not exactly  -- there are discussions on the forum if you look up Follow Me.

Hard to give advice with no idea what  your present wiring is now.

Larry

[CountryFit]

Sorry for my insufficient info...

48v Voc
42v Vmp

24v (or 48v) has some advantage on charging/inverting for sure.  I have thought about it.

the problem i have is uncommon - i don't use a starting battery, its role has been replaced by 1200ah house battery. in 12v system, it's easy that i have all the controls in place to bridge over. if i were to go with 24v or 48v, what kind of converter is able to provide 1400 cca at 12v to start my cummins engine? i attempted and gave up.

I heard "Follow me" thing, but I though it is on a separate array of panels.  Correct?

Thanks!

[ClassicCrazy]

Yeah I wouldn't try to use a converter to start the engine.  Just for loads.
You could however have a separate starting battery that you keep charged from  your main battery pack.

Larry

[CountryFit]

Yup, adding a standalone starting battery is another option...
(Funny thing is - see this photo http://www.irv2.com/forums/blog_attachment.php?attachmentid=486&d=1523468900 - in my battery bay, at the left where the yellow cells are at, the starting battery used to be there.  To make a room for a bigger lithium pack, I removed the starting battery and re-routed the cables as what you are seeing on the back panel (white board).  Am I going to make a complete circle back to the original point ? )

[ClassicCrazy]

ha ha - yeah lots of engineering to do there with your limited space.
What are you using for BMS for your lithium cells ? What setpoints do you use on the Classic ?

Larry

[ClassicCrazy]

I am also curious about the wiring on that shunt ?
Seems like too many connections on one side of it .

Larry

[CountryFit]

i use a chinese made bms on all strings (4 cells makes a 12v), to the left of the pic (not shown).  in the past 5 years, i have found no off balance on the cells in series.  it did happen among strings, i.e. in parallel. 

the shunt - two big cables from battery negative terminals to one end; all others either from solar controller negatives or from inverter/chassis to the other end.

[boB]

You will need to separate the PV into two arrays to use two controllers.  At least separate PV negative wires, that will reduce it from 4 to 3 wires.

The reason you can't parallel the two controllers is that they are two independent DC to DC converters (buck) and each track separately.  There is a high danger of hurting one or both controllers if they are paralleled on input and output.  The current circulates around in a circle from output to input is the reason for that.

The "Follow Me" only coordinates the charging cycles of multiple controllers...  i.e. when one controller goes from Absorb to Float, the other controller will do the same thing so you don't over-absorb the batteries.

boB

[CountryFit]

thanks boB!   this question has been hanging in my mind for a long time.  i am grateful that you explained.

in one discussion board, someone said because MPPT controller needs to ping the array so to know the voltage.  well, i did an experiment and i couldn't prove that was true - i added a diode before the controller, i.e. the current could only flow from the array to the controller, not other way around.  the result - the controller worked just fine and the battery got charged.

now, "The current circulates around in a circle from output to input is the reason for that. "   i am not sure i can picture it in my mind.  the output of one controller is connected to the battery,  how does its current route back to the input of the other controller?  i really like to test this out, but need to know how to do the wiring.  please lighten me up!

[mike90045]

MPPT controllers Sense and Adjust their internal loading of the PV array.
If you add a 2nd one, they will start to fight each other for control, and one will loose.
Either split the array, or increase battery voltage, or you could spend $600 for another controller and parallel wire it in with your diode and see how it works out.

[Resthome]

Trust boB's reply he designed the Classic.    And for what it is worth the Battery terminals on the Classic provide the power to operate the Classic and they also provided the charging current to the batteries.

The Classic also opens the circuit to the PV Array to measure the VOC of the Array every 3 minutes and if you tie two controllers to the same array that function will not work as one controller would have the Array loaded while the other would try to unload the array to read the VOC.

[CountryFit]

Thanks Mike and John!  A lot of good info in this thread!  Really appreciate it.

I re-read boB's "The current circulates around in a circle from output to input is the reason for that", I think I have a better understanding now - it meant within the same controller the current from output routes back to input...  Perfect!

[boB]

Right.  Current is actually created at the input from the battery.  This is a property of synchronous buck converters. (Google that for more info)

Turns into what they call a boost converter.  This topology is only done to increase the controller's efficiency.  Not all MPPT controllers do this.

Adding a diode will definitely fix that issue...  The diode will add a slight loss...  BUT, it still "most likely"  will not work because each controller will do its own tracking and you may most likely end up with less than optimum output from the array.  One controller will operate properly but then limit the output, (too much power for one controller), then the 2nd controller will bring the array down when it tries to track the already operating PV array, then controller #1 will open its input up to take a Voc reading and lower its input voltage to track (or whatever method it used to track MPPV), then controller #2 will current limit, back and forth, on and on and on.  A true MPPT nightmare with lower than expected or wanted output energy.

All this is the "fighting" they speak of.

boB

[CountryFit]

Big thanks boB!!  Took so long to realize how MPPT controller works.  Phew!

On a motorhome, unlike in a home, routing a wire from combiner box on the roof down behind the fridge to the storage bay, then moving towards the back, crossing over the rear wheel and finally landing in battery bay, is a pain.  I'd like to exhaust all the possibilities before going that route... 

Is there a MPPT controller made to output 130-150a?  I know PT-100 can output 100a, not much different from 96a.  If there is no such a thing, does Midnite or any other manufacturers anyone knows of have a plan to make it in the future?