Charge Controller Question: Uneven number of modules in series strings

[keith_allen53]

I have a situation where I have an uneven number of pv modules that I wish to connect to a single MPPT charge controller. In a single series they exceed the rated power of a given MPPT controller. (I wish to use series wiring because the wire gauge requirements are less) Question 1: What is the accepted practice of handling an odd number of panels as is pertains to connecting with an MPPT charge controller? Question 2: Is a second charge controller the only option? Question 3: Can a (cheaper) PWM charge controller be used for the odd-man-out panel while the remaining panels use the MPPT?

Thanks for the input. I'm glad I found this site!

Cheers, Keith

[Naptown]

Well how many do you have first off?

[billvon]

Question 1: What is the accepted practice of handling an odd number of panels as is pertains to connecting with an MPPT charge controller?

Well, accepted practice is "don't do it." It probably won't break anything but it will likely give poor results.

Question 2: Is a second charge controller the only option?

It's a good option _especially_ if you are over a single charge controller's current limit.

Question 3: Can a (cheaper) PWM charge controller be used for the odd-man-out panel while the remaining panels use the MPPT?

If the voltages work out, sure. Beware of controller grounding issues; run separate power and return lines for each string and the odd panel.

[keith_allen53]

I have 5 panels. I would like to use Kyocera 315 watt panels (Vsc = 50, Vmp = 40, Imp = 8 and Isc = 9) The batteries are 48v.

Thanks Nathan. I expected the #1 and #2 answers. It was a relief bouncing the question off others and receiving confirmation. The #3 answer was a relief. I was hoping it wouldn't cause any problems other than the ones you listed.

If anyone else would care to chime in feel free. I can use all the feedback I can get. Thanks!

One quick after-thought. Lets say the panels to the MPPT controller are shaded from the sun and that the single panel on the PWM controller was not. If I understand correctly how MPPT controllers work, this should not create a problem as the MPPT controller should be "off". Is that a correct understanding?

K..

[Sunking]

I have 5 panels. I would like to use Kyocera 315 watt panels (Vsc = 50, Vmp = 40, Imp = 8 and Isc = 9) The batteries are 48v.

Thanks Nathan. I expected the #1 and #2 answers. It was a relief bouncing the question off others and receiving confirmation. The #3 answer was a relief. I was hoping it wouldn't cause any problems other than the ones you listed.

If anyone else would care to chime in feel free. I can use all the feedback I can get. Thanks!

One quick after-thought. Lets say the panels to the MPPT controller are shaded from the sun and that the single panel on the PWM controller was not. If I understand correctly how MPPT controllers work, this should not create a problem as the MPPT controller should be "off". Is that a correct understanding?

K..

Will not work with 5 panels period because the panel voltage is too high unless you use 2 charge controllers or else a 250 or 600 volt CC.

[keith_allen53]

Reply to Sunking

Right, I suspected as much and Nathan confirmed. But two strings of two modules will work with the MidNite Solar CCs if I can put the 5th on a PWM controller. Do you agree?

Thanks, Keith

[Sunking]

Right, I suspected as much and Nathan confirmed. But two strings of two modules will work with the MidNite Solar CCs if I can put the 5th on a PWM controller. Do you agree?

You could do that if you wanted but I would first look at using Midnight Solar Classic 250.

[keith_allen53]

Sunking/Derek

Midnite Solar - in what capacity? I'm confused because I indicated I was already looking at Midnite Solar for two strings of two modules.

Cheers, K..

[Sunking]

The 250 series is a 250 volt Vmp controller. I assumed you were looking at 150 volt series. My bad.

[keith_allen53]

I checked out the 250 series. But when the weather is cold the voltage can be surpassed. Their own site lists the configuration as "marginal" - I'm not sure what that means exactly. Any idea?

[hugin8]

I have a situation where I have an uneven number of pv modules that I wish to connect to a single MPPT charge controller. In a single series they exceed the rated power of a given MPPT controller. Question 1: What is the accepted practice of handling an odd number of panels as is pertains to connecting with an MPPT charge controller?

Why couldn't you do a parallel-series combination:
2014-05-21_IA29_SolarDiagram_Rev0b.png
(P1 || P2) + (P3 || P4) + P5
Where “||” = Parallel, and “+” = series. Or, “two sets of parallel panels, in series with a single panel”

For example, with five 250 W panels, where the voltage and current at maximum power of each panel are V_mp = 30.9 VDC, and I_mp = 8.1 A, the array voltage and current are:

1) Start with parallel/series configuration
(P1 || P2) + (P3 || P4) + P5
2) Plug in voltage and current for each panel
=[(8.1A @ 30.9V) || (8.1A @ 30.9V)] + [(8.1A @ 30.9V) || (8.1A @ 30.9V)] + (8.1A @ 30.9V)
3) The current adds for the panels connected in parallel
=(16.2A @ 30.9V) + (16.2A @ 30.9V) + (8.1A @ 30.9V)
4) The voltage adds for the panels and sets of panels connected series
=40.5A @ 92.7V

[Wy_White_Wolf]

Why couldn't you do a parallel-series combination:
[ATTACH=CONFIG]4221[/ATTACH]
(P1 || P2) + (P3 || P4) + P5
Where “||” = Parallel, and “+” = series. Or, “two sets of parallel panels, in series with a single panel”

For example, with five 250 W panels, where the voltage and current at maximum power of each panel are V_mp = 30.9 VDC, and I_mp = 8.1 A, the array voltage and current are:

1) Start with parallel/series configuration
(P1 || P2) + (P3 || P4) + P5
2) Plug in voltage and current for each panel
=[(8.1A @ 30.9V) || (8.1A @ 30.9V)] + [(8.1A @ 30.9V) || (8.1A @ 30.9V)] + (8.1A @ 30.9V)
3) The current adds for the panels connected in parallel
=(16.2A @ 30.9V) + (16.2A @ 30.9V) + (8.1A @ 30.9V)
4) The voltage adds for the panels and sets of panels connected series
=40.5A @ 92.7V

Nope. P5 limits the string amperage to that of a single panel. Output would be 8.1A @ 92.7V or tha same as having 3 panels in series.

WWW

[hugin8]

Ah, Thanks!

[inetdog]

The problem is not that you have an "uneven" number. All even numbers and some odd numbers (like 9) are just fine.
You have a prime number, with no divisors other than itself and 1, and that is what limits your options.

You can either buy one more panel or keep that fifth panel as a spare.

[Sunking]

Five is a PRIME number and that leaves you only with two choices of all in series or all in parallel. 5 in series will more than likely exceed the controller maximum input voltage so that leaves you with all 5 in parallel.