Adding different type of panels to my RV

chiefk

I currently have on my RV two Uni-solar PV144 panels wired in series to a Morningstar 45A MPPT controller. The panels are rated at 33V Vmp, 4.36A Imp and 144 W.
I now have 2 new Renogy RNC-100D 100 W panels rated at 18.9V, 5.29 A.
My plan was to put these in series with the 2 existing panels for a total voltage of 103 volts which the controller can handle (150 volts max).
Now I am reading all kinds of precautions about mixing panels. Am I way off base with this idea? What should I do?

BB.

Re: Adding different type of panels to my RV

In general, you want to "match" the panel sizes within about 10% or better--If they miss-match by too much, you will get reduced output, or possible over heating.

For panels in series, you want panels to match Imp within ~10%.

For panels in parallel, you want them to match Vmp within ~10%.

In your case, the Imp (series) currents are 4.36A/5.29 A Imp. If they where ~1/2 amp difference or less--You can put them in series. Too large of difference which can cause loss of output.

For Vmp (parallel) voltages, 33V/18.9V Vmp. Double 18.9 to 37.8 volts (two panels in series)... You have a ~5 volt difference. Again, really too large of difference for comfort.

The problem with the two sets in parallel is that there are two Maximum Power Vmp "humps". If the controller finds the ~33 volt hump, you would get:

33 volts * (4.36A+5.29A) = 318.45 Watts
37.8 volts * (5.29A) = 200 Watts

Which local peak will the MPPT controller settle on--I don't know. You will not hurt anything by trying--So, you could experiment and see if they work well together or not.

-Bill

chiefk

Re: Adding different type of panels to my RV

BB. wrote: »

In general, you want to "match" the panel sizes within about 10% or better--If they miss-match by too much, you will get reduced output, or possible over heating.

For panels in series, you want panels to match Imp within ~10%.

For panels in parallel, you want them to match Vmp within ~10%.

In your case, the Imp (series) currents are 4.36A/5.29 A Imp. If they where ~1/2 amp difference or less--You can put them in series. Too large of difference which can cause loss of output.

For Vmp (parallel) voltages, 33V/18.9V Vmp. Double 18.9 to 37.8 volts (two panels in series)... You have a ~5 volt difference. Again, really too large of difference for comfort.

The problem with the two sets in parallel is that there are two Maximum Power Vmp "humps". If the controller finds the ~33 volt hump, you would get:

33 volts * (4.36A+5.29A) = 318.45 Watts
37.8 volts * (5.29A) = 200 Watts

Which local peak will the MPPT controller settle on--I don't know. You will not hurt anything by trying--So, you could experiment and see if they work well together or not.

-Bill

So it sounds like I am better off with my current setup. If I install these it sounds like I would be worse off than the beginning as I should be getting 288 watts right now, even though I never have. I think my best was a little over 200 watts.

Cariboocoot

Re: Adding different type of panels to my RV

chiefk wrote: »

I currently have on my RV two Uni-solar PV144 panels wired in series to a Morningstar 45A MPPT controller. The panels are rated at 33V Vmp, 4.36A Imp and 144 W.
I now have 2 new Renogy RNC-100D 100 W panels rated at 18.9V, 5.29 A.
My plan was to put these in series with the 2 existing panels for a total voltage of 103 volts which the controller can handle (150 volts max).
Now I am reading all kinds of precautions about mixing panels. Am I way off base with this idea? What should I do?

Welcome to the forum.

This is a bad idea for three reasons.
First of all the 150 Volt maximum input of the controller is based on Voc, not Vmp; the Voc of the panels will be higher than the Vmp. The Uni-solars are probably Voc ~40 and the Renogy about 22. So with all four in series the Voc of the string of panels would be 124 or so. Now add the increase in Voc from colder temperatures and it could exceed the 150 Maximum. Yes, panel Voltage goes up in the cold.
Second is the current limiting of the Imp. The Uni-solars at 4.36 Amps will limit the Renogy panels to that maximum, effectively reducing the power they will output. As such they would be (18.9 * 4.36) 82 Watt panels.
Third the higher the array Voltage in respect to system Voltage the lower the efficiency of the charge controller.

It's not that it won't work at all, it's just that there is a definite risk of exceeding the controller's input maximum and certain less power than predicted due to the current limiting and Voltage difference.

Frankly those Uni-solars are oddballs. You would probably be better off getting rid of them and utilizing different panels altogether to get the Watts you need.

BB.

Re: Adding different type of panels to my RV

I don't really know--The above are sort of worst case possibilities. It will not hurt to try.

Trying to match panels purchased at different times is one of the questions we get here a lot. And there are no secrete solutions that will mix/match panels efficiently and cheaply.

Panels have dramatically reduced their prices over the years. And sometimes it is just better to sell/give away the old panels and get a set of new panels that meet your needs.

For a 45 amp MPPT charge controller, the maximum "cost effective" array on a 12 volt battery bank is:

45 amps * 14.5 volts charging * 1/0.77 panel+controller deratings = 847 Watt "cost effective" maximum array

Since you already have the MPPT charge controller, looking at "GT type solar panels (our host)" (grid tied panels have a "non-standard" voltage for charging battery banks) at ~$1.00 per watt (check price shipped to your door, shipping a couple of large GT type panels can be very expensive).

Of course, with an RV, you have limited roof space and you not be able to fit the larger format panels. The smaller panels (~140 Watts) are closer to $2 per Watt (excluding shipping)--But sometimes are what you need to fit your application. The smaller panels (~140 watts an less) can ship via UPS/etc. ... The larger panels usually need to ship via truck.

-Bill

chiefk

Re: Adding different type of panels to my RV

Cariboocoot wrote: »

Welcome to the forum.

This is a bad idea for three reasons.
First of all the 150 Volt maximum input of the controller is based on Voc, not Vmp; the Voc of the panels will be higher than the Vmp. The Uni-solars are probably Voc ~40 and the Renogy about 22. So with all four in series the Voc of the string of panels would be 124 or so. Now add the increase in Voc from colder temperatures and it could exceed the 150 Maximum. Yes, panel Voltage goes up in the cold.
Second is the current limiting of the Imp. The Uni-solars at 4.36 Amps will limit the Renogy panels to that maximum, effectively reducing the power they will output. As such they would be (18.9 * 4.36) 82 Watt panels.
Third the higher the array Voltage in respect to system Voltage the lower the efficiency of the charge controller.

It's not that it won't work at all, it's just that there is a definite risk of exceeding the controller's input maximum and certain less power than predicted due to the current limiting and Voltage difference.

Frankly those Uni-solars are oddballs. You would probably be better off getting rid of them and utilizing different panels altogether to get the Watts you need.

I just added up the Voc's = 137.4 V. I really don't want to get rid of the uni-solars since I got them so cheap last year, 87.00 each brand new and they are now glued to the top of my RV.

Cariboocoot

Re: Adding different type of panels to my RV

chiefk wrote: »

I just added up the Voc's = 137.4 V. I really don't want to get rid of the uni-solars since I got them so cheap last year, 87.00 each brand new and they are now glued to the top of my RV.

137.4 is pretty close to 150; gets cold and one morning you're over the top. You don't even have a 10% margin there.

You could also just get a cheap PWM controller to put the Renogy panels on. Needs to handle about 15 Amps is all. Something like this Sunsaver: http://www.solar-electric.com/ss-20l.html

And yes it will work together with the TriStar.

chiefk

Re: Adding different type of panels to my RV

Cariboocoot wrote: »

137.4 is pretty close to 150; gets cold and one morning you're over the top. You don't even have a 10% margin there.

You could also just get a cheap PWM controller to put the Renogy panels on. Needs to handle about 15 Amps is all. Something like this Sunsaver: http://www.solar-electric.com/ss-20l.html

And yes it will work together with the TriStar.

I was thinking the same thing. I just have to add a lot more wiring that I didn't really want to do. And then I guess I would have to use some hefty wiring for the 12 volts down to the controller and battery.

Cariboocoot

Re: Adding different type of panels to my RV

chiefk wrote: »

I was thinking the same thing. I just have to add a lot more wiring that I didn't really want to do. And then I guess I would have to use some hefty wiring for the 12 volts down to the controller and battery.

How much distance between where the panels will be and the controller?

chiefk

Re: Adding different type of panels to my RV

Cariboocoot wrote: »

How much distance between where the panels will be and the controller?

Probably around 20 feet at least if I put the controller real close to the batteries.

Cariboocoot

Re: Adding different type of panels to my RV

chiefk wrote: »

Probably around 20 feet at least if I put the controller real close to the batteries.

You could use 10 AWG for the two panels in parallel over that distance.

chiefk

Re: Adding different type of panels to my RV

I finally decided to make it a separate system. I have on order a Morningstar Sunsaver 15A MPPT controller and will wire the panels in series and run #10 wire to the controller.