Mixing Panels (open circuit voltage)

[Croft]

I have one 125 watt panel on the roof of my RV and want to add a second panel, a 100 watt Coleman. The open voltage on the first panel is 19.56 (measured with voltmeter) and the new panel is 21.25 (measured with voltmeter). I think I know the answer to my question but am I correct in thinking there is no problem connecting these two panels in parallel (+ to + and - to - ) for a combined output of 225 watts? I have a 30 amp controller feeding two 6 volt golf cart batteries. Thanks!

[Sunking]

You can connect them in parallel, but the power will not be 225 watts, something less. You now changed your 21 volt panel into 19 volts. If you have a PWM controller as I suspect your wattage will be more likely around 150-160 watts.More than the 65 watts you did have using the same controller.

If you have a means to measure the charge current to the battery. Multiply the current x the battery voltage to find the wattage.

Watts = Voltage x Current.

[bcroe]

With a relatively inefficient PWM controller, the voltage difference won't affect the charging current. If you had a more
efficient MPPT controller, Vmp would need to be compared. If they are as far apart as the Voc, you will be operating
in between at lower efficiency than a close match. Bruce Roe

[Croft]

Thanks for the prompt answers. I understand there will be some loss created by using mismatched panels but our needs are limited. Two or three days boondocking or leaving the rig for a few dayswith no hookups and just needing enough to keep the systems running and the fridge running on propane. I will be up on the roof mounting the panel tomorrow!

[Croft]

For what it is worth the 100 watt Coleman panel was on sale at Canadian Tire for $199 Canadian. ($153 US). They claim their regular price was $525 Can ($403 US).

And it looks like they actually were a good deal. The sale is over and they are back up to $1060 CAN ($814 US) for a pack of two.

[Sunking]

It is not the panel, it is the controller that has you hosed. A good 200 watt GGT panel is less than $200 with a $200 MPPT controller is equal to a 300 watt PWM system.

[Croft]

It is not the panel, it is the controller that has you hosed. A good 200 watt GGT panel is less than $200 with a $200 MPPT controller is equal to a 300 watt PWM system.

So SunKing (or anyone), give me a hand here. Given the panels I have (see OP) which controller should I be looking at? The one I have, a Sunforce 30 Amp Solar Charge Controller I bought at Camping World in 2009 for $79 owes me nothing. What is a good replacement and how many watts could I expect to get out of my setup? Thanks, I need some constructive advice here.

[SunEagle]

So SunKing (or anyone), give me a hand here. Given the panels I have (see OP) which controller should I be looking at? The one I have, a Sunforce 30 Amp Solar Charge Controller I bought at Camping World in 2009 for $79 owes me nothing. What is a good replacement and how many watts could I expect to get out of my setup? Thanks, I need some constructive advice here.

I also own one of those Sunforce 30amp charge controllers and while I felt it was a good buy at $75 I found that because it is a PWM type charger it is less efficient then a MPPT type once you exceed 200 watts of panels.

My current portable system has 5 panels that total ~ 420 watts, but because of the way a PWM CC uses that wattage I really only have about 300 watts of charging ability. That is ~ 30% loss of panel wattage that I have purchased and really do not get anything out of it.

Look into a quality MPPT type CC and you will get more charging capability out of your solar panels.

[Sunking]

So SunKing (or anyone), give me a hand here. Given the panels I have (see OP) which controller should I be looking at?

Not saying your Controller is bad, it is PWM. Just the nature of the beast. With PWM OUTPUT CURRENT = INPUT CURRENT. It is just the way they work. But here is the problem. Take a look at your panel specs and note Imp or Current at maximum power. Your 100 watt panel is around 5.3 amps and your 125 watt panel is around 6.9 amps right? With your PWM you must connect them in parallel so the current adds. 5.3 + 6.9 amps = 12.2 amps. OK into a 12 volt battery Watts = 12 volts x 12.2 amps = 146 watts. Don't even think about wiring those two panels in series with PWM. Current would be limited to 5.3 amps. That means 64 watts wired in series.

MPPT OUTPUT CURRENT = PANEL WATTS / BATTERY WATTAGE. Huge difference. 225 watts / 12 volts = 18.7 AMPS. You went up from 12 Amps to 18 amps. Now the thing with MPPT is you can wire th epanels in series IF your Imp rating is equal, yours i snot and still limited to 5.3 amps. So in series you are looking at a panel wattage reduction down to about 190 watts or so. But you can still wire them in Parallel. Take a look at the Vmp rating, ignore Voc. They both should be roughly equal. Take the lower voltage of Vmp, and multiply it by the 2 panel Imp current together. So lets say Vmp = 17 volts and Imp = 12.2 amps gives you 207 watts. That would give you 207 / 12 = 17 amps.

[Croft]

Thanks for the explanation. I have done a little research into MPPT technology myself and now understand the difference much better than I did previously and can see the huge improvement in efficiency with the MPPT over the PWM. I recall back when I was installing the original panel and looked at an MPPT unit but it was much more expensive at that time and I could not or at least did not justify the difference in price, something I am now ready to do.

I have been reading up and watching YouTube videos on the MPP Solar PCM-3012 from Taiwan. These are currently selling on Ebay for $126 US with free shipping. They seem to be true MPPT and also seem to be well built. Does this seem like a reasonable unit for me, considering that US suppliers do not ship to Canada for free?

[inetdog]

There is a credible review (even though the language is a little off) at https://www.youtube.com/watch?v=6q2ICp7acvA that shows Vout > Vin.
The review in this Forum https://www.solarpaneltalk.com/forum...ed-mppt-tested casting doubt on the MPPT behavior is flawed because the CC output is being limited by the battery SOC.
It is not a "name" company, but this unit seems to be OK.

[Croft]

I did watch that video as well as this one, https://www.youtube.com/watch?v=kTDKogV7Djk.It is a bit long but the reviewer obviously has a technical background. He pointed out the deficiencies in some of the materials and in the software design but was over-all impressed with the unit's quality and performance. I like the fact that these MPPT controllers actually use more of the power coming from the panel. The $126 price point is acceptable to me and I might even be able to sell the old Sunforce controller on Craigslist as solar components are quite a bit more expensive over here on the west coast of Canada.

[SunEagle]

I did watch that video as well as this one, https://www.youtube.com/watch?v=kTDKogV7Djk.It is a bit long but the reviewer obviously has a technical background. He pointed out the deficiencies in some of the materials and in the software design but was over-all impressed with the unit's quality and performance. I like the fact that these MPPT controllers actually use more of the power coming from the panel. The $126 price point is acceptable to me and I might even be able to sell the old Sunforce controller on Craigslist as solar components are quite a bit more expensive over here on the west coast of Canada.

The sunforce is still an ok CC. It just does not get you all of the charging capabilities out of a solar pv system that exceeds 200 watts. You can still use it for a smaller portable system.

[inetdog]

The sunforce is still an ok CC. It just does not get you all of the charging capabilities out of a solar pv system that exceeds 200 watts. You can still use it for a smaller portable system.

It gets you only 2/3 of the charging capabilities of any panel or group of panels which is composed of nominal 12V (18V Voc) panels and gets you even less of the charging capabilities of any higher voltage panels. It is not the wattage of the system that makes a difference in the efficiency.
But, perhaps better stated, a PWM CC is cost effective for a smaller system where you have 12V nominal panel(s) and you can just increase the size or number of panels to get more output more inexpensively than you can get the same increase in output by buying an MPPT CC.
If you will be maximizing your panel watts/dollar by buying higher voltage, larger, grid tie panels (or even just one), then you need to go to MPPT.

[SunEagle]

It gets you only 2/3 of the charging capabilities of any panel or group of panels which is composed of nominal 12V (18V Voc) panels and gets you even less of the charging capabilities of any higher voltage panels. It is not the wattage of the system that makes a difference in the efficiency.
But, perhaps better stated, a PWM CC is cost effective for a smaller system where you have 12V nominal panel(s) and you can just increase the size or number of panels to get more output more inexpensively than you can get the same increase in output by buying an MPPT CC.
If you will be maximizing your panel watts/dollar by buying higher voltage, larger, grid tie panels (or even just one), then you need to go to MPPT.

I agree. Your explanation was better.

Anyone want to purchase a 30amp Sunforce CC.