24 volt panels

[justjim]

It does not matter what comes into a MPPT controller. What matters is what comes out of it.

[MN C Van]

Will it convert 12.8 to 14.7v?

[Blacktank]

hmm,so just need a mppt controller which are said to be better,a 350 w panel for $150 sure sounds good to me,planning on a roof mounted system and then a 1-200 w mobile system because if i can park in the shade i'm going to do it

[justjim]

Will it convert 12.8 to 14.7v?

The voltage the panel is putting out correlates to the amps it puts out. Any panel putting out 12.8v is going to be so far below its norm that I doubt it would have any amps backing it up. it would have to be extremely low light for even a 12 volt panel to be producing such low voltage. I suppose in theory the buck converter in a MPPT controller would do it but the output would be nil.

[justjim]

Gary

A 350 watt panel should produce 25 amps or so at 12 volt. You would want a 30 Amp controller.

[Blacktank]

see i have been passing over 24v on craigslist,can also contact the local solar companies for when they have an leftover panel or two from residential projects,solar is well down on the priority list but if a deal pops up...

a quick google look and 300w 24v for $200-250 which is about the savings over 12v for a ok mttp controller

[frater secessus]

confirming i need 12 volt panels and 24 wont work,or will they?

Good information already, so I'll approach it from another angle.  

With MPPT it is common to use 24v (or 20v) panels with 12v bank, as has been described already.

Folks often say you can't use a PWM controller with 24v/20v panels and a 12v bank.  That's not exactly right;  you can but it won't be able to make full use of the panel.  In this scenario the panels will be run at battery voltage (Vbatt), yielding about half the panels' rated power.  The silver lining, should we need one, is that at half power there will be no temperature derating.  :-)

20v+ panels tend to be cheaper by the watt, and tend to be easier to find in high watt ratings.  The higher voltage is an antidote to those edge cases where MPPT output drops below PWM (12v poly  + high ambient temperatures + high Vabs settings + MPPT efficiency losses).

[justjim]

I have never seen a test where high voltage and MPPT loses out to PWM in actual output to the battery. In fact the Best PWM does is come close in high temps. All of my Panels are Polys. I might see some derating in excessive high heat but I am talking a few amps at 100 degree temperatures. It has been a discussion for years on RV.net but actual side by side test do not prove it out.

I guess it is time to hitch the computer up to the controllers and break out the MSview. It will show me any voltage drop at high temperatures. It would take a huge drop in voltage before a PWM system that loses 30% of its power automatically will ever match it.

[MN C Van]

LoL, so the blinking LED SAYS its charging, and the panels are putting voltage rapidly varing from 16-22v, but bat voltage is stable at 12.92- so I'd guess my theory is academic.
Odd, the charge LED goes off when i remove the multimeter leads, then goes back on ...

[frater secessus]

I have never seen a test where high voltage and MPPT loses out to PWM in actual output to the battery. In fact the Best PWM does is come close in high temps.

I didn't claim that.  Here is what I wrote:

"The higher voltage is an antidote to those edge cases where MPPT output drops below PWM (12v poly  + high ambient temperatures + high Vabs settings + MPPT efficiency losses)."

It would take a huge drop in voltage before a PWM system that loses 30% of its power automatically will ever match it.

There is no 30% MPPT advantage in the case I described.

.