Solar Help

X2 though I have a 15v panel. It is designed to be used without a controller.

Jim

Hi BFL13,

My point is that 17 volt panels are better than 22 volt when using a PWM technology charge controller, so he has nothing to worry about if say the panels were 16.5 volts.

It is true the output voltage is controlled by the battery on pwm.

It is true that lower voltage so long as it is around 17 is better when using pwm.

It is true that with 36 volt panels one would be nuts to use pwm. MPPT would be the way to go then.

I've never seen a 15 volt panel. That would be a rare bird indeed.

PT is confusing matters by not explaining. Panel voltage is the same as battery voltage with a 12v system. As battery voltage rises the spread between battery voltage and panel Voc spec voltage shrinks, reducing amps.

With lower battery voltage (and so panel voltage) the spread is large and amps can flow.

However, you don't want those lower voltage panels they had for a while with only 32 cells that were "15v" instead of "17v" like 36 cell panels. those lower voltage panels could not charge RV batteries to 14v because the spread was too small between 14 and 15 so they went to 17 to get more spread so battery voltage could rise to where it needs to go.

BTW, with a B and roof real estate "issues" you might be better off with 24v panels and MPPT for the amps you can get in that space, but it will co$t you for that MPPT. OTOH a B can't carry many batteries, so you don't need that many amps perhaps.

Hi MickD,

If you are doing a PWM controller volts being down on the panels is a good thing.

The volts should remain the same 21ish for all 12v panels. You may be confusing 24v panels with 12v panels. Note that Isc on 24v panels is half what it would be for equal wattage 12v panels (natch! :) )

You don't use PWM with 24v panels and a 12v battery system, you use MPPT (big bucks!) controllers with 24v panels to get 12v out to the batteries. Also the MPPT makes its own amps so the 24v panel amps specs mean absolutely nothing

You don't use Imp when on PWM. Use the Isc with 12v panels as expected max amps (except in edge of cloud effect when amps spike higher than Isc)

I have a pair of 100w 12v panels that get 6.3a each as Isc. Together (in parallel) they make 12.6a using the one PWM controller. If I had a single 12v 200w panel I would get the same 12.6a from the same controller.

So now you are asking, "How in heck can I figure out my expected max amps when using 24v panels where the amps spec means nothing and MPPT?"

We have reports that 235w 24v panel and MPPT gets 15amps pointed at the sun. This is about the same as you would get with 12v and PWM with 235w worth of panels. "How can that be when MPPT gets you way more amps than PWM?" you ask.

it seems that people might be seeing those half-amp specs on the 24v panel and seeing them double when using their MPPT controllers making 12v output and they think that MPPT is twice as good as PWM? :) who knows? ( I think MPPT is silly for the money it costs unless you have special circumstances, but never mind me!)

Several have said that I should get 2 panels to total the necessary watts needed. I have no problem with that but when I look at the specs for lower watt panels the IMP goes way down also the volts. What is the effect on charging the batts? Also that means more brackets and more wire and more holes in the roof.

Mick

Hi BFL13,

You just don't camp in weather cold enough for it to matter. Or hot enough. Mena could fry his marvelous battery bank without temperature compensation.

It does matter which battery post the temperature probe is on.

Some controllers have temp comp but not remote, so the temp is where the controller is. However, since the controller should be mounted close to the batteries if possible, this can make the temps "close enough" for ambient at the battery.

So far as getting the temp right for inside the battery, that is a whole other thing. Not clear that attaching the temp probe to a battery post does that either.

From what I have seen, temp comp doesn't matter while camping anyway. That would be more of a Float, storage requirement unless you are camping in winter in Regina or in summer in Death Valley :(

Hi MickD,

Any controller that has temperature compensation at the battery will meet your needs.

BFL13 wrote:

What does this mean?
"• Industry’s lowest voltage per watt rating
Delivers the most cost-effective installs"

Maybe they meant low price per watt. Who cares... It's a Chineglish...

I wholeheartedly agree with suggestions to install 2*90W over one 180, wherever possible. Handling a big panel on a small roof can be a challenge. It is heavy and big. You think it fits, and then suddenly realize that you don't "fit" on the roof next to that panel.