Questions about solar power

[Mike Magee]

It has been my understanding that a PWM controller discards the additional voltage, whereas a MPPT controller will utilize the higher voltage to push more current (with some internal losses due to power consumption of the MPPT circuitry). Which would seem to suggest that a 21V, 3A input would be turned into something like a (roughly) 14V, 4A output. Yes, no? Discussion?

[blodn1]

A charge controller prevents the solar panel overcharging the battery. The MPPT advantage doesn't go into effect until the batteries are so low that more amps can be forced in at a higher voltage. To get that small increase in wattage, you pay a lot of money.

You'll also have a lot of loss the farther from the battery all your equipment is - that's one of the reasons I was arguing about putting the batteries on the wheels and extending the cables. The voltage drop over the system would be considerable.

I'm still planning to put a couple of panels on the roof of the Snoozy, connected to a PWM under the bed and very close to the batteries - using heavy wire for less resistance. Haven't figured out the size of the system, yet, but I'm loving your calculations of use:-) They're real world help for my system, so Thanks!

[Carol H]

Originally Posted by blodn1

You'll also have a lot of loss the farther from the battery all your equipment is - that's one of the reasons I was arguing about putting the batteries on the wheels and extending the cables. The voltage drop over the system would be considerable.

I would be surprised if anyone here was to argue with you over the fact that its best to put a solar charge controller as close to the battery as possible (mine is actually in a water proof box connected directly to the battery box) and most installs I have seen here have been done with the solar controller as close to the batteries as possible.

I believe the suggested made to you & which you argued against was in reference to your concern for the location of the "electronics" as you referred to them with no mention of it actually being a solar controllers location you where concerned about. As such the assumption was made that the "electronics" you were concerned about was your power converter. On many trailers the converter is more often than not located at the opposite end of the trailer from the batteries thus the reason the suggestion was made.

[blodn1]

I think I also mentioned electrical "stuff" :-)

In the Snoozy, converter (and stuff) are located under the bed in front. With the batteries on the tongue and the other electrical operations under the bed, solar's a relatively easy add. When I was at the factory, Richard and I discussed a way of glassing in the cable run through some sort of conduit so that it would be an integral part of the trailer and would take a linear route to the controller, making it a shorter run.

As I've said, though, I'm open to suggestions.

PS "Argue" was used in the sense of debating, not fighting.

[MCDenny]

I finally found an understandable discussion of PWM vs MPPT controllers. It's a white paper written by Morningstar, leading maker of both types. Spoiler alert: It says PWM is recommended where power is low <200 watts and temperature is warm.

http://www.sunwize.com/documents/Mor...r-jf_12-12.pdf

[Jon Vermilye]

I just added a review of Solar Blvd's 200 watt folding panel at my website.

[Briantb]

In as much we are talking a small trailer unless you park the panels a long way from the trailer, line loss while always an issue will not be major.. You do need to be primarily concerned about cable size from the panel to the charge controller
We have the folding 100 Watt from Solar Blvd and use the stock cheap controller and connecters that came with it.. Works well for our purposes, maybe not so for others.

Now for our house system. Different story.. Mppt controller and wiring sized specifically to distance from panel to charge controller.. Only a couple of feet from batteries and inverter.

[MCDenny]

Originally Posted by Jon Vermilye

I just added a review of Solar Blvd's 200 watt folding panel at my website.

Thanks Jon. (And thanks for posting a link to my blog. I get lots of visitors coming form Lakeshore images)

You have a 160w panel but measured 8.5 amps at, I assume, 12.7 volts for a fully charged battery. That's 108 watts. What's up with that?

[blodn1]

Spoiler alert: It says PWM is recommended where power is low <200 watts and temperature is warm.

"Optimal input voltages for most MPPT controllers outputting to a 12-volt battery is in the 34-50 volt range." [from: ]RV Electrical

[Carol H]

Originally Posted by MCDenny

Spoiler alert: It says PWM is recommended where power is low <200 watts and temperature is warm.

That pretty well supports what our local solar supplier told me when I was in talking to him in regards to upgrading my system a few weeks ago.

[multi-task]

Originally Posted by MCDenny

Thanks Jon. (And thanks for posting a link to my blog. I get lots of visitors coming form Lakeshore images)

You have a 160w panel but measured 8.5 amps at, I assume, 12.7 volts for a fully charged battery. That's 108 watts. What's up with that?

To calculate panel watts use panel voltage. In this case the panel specs show it would sit around 18v, so thats 18 x 8.5 = 153w. His panel is operating close to perfect.

[MCDenny]

Originally Posted by multi-task

To calculate panel watts use panel voltage. In this case the panel specs show it would sit around 18v, so thats 18 x 8.5 = 153w. His panel is operating close to perfect.

Except that the voltage isn't 18 volts, it's the battery voltage (as I've learned since the my reply asking about the watts difference).

Seems this is the dirty secret of 12v battery charging with a solar panel. The rated power is at the max voltage, usually 17 or 18 volts, but when connected to a battery it puts out its max current but the voltage is whatever the PWM controller is calling for to charge the battery. Probably 13.6v for float and 14.4 for bulk charging. So the 160 watt panel would only be able to put at most 8.5 x 14.4 = 122 watts into the battery. 77% of its 'rated' power.

An MPPT controller could do a little better but comes with it's own electrical overhead and great price difference.

[blodn1]

Like anything tested, there have to be standards. In the case of solar panels, I think the standard test temperature is 77 degrees. There are also other standard conditions (I can't remember all of them) including the number of hours of optimal sun angle. No panel is going to have all the standard conditions happening all the time, so there are many inefficiencies in the system, causing lowering of the "rated" values - just like MPG for cars.

[multi-task]

Originally Posted by MCDenny

Except that the voltage isn't 18 volts, it's the battery voltage (as I've learned since the my reply asking about the watts difference).

I purposely left out the controller/battery from my calculation as I didn't spend time finding out what kind of controller the panel had. Yes, with a pwm controller it will drag the voltage down, this is common knowledge. Sorry, I thought you were interested in panel watts.

Originally Posted by MCDenny

An MPPT controller could do a little better but comes with it's own electrical overhead and great price difference.

There's no real overhead if you buy a real mppt controller. My Morningstar controller is 97% efficient and self consumes 35mA. Cost really depends on what you are trying to accomplish. I have two meters and mine is pretty much right on spec. But I also use a 42v panel, most people aren't.

Originally Posted by blodn1

Like anything tested, there have to be standards. In the case of solar panels, I think the standard test temperature is 77 degrees. There are also other standard conditions (I can't remember all of them) including the number of hours of optimal sun angle. No panel is going to have all the standard conditions happening all the time, so there are many inefficiencies in the system, causing lowering of the "rated" values - just like MPG for cars.

Exactly. This is why NOCT ratings are more important than STC ratings and also why +- tolerance on panels should be taken into account. If a panel has a tolerance of +-5% you can almost bet you're getting -5% most of the time. A big problem is that lower end manufacturers don't even provide most of this data so you're stuck with STC ratings.

[RogerDat]

This goes to an earlier post. The basis I read to use is a panel generates at 50% output for 1/2 the daylight hours and the other 1/2 at full output.

Really when you think about it if your battery loses say 10% per day it would still take 5 days to get down around 50% charge. If your solar cuts that to 5% discharge a day your going to run out of water before electric :-)

Calculations assume that after 6 days you need to be at 100% charge, which you don't actually need. You just need to still be above 50% charge. If you use 40 ah and put back in 30 ah how many days will it take before your batteries are at 50%?

My mistake I re-read you post, you are assuming your battery will be run down after 6 days. At least I think you are.

[MCDenny]

Originally Posted by RogerDat

This goes to an earlier post. The basis I read to use is a panel generates at 50% output for 1/2 the daylight hours and the other 1/2 at full output.

Really when you think about it if your battery loses say 10% per day it would still take 5 days to get down around 50% charge. If your solar cuts that to 5% discharge a day your going to run out of water before electric :-)

Calculations assume that after 6 days you need to be at 100% charge, which you don't actually need. You just need to still be above 50% charge. If you use 40 ah and put back in 30 ah how many days will it take before your batteries are at 50%?

My mistake I re-read you post, you are assuming your battery will be run down after 6 days. At least I think you are.

Yes, we use about 40ah per day so 3 days takes down to 45%. I'd like to double that time so need about 20ah per day from the sun. A 100w panel would put 5a into the battery for (according to the solar tables) 5 hours where we generally camp in SW Florida in January.

We like primitive camping at Myakka River State Forest. We can replenish the water by towing the trailer about a half mile to the ranger station but would need a second 100w panel to get all our power from the sun. We can also leave and go to a full service site for a day or two. We have a 30 amp charger so can easily go from 50% to 100% with an overnight plug in.

I'm leaning toward trying one panel first but getting a controller large enough to comfortably handle two.

PS: We camped five months last winter and just finished our second month this year. I'm really enjoying the process as we learn more about camping and what we want from the Lil Snoozy. Probably a month doesn't go by that we don't make some improvement. It fits us far better now than when it was brand new.

Refrigeration was our last big change after a lot of study and now solar is an even more complicated challenge and I can see a propane furnace in our future next year.

I just got an slick outdoor shower (total cost $20) this afternoon. I'll show you the details in another thread.

[Carol H]

Originally Posted by MCDenny

We can replenish the water by towing the trailer about a half mile to the ranger station.

I just carry portable water jugs with me and top off the water tank as needed. Also carry a portable 8 gal tank for draining off the grey or black water as needed. Would hate to have to move the trailer just to deal with water issues.

[RogerDat]

People have done some nice dual panel installs with a fixed panel and portable one to supplement. Totally agree getting a controller that allows for expansion just makes sense.

I'm just in the scoping out stage for solar. So for now I'm following all the solar threads with interest. 40 ah a day seems fairly high, do you run a 12 volt compressor fridge by any chance? I had one of those in the past recall it could use a lot of my RV battery.

[MCDenny]

Originally Posted by RogerDat

People have done some nice dual panel installs with a fixed panel and portable one to supplement. Totally agree getting a controller that allows for expansion just makes sense.

I'm just in the scoping out stage for solar. So for now I'm following all the solar threads with interest. 40 ah a day seems fairly high, do you run a 12 volt compressor fridge by any chance? I had one of those in the past recall it could use a lot of my RV battery.

Yep, just put in a 4.2 cu ft Truckfridge 12v compressor refrigerator. By itself it uses between low 30s and high 30s amp hours per day depending on outside temperature. I learned it makes a big difference if I can keep the sun off the fridge side of the trailer.

3 hours of interior lights (all LED) take another 3ah or so

[multi-task]

Originally Posted by MCDenny

Yep, just put in a 4.2 cu ft Truckfridge 12v compressor refrigerator. By itself it uses between low 30s and high 30s amp hours per day depending on outside temperature. I learned it makes a big difference if I can keep the sun off the fridge side of the trailer.

3 hours of interior lights (all LED) take another 3ah or so

I also use a DC fridge. My experience with a smaller 75w panel was not great. It could push 4amps in on a 'perfect' day. Problem was, most days aren't perfect. So it was averaging around 3 amps. When the fridge cycled on it used 2.2, so now I was left with less than an amp going in. It slowed charging considerably, even worse on very warm days when it cycles on more.