Questions about solar power - Page 3 - Fiberglass RV


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Old 03-06-2014, 07:49 AM   #29
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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
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Old 03-06-2014, 11:04 AM   #30
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Originally Posted by MCDenny View Post
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.
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Old 03-07-2014, 12:30 PM   #31
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Originally Posted by MCDenny View Post
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.
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Old 03-07-2014, 01:35 PM   #32
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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.
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Old 03-07-2014, 02:20 PM   #33
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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.
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Old 03-07-2014, 02:48 PM   #34
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Originally Posted by MCDenny View Post
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.

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Originally Posted by MCDenny View Post
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.

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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.
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Old 03-07-2014, 03:32 PM   #35
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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.
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Old 03-07-2014, 04:27 PM   #36
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Questions about solar power

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Originally Posted by RogerDat View Post
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.
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Old 03-07-2014, 04:54 PM   #37
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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.
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Old 03-07-2014, 05:07 PM   #38
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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.
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Old 03-07-2014, 08:38 PM   #39
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Originally Posted by RogerDat View Post
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
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Old 03-08-2014, 12:08 AM   #40
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Originally Posted by MCDenny View Post
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.
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Old 03-08-2014, 07:09 AM   #41
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From Solar Charge Controller Basics
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The obvious question then comes up - "why aren't panels just made to put out 12 volts". The reason is that if you do that, the panels will provide power only when cool, under perfect conditions, and full sun. This is not something you can count on in most places. The panels need to provide some extra voltage so that when the sun is low in the sky, or you have heavy haze, cloud cover, or high temperatures*, you still get some output from the panel. A fully charged "12 volt" battery is around 12.7 volts at rest (around 13.6 to 14.4 under charge), so the panel has to put out at least that much under worst case conditions.
*Contrary to intuition, solar panels work best at cooler temperatures. Roughly, a panel rated at 100 watts at room temperature will be an 83 watt panel at 110 degrees.
The charge controller regulates this 16 to 20 volts output of the panel down to what the battery needs at the time. This voltage will vary from about 10.5 to 14.6, depending on the state of charge of the battery, the type of battery, in what mode the controller is in, and temperature.
This is why I claimed that you need higher volt panels - the volts are the amp-pushers. If you don't get 16-20 volts from that 17 volt panel, the battery will never fully charge. As the battery gets close to full, it needs higher voltage to push in the remaining charge. Just like squeezing into a tight pair of pants - need to jump higher with more force to reach the goal.
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Old 03-08-2014, 12:45 PM   #42
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"As the battery gets close to full, it needs higher voltage to push in the remaining charge."

Er, no. First, solar panel output will always be in the same, fairly limited voltage range. (This is classical physics, Einstein's photoelectric effect.) The only exception to this rule is when the panel isn't getting a lot of sun and the panel isn't producing much power. So, even when your 100w solar panel is only producing 50, 25, or even 10w, it's still producing that power at about 17.5 volts.

Beyond that, batteries have a charging cycle that runs through three or four phases: bulk charging, absorbtion, equalization (the optional phase), and float, and none of these charging phases ever requires more than the 17.5 volt output from a solar panel.

During the initial phase of battery charging, when the battery is run down, it'll take in the full output -- volts and amps -- of a battery charger or single-panel solar panel system without over-heating or damaging the battery. This is the "bulk" charging part of the cycle, when the battery's resistance to charging is almost zero and the charging system's full output is converted almost directly into battery charge.

Once the battery gets to about 60-75% charge, the battery's internal resistance to charge increases and, unless the charging voltage drops to 14.5-14.9 volts, the excess energy will go into creating battery-damaging heat. This is the "absorbtion" phase, when the battery's resistance gradually climbs and the number of amps the battery can accept as charge declines until the battery reaches around 95% charge. It's this part of the charging cycle where an MPPT charger can make a big difference, increasing the actual current passed to the battery by 15% or so. More on that later.

When the battery's resistance flattens out at about 95% of full charge, batteries (AGM batteries in particular) can still accept just a little bit more charge, but the charger really has to work to make this happen quickly. Some charging controllers oblige them by boosting their output voltage just a bit, by a couple volts or so, for a short period of time, packing that last little bit of juice into the system in the shortest period of time. This is the equalization phase, when the charge stored in the battery's cells is "equalized," and spread evenly over the entire surface of the battery's lead plates. This charge phase has to be kept short, or the battery will overheat and become damaged.

The last phase is the "float" phase, where the charger pushes around 13.6 volts into the battery to prevent its gradual self-discharge, but, like the equalization phase, it'll also pack more charge into the battery if it has remaining storage capacity. It'll just happen much more slowly.

If you're a tech type, you can read details on how this works on the BatteryTender website, here: http://batterytender.com/resources/battery-basics.htm

Here's the difference between a less expensive Pulse-Wave Modulation (PWM) solar charge controller and an Maximum Power Point Tracking (MPPT) controller:

When the battery charge hits 60-75% and it's time to enter the absorbtion charging phase and reduce the charging voltage, PWM controllers reduce the voltage by rapidly switching the power coming from the panels on and off, so they're only fully "on" about 85% of the time, so the average voltage passed to the battery is about 14.6 volts. This means you lose that last 15% of the panel's charging capacity. When the battery charge hits 95% and it's time to enter the float phase, PWM controllers reduce the "on" part of the cycle to about 78% of the time, so 22% of panel capacity is lost.

I do not know of any PWM controller that performs an equalization step.

MPPT controllers take the full output of the solar panels and run it through a special inverter/converter circuit that pumps out that same 14.6 volts, this time without turning the panels on and off, so you get the full charging potential of the panels. They do this same thing throughout the absorbtion, equalization, and float charging phases.

So, here's the question: Is an MPPT controller worth it? Here's my take:

IF your expected daily solar panel output comfortably exceeds your daily energy consumption, no. You already have excess charging capacity and don't need an ultra-efficient charger.

IF your expected average solar panel output is much (25%) less than your daily energy consumption, no. Your solar system will only rarely enter the absorbtion part of the charging cycle, and you're just throwing money at buying charge controller features you'll never need.
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