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ppine

Northern Nevada

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Posted: 12/17/20 09:20am Link  |  Quote  |  Print  |  Notify Moderator

Forget used panels.
forget cheap panels and controllers.
Then you just have to buy more of them.

pianotuna

Regina, SK, Canada

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Posted: 12/17/20 01:37pm Link  |  Quote  |  Print  |  Notify Moderator

SJ-Chris wrote:



I was able to test each 250w panel. They were producing at 37V and 9.1amps. Seemed very healthy. I'm curious if anyone can comment on the likely output of these to the batteries from the MPPT controller after the controller converts it to be used on my 12v system... Is there a simple equation for this?

-Chris


Hi Chris,

I would definitely go with 500 watt systems.

I'll be interested in your real life results.


High end MPPT charge controllers "steal" about 5% of the output.

500 watts X .95 = 475 watts X 5 hours = 2375 watt-hours (in perfect solar conditions).

Batteries "steal" 10%. I.E. use 100 amp-hours and return 110 amp-hours.

2375 X .9 ~= 2137 watt-hours sent to the battery.

Depending on the type of battery (a single 100 amp-hour) acceptance rates are, at 85% state of charge 12.5 amps

12.5 x 12 volts = 150 watts for each 100 amp-hour battery.

In SiO2 that is doubled to about 300 watts.

In Li the battery management systems typically limit charging to 1C or about 1200 watts.

So the answer to your question really depends on the type and number of battery in use.


Regards, Don
My ride is a 28 foot Class C, 256 watts solar, soon to have SiO2 batteries, 3000 watt Magnum hybrid inverter, Sola Basic Autoformer, Microair Easy Start.

BFL13

Victoria, BC

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Posted: 12/17/20 02:19pm Link  |  Quote  |  Print  |  Notify Moderator

"I was able to test each 250w panel. They were producing at 37V and 9.1amps. Seemed very healthy. I'm curious if anyone can comment on the likely output of these to the batteries from the MPPT controller after the controller converts it to be used on my 12v system... Is there a simple equation for this?"
----------
if the panels are aimed at the sun at high noon, your 500w loses 10% from panel heating, so now at 450w. Line loss from panels to controller using proper wire should be under 3%, so call it 2% off the 450, so now at 439w input watts to controller. Say controller is 95% efficient doing 24-12, so lose 5% of 439 = 22w so output watts = 417w

Amps to battery (assuming no other loads) = output watts / batt v, so
it depends on batt v at the time what amps are. (there might be some more loss on the wires from controller to battery too, but we are pretending that is no loss for this example)

417/13v = 32 amps
417/13.5 = 31 amps
417/14 = 29.8 amps

With 500w of PWM aimed at high sun for full Isc (you get about the same amps to battery as the Isc and no loss for panel heating since Isc does not drop with heat (it goes up a bit)--expect about 31 amps

31 = 31. but as previously mentioned that is with panels aimed at high noon on 21 June in say, Texas.

You need another calculation for say, 9am with flat on roof panels on 21 Sep in, say Seattle, and another for 21 Dec.

All you can do is stick those panels up there and you get what the weatherman will allow that day for your latitude and calendar date. Looks like you got a very good deal on those 250s!


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SJ-Chris

San Jose, Ca

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Posted: 12/17/20 03:36pm Link  |  Quote  |  Print  |  Notify Moderator

BFL13 wrote:

"I was able to test each 250w panel. They were producing at 37V and 9.1amps. Seemed very healthy. I'm curious if anyone can comment on the likely output of these to the batteries from the MPPT controller after the controller converts it to be used on my 12v system... Is there a simple equation for this?"
----------
if the panels are aimed at the sun at high noon, your 500w loses 10% from panel heating, so now at 450w. Line loss from panels to controller using proper wire should be under 3%, so call it 2% off the 450, so now at 439w input watts to controller. Say controller is 95% efficient doing 24-12, so lose 5% of 439 = 22w so output watts = 417w

Amps to battery (assuming no other loads) = output watts / batt v, so
it depends on batt v at the time what amps are. (there might be some more loss on the wires from controller to battery too, but we are pretending that is no loss for this example)

417/13v = 32 amps
417/13.5 = 31 amps
417/14 = 29.8 amps

With 500w of PWM aimed at high sun for full Isc (you get about the same amps to battery as the Isc and no loss for panel heating since Isc does not drop with heat (it goes up a bit)--expect about 31 amps

31 = 31. but as previously mentioned that is with panels aimed at high noon on 21 June in say, Texas.

You need another calculation for say, 9am with flat on roof panels on 21 Sep in, say Seattle, and another for 21 Dec.

All you can do is stick those panels up there and you get what the weatherman will allow that day for your latitude and calendar date. Looks like you got a very good deal on those 250s!


Great info! Taking into account all the potential losses here and there that you mention, the 500w drops down to closer to 417w under "ideal conditions". Well, we know that we will almost NEVER be in "ideal conditions" so the panels will produce less than 417w most of the time. That makes me think that the 30amp MPPT controller is going to work perfectly and it will most likely NOT limit the charging probably ever. I was not "worried" but was just a little bummed thinking that the 30amp controller might actually cut off the maximum amount of amps provided by the panels sometimes, but it sounds like max amps out will likely be somewhere between 25-30amps 99% of the time. Good enough for me! :-)

Too much holiday stuff going on/planning right now to do the install without my wife killing me...lol. So I'll likely tackle installing solar onto my 2 RVs in January (one with a 200w WindyNation sytem which cost ~$280, and the other with a 500w SanTan Solar used panel system which cost me $225). When I do, I will run some tests and collect some data and report back. I'm also going to lay out my 200w WindyNation panels such that I can easily add a 3rd 100w panel if I decide to do so later (...the controller can handle up to 300w).

Testing fun... My battery bank on each RV is ~200AH. I will have an Aili battery monitor on each RV when I do the testing. (I already have it installed on one and love it). I plan on doing the following test: Drain each RV battery to 50% at nighttime. Then 3-4 times throughout the next day I will record the Amps flowing into the battery and the SOC% as measured by the battery monitor. I will do one day of 200w WindyNation PWM vs 250w SanTan MPPT, and another day with 200w WindyNation PWM vs 500w SanTan MPPT. Yes, I'm a bit of a nerd...lol

Yes, I think $35 per 250w panel was a pretty good deal [emoticon]
(But it was only possible because I happened to be visiting someone who lives ~10 miles from SanTan headquarters...otherwise I'm 700 miles away and shipping makes them impractical). But if you go look on your local craigslist you will likely find some very good deals on used 200-400w panels locally that you can pick up. Just be sure to bring a volt/amp meter and test them.

For my case:
- New WindyNation 200w system cost: $1.4/watt
- Used (panel) SanTan Solar 500w system cost: $0.45/watt (68% less)

I'm guessing that with 500w of solar and 200-300AH of battery bank, I'll never have to worry about keeping my batteries fully charged. Now, I'm gonna go and create a new post: How to use MORE battery power when camping!

Chris


San Jose, CA
Own two 2015 Thor Majestic 28a Class C RVs

pianotuna

Regina, SK, Canada

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Posted: 12/17/20 03:52pm Link  |  Quote  |  Print  |  Notify Moderator

Hi Chris,

It is easy to use the "excess" energy, if you have an electric/gas water heater, and 2000 watt inverter. Simply turn on the water heater, after the batteries are full.

To turn the watt hours and watts in my post to amps and amp-hours, simply divide by 14.8 volts.

My guess is that with 500 watts, the solar harvest will max out about 135 amp-hours.

SJ-Chris

San Jose, Ca

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Posted: 04/06/21 03:54pm Link  |  Quote  |  Print  |  Notify Moderator

Wanted to post a follow-up on this "inexpensive panels" thread...

I finally got around to installing the 500w system on my RV with two 250w panels. I'm happy to say that the SanTan used solar panels (~10yr old) are working beautifully and cranking out plenty of power.

Here's a good link to the install project:
https://www.rv.net/forum/index.cfm/fuseaction/thread/tid/30217540.cfm

I'll be adding a 3rd lead acid battery soon making my battery bank 300AH. Then, as a test I'll drain the batteries at night to 50% (-150AH) and I'll let you know how much the 500w system gets them back up to by the end of the next sunny day.

I liked this setup so much, I've decided to ditch the brand new 200w Windy Nation syetem and just install an identical 500w system (two 250w SantTan panels and a 30amp MPPT controller) onto my 2nd RV. If anyone lives in the CA Bay Area and wants a good deal on a brand new 200w Windy Nation system, feel free to send me a private message.

Happy Camping!
Chris

* This post was edited 04/06/21 06:05pm by SJ-Chris *

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