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Old 03-11-2016, 01:37 PM   #11
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The more I measure, think and price out options, the more I think I am going with 24 volt panels and MPPT.

Check my logic here:

I haven't done a proper power audit, but figure I am going to run 2 Golf Cart batteries and upgrade to 4 if 2 don't cut it, although 2 should based on what I have looked at. With that in mind, I am sizing my Solar (at least upgradable) to enough to run 4 batteries. To me, with what I have read, 4 batteries is around 400A-hrs, and solar, by rule of thumb would be around 400W to maintain the 400A-hrs. This puts me into a Tristar 45 (if I go Morningstar), which has a top capacity of 600W. If I got to 600 W, I'd be great with 4 batteries and it would be ok on less sunny days.

Looking at costs for 600 watts of system (I'll call this "nominal" since I can't get to 600 watts exactly with 24 volt panels) I find the costs come out nearly equal. The savings in panels for a 24 volt panel system are offsetting the cost of the controller. This was just looking at panels, controller, monitor and temperature compensation. The were within 10% of each other and the higher voltage seems to have a few benefits that would be good. Primarily, not much worry about wire size upgrades.

So I am really heavily considering buying a couple 260W or 285W panels (Solar Blvd seems to have good prices if I can't find locally) and a Tristar 45 MPPT. Really thinking about mounting one panel on the roof and keeping the other stored to be used as portable for the heavily shaded sites. This will give me good flexibility. I want to mount the panel towards the front of the roof since that end of the trailer is least likely to be shaded.

The other thought with buying so much wattage right now, even when I don't need it, is that the panels will be matched and I won't have to worry about it. I'll have it all now.

Am I crazy in my line of thinking?
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Old 03-11-2016, 01:40 PM   #12
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Quote:
Originally Posted by Mustang65 View Post
I personally think that it is overkill to worry about voltage loss if you are using the proper size cable for the job.
Isn't calculating the voltage drop the way to properly size the cable?
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Old 03-11-2016, 01:49 PM   #13
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Quote:
Originally Posted by Subaru297 View Post
I'd be interested in the correct formula.

Thanks
Ok, here you go....

To calculate it out, you need to know a few numbers (resistance). You can not just add up all the lengths of your 3 cables and run it against an online cable calculator because your circuit is a series-parallel circuit. You have 3 legs 1(leg from controller to junction box) 1 (leg from panel #1 to junction box), 1 (leg from panel #2 to junction box). Look up the resistance for that cable size (the numbers are stated in resistance for each foot). Multiply the length of each leg * the resistance per foot number and write it down. Once you have the resistance for each leg, the formula will be a series parallel calculation.

Calculate Resistance in parallel (Leg#2, Leg#3) first:

1
_______
1 1 (this is 1/R1 + 1/R2.... it will not allow me to put spaces)
__ + ___
R1 R2

Now calculate TOTAL Resistance of the circuit in series (leg #1 + Combined parallel total from above calculation of (Leg#2, Leg#3):

Rt (Total):

Resistance of Leg#1 + Calculated Leg (Leg#2, Leg#3) = Total cable resistance

You now have the Total Cable Resistance Rt

V (volts)/ rt = Voltage drop

8AWG:
1000' of cable = 0.6282 Ohms
1' of cable = .0006282 Ohms

Or once you cut the cables to their required lengths you can take an Ohm meter and measure the resistance of each of the cables.

Have fun, just use 8AWG for the two panel legs and a 6AWG down, this should allow for future panels

Don
Really had to dust off some of those unused brain cells...
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Old 03-11-2016, 02:40 PM   #14
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Like I said, Mustang would be the best bet to answer
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Old 03-11-2016, 04:06 PM   #15
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I take it back. I don't want to know
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Old 03-11-2016, 06:46 PM   #16
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Cdash... How would you plug in your portable panel?
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Old 03-11-2016, 07:51 PM   #17
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Originally Posted by klassic View Post
Cdash... How would you plug in your portable panel?
Get or make some cables with MC4 on one end and terminal rings on the other end and connect to a lug or terminal block near the controller. Could use a wing nut on the lug to put it on and take it off, or just leave it attached and coil up the cable when the portable isn't in use. I'd connect the roof top solar to this same lug to combine them and then run to the controller.

Seems like an easy way to do it.

Other thought is to use the "solar on the side" connector and just plug it in there and then I wouldn't have to remove anything from the lug.
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Old 03-11-2016, 08:04 PM   #18
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Side solar goes direct to the battery
You would be at 24v.
And from what I am led to believe the side solar portable panel has a small controller of some sort??
Onefastdaddy has a schematic from Jayco of the size side solar posted.
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Old 03-11-2016, 08:15 PM   #19
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Quote:
Originally Posted by klassic View Post
Side solar goes direct to the battery
You would be at 24v.
And from what I am led to believe the side solar portable panel has a small controller of some sort??
Onefastdaddy has a schematic from Jayco of the size side solar posted.
I found a MC4 cable connector that converted from a MC4 connector to a pigtail and I connected my cable run to the pigtail end. If you can't locate the MC4 to pigtail converter by a short MC4 to MC4 extension cable and cut it in the middle and wire it to your connecting cables.

Don
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EnGenius WI-FI extender, D-Link wireless (n) modem
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