Re: [A/S] Solar Panel Suggestions

["Maurice Riggins <maurice.riggins@xxxxxxxxxxxxxxxx>" <maurice.riggins@xxxxxxxxxxxxxxxx>]

Solar panel output voltage drops as the panel temperature rises. 
That's true even of the flexible panels, which aren't as bad as 
others about losing voltage due to heat (but they also start out with 
a little lower voltage to begin with).  The temperature rises 
significantly from the black surface absorbing heat from the sun, 
just like a solar collector for hot water.  Putting one of these flat 
on the roof with no space under it for cooling increases its 
operating temperature which reduces its output.  And I've gotta 
believe the hotter it runs, the shorter its life.  These are 
certainly tempting if you're looking for something to do away with 
mounts.

From the 5W panel you used as an example, I assume you're just 
looking for something that will maintain the batteries when the 
trailer is parked, without having to plug the trailer in.  While 5W 
may do one battery, you need twice that for two.

Phantom loads can consume more than the smaller panels put out.  For 
example, the LP detector, CO detector, and refrigerator circuit board 
(even on gas) can each draw 200-300 milliamps each,  that translates 
into 5-6 amp-hours per day for each of these.  That's in addition to 
needing amp-hours to replace the self-discharge of the battery.  And 
whatever you do, don't forget to turn off the batwing TV antenna 
amplifier!

On a really good day, a panel laying flat (with cooling underneath) 
puts out an average of 70% of its peak current rating or peak wattage 
over about a five hour period.  For each of the above phantom loads, 
you need about 5.5AH per day.  I've read you'll also need about 1 amp-
hour per day for each 100AH of battery capacity to account for self-
discharge.

Two 100 AH batteries w/no phantom loads (you disconnect the batteries 
from everything but the solar panel) = 2AH/.70 = 2.9AH/5 hours = .6A 
X 17 volts = 10 watts of panel

One phantom load (i.e. LP detector) with two 100 AH batteries = 
7.5AH/.70 = 10.7AH/5 hours = 2.14A X 17 volts = 36 watts of panel

Two phantom loads (i.e. LP and CO detectors) with two 100 AH 
batteries = 12.5AH/.70 = 17.9AH/5 hours = 3.6A X 17 volts = 61 watts 
of panel

Although these numbers don't account having extra panel capacity to 
make up for cloudy/rainy days, you don't really have to allow for 
them IF the current draw is only going to be self-discharge and 
phantom load currents, i.e. an amp or less.  At this low of a 
discharge, the battery capacity is more than the rated capacity, just 
as with more than 5 or so amps per battery, the battery capacity is 
less than the rated capacity.  I've heard of people with 25W panels 
getting by without disconnecting the LP detector (with no CO 
detector) but that will depend on your local temperatures and weather 
patterns.

As far as a charge controller, you generally don't need one if the 
panel's peak current output is less than the total battery capacity 
divided by 60.  For two 85AH batteries that's 170/60=2.8A (about 48 
watt panel at 17 volts).  Also keep in mind that the prewired cord on 
the small panels usually is too short to reach any solar prewire down 
in the refrigerator compartment, so you may wind up installing a 
weatherproof junction box in the refrigerator vent to extend the wire 
down to the compartment.

The other downside to these flexible panels is that they are 
expensive per watt and take up a lot of real estate on the roof.  
This is a consideration if you ever want to upgrade.  For the same 
price as the flexible 32W panel (which if installed on the roof deck 
with no circulation under it may only put out the same as a 25W 
panel) you can buy a 50W panel that's less than half the length.  
Down the road, you have space for another 50W panel and would have 
about 4 times the output, for only about twice the price (yeah, gotta 
buy mounts) in the same roof length.

Just some thoughts...
--
Maurice
WBCCI 5446