Planning Small RV System

SpiderMan

New guy here.

First I want to say a huge "Thanks!" for the wealth of information shared here. I see a lot of selfless dedication from the regular posters. I've spent many hours educating myself and I think I have a good handle on what I need, but still have many questions. BTW, my background is engineering, but not electrical.

For starters, any thoughts about buying PV panels from Craig's List? My system needs basically 200-300W, which is all I have room for. Buying from NAWS would get me 2@ 140W panels shipped for about $750. As an alternative, I see a listing on Craigs List for a higher voltage Sharp 240W panel for about $350 (local pickup). Being a basic cheap-skate, this is very attractive, even after factoring in the bump to an MPPT controller. Based on the listing, I suspect it's an installer getting rid of random left-over stock.

I wouldn't expect any warranty support if there's a problem, but I'm not sure how much of a gamble this would be. How often do panels go bad in the real world? I'd feel better parting with my cash knowing first that it works. Would this be as simple as measuring Voc and then shorting the leads to measure Isc? (My DMM includes a clamp-on DC ampmeter). Would shorting like this damage the panel? Would I need to hook it up to a load? I haven't yet decided on which charge controller. This panel runs at about V-mp 30V; any thoughts on how to jury-rig an appropriate test load?

Thanks,
Garet

BB.

Re: Planning Small RV System

I understand and agree that buying leftover stock from local installers can really save a lot of money. Shipping a few panels at a time (especially >~135 watts or so) can be very expensive (you are shipping very thin tempered window glass in thin aluminum frames). Talking with NAWS or other vendors about optimum shipping/panel options is about the best you can do over the Internet.

To test panels, there are several things to look for. The first two will tell you 80-90% confidence the panel is good. The last gets you to the 95+% confidence:

1. Open Circuit Voltage. Voc--Basically connect a volt meter to a cool panel and uncover. Read the voltage--If Voc~specification, then OK (note, hot panels will drop Voc at ~0.36% per ^oC over 25^oC--So hot panels laying in sun will read upwards of 5-10% low).
2. Short Circuit Current. Isc--On a clear day, between ~10a-3pm, with panel pointing at sun, you should measure with a current meter (dead short on panel) of ~0.7<Imeasured<Isc... Note that this measurement is not very temperature sensitive but is directly proportional to the amount of sunlight hitting the panel. If you have clouds, Im will be much less. If you have snow/sand reflecting onto the panel, you may measure more. If you have multiple panels and take quick measurements, they should all be within ~10-15% of each other. If you find a panel that is 50% or less, then it is probably bad.
3. Vmp/Imp--Note that there are failures that can pass tests #1 and #2--So, the next thing to do is to load the panel somewhat near its Vmp/Imp operating point. That may be one or two car batteries (12 volt/24 volt/etc.) or it may be lights/power resistors. If you can load the panel and measure its operating voltage/current and it seems to be performing (i.e., a 12 volt panel with Vmp~17.5 volts, and measured current in the ~50-80+% range--the panel is almost for sure OK).

Note, for larger panels/current measurements that exceed ~10 amps--You should get a DC Current Clamp Meter--It is much easier and safer to use when measuring DC (and most do AC too) currents. Something like this one for $60 is "good enough" and has the other common DMM measurement options too.

Otherwise, when you inspect the panels--look for scratches in the glass (tempered glass can shatter very easily if scratched). Also look for brown/black spots on the panel. And over the junction box, on the glass side of the panel, look for any heat related browning or delamination. Note, if these are used panels, you may need to use some glass cleaner to get dirt off the panel to look for browning/blackening. The leaves+water from trees in my area make very similar brown stains on the glass surface.

If you are buying "A" grade panels, there should be a name tag and UL/CSA/TUV or other NRTL type approval marking.

If you are getting used panels (obviously installed outside for years)--There have been some panels that have failed after 5 years and are being replaced. Some much older Kyocera's were factory replaced, as were/are a bunch of BP Solar panels (~20+% of mine went bad after 5 years). If the person you are dealing with does not seem "right"--You may want to walk away from the deal.

Anyway, my two cents worth of advice.

-Bill

SpiderMan

Re: Planning Small RV System

Thanks Bill, that helps quite a bit.

Use the batteries as a load. Got it. I've been thinking of them for so long as a source of power, not a load.

Garet

niel

Re: Planning Small RV System

don't forget that full batteries aren't quite as good of a load as those that may be down to say 70% soc.

another thing to note is that these panels have a good bit of umph to them. now there's a word eh? just aiming them and then shorting them into a meter can cause quite the spark with a large inrush of current and can damage or, for a better description, weld metals. have a means of connecting/disconnecting like through a switch or have somebody put something over the pv for cover with removing and replacing as needed.