Solar education

Author: Dave
Date: 01.28.26 - 8:18pm

Ok, so I got a little more than i bargined for..

I bought 10 "tested" panels off of a guy on facebook marketplace. He showed a pict of pallets of panels. I figured he was an installer or a dealer. I knew 0 about solar panels so I was trusting. $50 each for used 235W panels.

First up, they all came with the leads cut 1" out of the junction box. The bullect connector inside propiritary. Not supposed to solder this, so I had to do some testing and come up with my own butt crimp that latched onto the existing wire coming out of the bullet, and still fit inside the junction box. Ok cool i figured it out. (I also found some old new stock on ebay of exact sample junction boxes with the perfect connectors so I had 6 panels covered right there)

Next, i figured Id test the panels to see which ones i wanted. I only needed 6-8, i figured id be safe buying 10 just in case. Here were my test numbers.

Panel/String #	Pmax (Watts)	Vmp (Volts)	Imp (Amps)	Voc (Volts)
1	200	29.17	6.85	36.8
2	145	25.6	5.66	35.25
3	161	29	5.57	37
4	117	24.6	4.8	36.72
5	103	25.3	4.1	36.31
6	135	26.2	5.13	36.5
7	178	27.3	6.5	36.95
8 (Faulty)	60	24	2.59	36.1
9	151	28.2	5.3	37
10	198	29.5	6.85	37.5

oof! Now talking with teh guy turned out he lived in a tiny home with 4 other families on teh same property. Immigrant. I dont think he was a dealer or installer. I believe he was actually acting like a broker and took a pict of some other guys pallets of solar panels! His testing was probably open voltage only..obviously not under load ampage. Only 3 of these panels are decent and some are outright junk!

So i figured I would screw around. I took the 60 watt panel and snipped out the diodes and retested to see if one had shorted and was sucking current. Nope. I then meansured the diodes on the bench, all fine. Ok..so my solar tester needed more than 12v to turn on, so I disconnected the strings at the junction box and differential tested the different cells of strings

String Terminals	Pmax (Watts)	Vmp (Volts)	Imp (Amps)	Voc (Volts)
1 - 3 (Full)	48W	23V	2.1A	35V
1 - 2	33W	15V	2.3A	23.4V
2 - 3	30W	15V	2A	23V

Since my meter couldnt test one string on its own, i ended up ordering a dc ampage clamp meter. I also build a load bank of 3s3p halogen bulbs.

I will add those results once i get the new meter in. My current one wont do DC current. So what can I do..what can i do.

Well how about I hook a power supply up to each string of cells and pump a couple amps into them and see what heats up on the thermal camera?! Bingo!

Check out these results...

Image 1 shows a nice even distribution, the current is making it to all the cells and their interiors. The other two images, only the bus bars are really heating up. The cells arent getting much electricity. Seems like they arent working right. This is mirrored in the differential outputs measured above. #3 must be the worst.

What surprised me is that the panel visible looks perfect. No corrosion on the bus bars, the junction box was clean. It looked like a great panel, but...total junk! Also remember Voc no load testing also means nothing. You will get Voc at panel rating even on a cloudy day. You must know the amp output and if current will collapse the instant you apply a load. Voltage no load is just a saturation state.

When I can load test each string independently then it will be more clear.

So I got an education...I think I am going to sell off all these panels as a lot for $100 or something and just buy new ones. I need 120v for my charger to start, which means I would have to start using my mid panels. Not worth the labor of hanging them. bleh!

but i wouldnt have learned this any other way..

Here is the solar cart i put together for it.