TLDR: "You get what you pay for!"
This "500W Solar Panel" was advertised online (Shopee) for 1098 Php (~19 USD).
Curious, I ordered two of them. With shipping, the price was still under $39.
The two solar panels arrived about 8 days later and I hooked each of them up to a 100A charge controller.
One of the panels immediately began sending 18V to its charge controller; while the other one would sporadically send power, then stop.
Knowing that 99% of the time, the culprit for these problems lies within the MC4 connections, I dismantled the one on the finicky panel's PV wire.
As soon as I got down to the ferrule inside the MC4 connector, the PV wire crumbled and detached from the ferrule.
The inside of the wire contained corroded tin flakes around old and brittle copper strands.
It was so bad that I couldn't strip the wire and replace the MC4 connector.
The junction box was in the same, sad shape as the wires.
I ended up clipping the old wires and cleaning up the contact points as best I could.
With no soldering iron available, I attached some spare cigarette lighter wire (rated for 15V-18V) to the contact points a shrink wrapped around the exposed old metal contacts.
Finally, I attached new MC4 connectors to the new wires.
I was eager to see if my DIY junction box setup was successful, so I hooked up the appropriate probes to the inside of the MC4 connectors.
To be honest, I wasn't expecting it to work, but...
...my "first time DIY makeshift junction box setup" was pumping out 18.35V!
(Hoo-Yah! DIY Rules!)
Now the "finicky panel" is no more and working as well as its twin brother solar panel.
One thing still bugged me about this project: "Seriously, 500W solar panels for $19? Really?"
Even though the back information plate states that the panels are "SUNPOWER-600W", the panels are definitely NOT "600W" or even the "500W" solar panels that I originally ordered.
The specifications listed in the advertisement boldly says it all:
Voltage: 18V, Open Circuit voltage: 21.6V, Short circuit current (or ISC): 0.83A
These specs are NOT for 500W solar panels.
They are for panels rated 300W or less!
At 18V and ~10A, each one of my panels is sending 180W to its corresponding charge controller.
Oh well, I guess I shouldn't complain.
I mean, I only paid $19 for each solar panel.
And each one is functioning like a 200W+ panel.
Submitted FYI.
May you and yours be well and loving life today.
In Lak'ech,
JaiChai