Solar Panel efficiency when segments all in series and some 'blocked'


Newly Enlightened
Jan 18, 2015
Hi all,

I'm cycling around the world and recently bought a cheapo 20v 10W solar panel on aliexpress. On paper it looked ideal - lightweight with no glass, but the epoxy top scratches easily. I noticed when I first got it that hiding even one of the 72 segments would cause the power output to seriously drop. Hiding three would drop the output below 5v. Having this solar panel on the back of the bike doesn't work as the odd shadow causes it to fail.

At the moment I've got it wired to a switch-mode 5v regulator. If I'm lucky I can get 500mA at 5v when I should be getting 2A. I believe this is due to the panel being slightly scuffed on top.

Am I right in saying this is due to high resistance? It looks like I can probably rewire the panel to have the four large strips in parallel but even then this may not be enough.

Or should I give up and get one with a glass top?



Steve K

Flashlight Enthusiast
Jun 10, 2002
Peoria, IL
Losing power due to one of the series cell being shaded is a common problem, I've heard. The various solar power forums should be a good place for help... I seem to recall seeing circuits that would disconnect the cell from the string and connect the adjoining cells together. This probably isn't an option for a smaller panel.

The choice of the load will affect the power you can get out of the panel. Hmmm.... where to start?? Well, the solar panel can be modeled as a voltage source with a series resistance. Depending on the amount of sun hitting it and the temperature, the voltage of the voltage source and the size of the resistance will vary considerably. Getting the most power out of the panel will require adjusting the input resistance of the load (i.e. power supply) appropriately. You might want to search on the phrase "maxiumum power point tracking".
The basic issue is that if you want to have the load draw the most power from the panel, the input resistance of the load will have to be equal to the internal resistance of the panel. ... and the internal resistance of the panel is constantly changing.

In my professional life, I have designed a circuit to do the "max power point" tracking function. Kinda fun, and it did teach me how complicated it can be to use solar panels. In my personal life, I've used small panels (i.e. not the 3000 watt panels that I dealt with at work) to charge a battery. For this, I just hooked the panel up to a battery with an appropriate voltage and enough capacity that it couldn't be damage by trickle charging it with the panel.

Your 20v panel is probably designed to produce around 12v while delivering the max power. As such, your best bet would be to hook it up to the battery (with a series diode to prevent the current from flowing back into the panel when it isn't generating power). The 5v regulator can then be powered from the 12v battery.
The downside is that the battery adds weight, especially if it is a robust type, such as nicad.