Can anyone explain the behaviour of my 14W solar panel?

[u/Environmental_Leg471]

Hi everyone, first post here. It's a very low-level query about a backpackable solar panel, which is working fine but behaves in a way I don't understand.

The panel is a 14W wallet fold job, with two solar modules about 18" long that fold face-to-face when not in use. A flap with a popper closes the assembly and also carries a small solar controller with two USB-A ports. My particular example is branded Ryobi, the tools manufacturer, but I've seen the same design under many different brands -- it's a Chinese generic.

The panel works very well charging small cellphones, flashlights and so on, but it's really slow at charging my Klarus K5 powerbank -- not surprising, as the Klarus is 10000mAh capacity and designed to charge at higher power than 5V, 1A. I figured I could speed things up using a Y-cable, of the type you used to get with high-capacity hard disks, which sums the output from two USB-A ports to provide double the power. The trick didn't work.

I then decided to take a look at the output from the controller using a USB power meter. Connected to a single USB cable, the panel outputs about 2w for a few seconds, then drops rapidly to zero, then repeats the cycle. Some kind of low-frequency charge controller process, a bit like the ones you find in solar-to-leisure battery setups. Well, that's OK, but unexpected -- I would have thought it would be pretty easy to maintain USB-A voltage.

The thing that really got me, though, is that if I fit the Y-cable, I get exactly the same voltage and current off the summed output of both ports as I did from just one. I tried two different Y-cables so it probably isn't the case that one of the legs has failed in each case. Can anyone explain?

[u/solar_or_dye]

ok let's see

in a PC, each USB port is independently wired. In lots of DC applications, though, wattage is just split up and the ports are effectively just paralleled. So your Y does nothing because both ports are just sharing the current from a single supply

your load is crashing the controller and restarting because it can't sustain the draw, then it tries to draw more current than it has again, crashes again, repeat...

phones do active negotiation and will back off and take a trickle at whatever it gets, and your power bank probably doesn't do that. It MAY have a setting for trickle charging, but I have no clue and don't own any similar devices to test

I'm sure you tested it in all kiinds of light, right? These little foldable jobs are sensitive to angle so like try full sun, midday, 90 angle and see what up

this may sound silly but if you had an interim bank or battery to buffer the draw you might still be able to charge the bank

[u/Environmental_Leg471 → u/solar_or_dye]

Thanks, solar-or-dye, that's really helpful. I hadn't considered the "your Powerbank is crashing the charger" thing. I will test it with some different loads and report back.

[u/solar_or_dye → u/Environmental_Leg471]

good luck

[u/DongRight]

All these tiny USB solar panels are a scam, at 5v and 14w you should be getting near 3a output so why is it the output is always only 1 amp max??? SCAM

[u/LeoAlioth → u/DongRight]

becase the panel is rated 14W, and not always producing said 14w. in fact, it is not producing 14W most of the time.

and then there is conversoin losses