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I normally am pretty good with electrics and they taught us about them in a few of my classes, specifically my into to engineering courses. I however don't know a ton about solar panels so this project is a frustrating adventure to me.

Basically we have 3 Items:

  1. 12GA copper stranded wire
  2. 18V 8W 450mA Solar Panel, Link
  3. DC-DC step down module, Link

So, I would think this would be a pretty simple task, solder some wire on the + and - of the solar panel and plug it into the two ports on the DC-DC converter. However after I do this, nothing lights up.

I measured the solar panel and it was at ~19.6V [+/- .5v] (unloaded reading on meter) so all seems well there.

Now, normally I would think the DC-DC regulator is bad. So, I have tried with a different one that I purchased for a laser project that is next on the bench. (link -> buyincoins/item/46768.html#.VVGPHoCy3ak) still doesn't light up at all.

My goal is to use this 18V, 8W, 450mA solar panel to charge a few mobile chargers that I have in the end, so it should ideally take 18V 450mA and make it 5V 2A or 1.5A (which seems reasonable to me, but even 1A is fine. I just want to sit it and forget to ensure the portable chargers are full)

Can someone please help me out here?

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3 Answers 3

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The power output of a solar panel depends critically on the output voltage and current. There is an optimum point, called the maximum power point. You could say it is the "sweet spot" for the panel.

If you try to draw more than the peak power current from the panel, the voltage will start to drop fairly rapidly. So, assuming the regulator works, I think you simply tried to draw more current than the panel could supply, thus causing the voltage to collapse. If the panel was not in a good sunny area and pointing directly at the sun, then you could try that to see if it helps.

There is nothing wrong with using a DC-DC converter, but you have to make sure it does not try to draw more current than the panel can supply. There may be some DC-DC converters which sense the maximum power point and scale back on output current to keep the panel in that region. You can google MPPT converter. See what you find.

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  • \$\begingroup\$ Yea, I was using a light to try to power the panel and just light up the controller. I put it in the sun and it started working as when I did this, it was night time. But now I am seeing voltage drops when the solar panel is overloaded, it drops to 4.4V then jumps to 5.2 causing inconsistent powering of devices.... it is kind of frustrating me now as i would think an 18v 8W solar panel would be able to charge a dinky little mp3 player, but it cuts out continuously... :/ is there any way to set it up so i have a battery as a buffer to fill up what is missing? \$\endgroup\$
    – Tab Carmel
    Commented May 13, 2015 at 21:28
  • \$\begingroup\$ I was thinking if I could put like a few 18650s in line then I could set it up so that they would charge back up when the device hits full using the solar panel. I have a rectifier save solar-panel drain already. \$\endgroup\$
    – Tab Carmel
    Commented May 13, 2015 at 21:39
  • \$\begingroup\$ It becomes complicated. In order to charge a lithium ion battery, the charger has to monitor the charge current. So generally, you need a charger with power path management. It has a battery connection, a system/load connection, and a power input connection. As long as either the power input (from solar panel) or battery is capable of supplying energy, it will power the load. If extra energy is available, it will recharge the battery. The fact that the power supply is a solar panel adds yet another complication. I think you need to spend some time with your favorite search engine. \$\endgroup\$
    – user57037
    Commented May 13, 2015 at 21:43
  • \$\begingroup\$ The solar panel can supply 8W under specified conditions. Maybe you will get 6 or 7 Watts under real world bright light conditions, but only if you use maximum power point tracking (MPPT). A typical USB device, if it draws only 500 mA needs 2.5W. So it SHOULD be enough, given the right DC-DC converter. If your USB device uses 1.5A, that is 7.5W, and it may be too much for the panel, even with MPPT. \$\endgroup\$
    – user57037
    Commented May 13, 2015 at 21:48
  • \$\begingroup\$ I see what you mean now. Well, I think I have a plan. years ago, i got this little power bank that can both take energy from the solar panel to charge the battery and a device at the same time. I will prolly pull it apart and use it's guts to make that happen. My main concern is that I wont be able to remake it with a larger bank of batteries in parallel since the original 6 year old Lipo battery is shot. I have done some googeling, to figure stuff out, but your right I do need to do more to ensure that what I want to do can be done, but with some info, google wasn't the best helper. \$\endgroup\$
    – Tab Carmel
    Commented May 15, 2015 at 4:44
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What is the loaded voltage output from the solar panel?

It's likely that the panel can't (at least where you're testing it) provide enough current.

My first suggestion is to ditch that DC-DC converter. The display alone will be sapping the majority of the available current, not to mention the display driver and whatever processor is controlling the output regulation. Get yourself a good old fashioned LM317 adjustable voltage regulator [Product site]. It can provide up to 1.5A and only needs two resistors to set the output voltage. It's also no doubt what's in your fancy converter. You don't need a display if (a) you've got your own multimeter, and (b) you're going to keep the voltage fixed.

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  • \$\begingroup\$ -1: It is not recommended to use an LDO when you have big difference between Vin and Vout, in this case 13V is pretty huge. With 450mA current it would waste 5.85W, the LDO won't bear that heat. \$\endgroup\$
    – Bence Kaulics
    Commented May 12, 2015 at 8:48
  • \$\begingroup\$ That was only a suggestion, not an edict; the true solution will depend on the answer to the question in the first line. And there's no shame in using a heatsink! \$\endgroup\$
    – CharlieHanson
    Commented May 12, 2015 at 9:20
  • \$\begingroup\$ It was just a remark. The heat is not the main issue it would be just another problem to be solved. (I did not say it could not be solved). But it would not be efficient, wasting energy instead of using it to charge. \$\endgroup\$
    – Bence Kaulics
    Commented May 12, 2015 at 9:31
  • \$\begingroup\$ With regards to the LM317, that looks like a 4 prong chip... do I just solder wires to the chip and call it a day? I work with computers everyday so it feels weird to use a single chip for everything. Do I still need a diode? Last, how do i regulate voltages with that? Bence, what did you have in mind to regulate the voltage? \$\endgroup\$
    – Tab Carmel
    Commented Jul 27, 2015 at 11:32
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If you wish to go with the LM317, you have 2 main options. 1 use variable resistors and get a adjustable supply or 2 do the math and fix the voltage and amps with set value resistors. I would suggest using a solar charger for 12v led-acid and let the battery smooth your output. Using li-po or li-ion is always a bit risky as they require much tighter control (such as charge balancing)and react far worse to of spec voltages and amps. Collect your power when you can and use it when you need it. Good luck

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  • \$\begingroup\$ Are you attempting to reply to the comments under @CharlieHanson's answer? \$\endgroup\$
    – Null ♦
    Commented Feb 15, 2016 at 16:42
  • \$\begingroup\$ Both I guess, I read some about the lm317 for a "lab power supply" I´m building but decided to go with a dc-dc converter instead. \$\endgroup\$
    – Anders_gbg
    Commented Feb 16, 2016 at 18:19

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