Portable solar and power banks best practices?

I admit, I’m starting this thread to highlight a comment I made here (hoping to get it noticed and get answers.)


I wound up getting the Big Blue but for my home, to charge small devices. I have a few questions on this topic.

1) If we’re not supposed to fry our phone battery by putting it in direct sunlight, why is it okay to fry the battery pack?

2) Is it a viable strategy to try to shield the gadget or battery pack from the sun by putting it, for example, under the solar panel or otherwise in the shade?

3) What happens if you plug more than one device into the solar panel? for example, what if you plug in two battery packs. Do they each get charged at half the rate? I’m sure it’s not that neatly broken out, but I’m trying to get the general idea.

4) I am a little bit confused about how to figure out what “perpendicular to the sun” means because the sun is a ball not a plane, to be simplistic about it. Could you explain? Also, what are best practices for propping portable panels up so that they are at the correct angle? .


  • Comments (10)

    • 8

      re: #4, it just means facing the sun head on – so the flat part with the panels faces the sun with the greatest surface area possible.  Technically this would be an offset tangent, but in order to work a solar panel has to face the sun, and it works best when facing the sun so it gets the maximum solar exposure.

      re: #2.  Yes.

    • 12

      re: #1 because the battery pack is much cheaper to replace than your phone.

      Another reason to charge the battery pack instead of your phone directly: the battery pack has a much lower standby charging current than your phone. To make up some numbers: if you phone uses 0.2A (A=amps) to run it’s charging circuit, and the battery pack uses 0.02A, the battery pack can successfully fill up (slowly) from 0.1A from the solar panel, whereas your phone would actually lose power under that amount of charging current. Then, when you move the power from the battery pack to the phone, it can move the stored energy much faster (say 2A), so that the phone can actually charge.

      re: #3 If your load (device, battery pack etc.) can’t charge with as much current as the solar panel can provide, adding another load may not reduce the current going to the first one. Again, to make up some numbers, if load C can charge at max 1A, and the solar panel is putting out two amps, you can plug in a second load C without effecting the charging rate of the first. Now if load B charges at max 1.5A, and you plug in a second load B, they will both be reduced to 1A.

      I highly recommend you get a usb power meter or two (I like this one: https://www.amazon.com/gp/product/B07SX18TWK/ref=ppx_yo_dt_b_asin_title_o07_s00?ie=UTF8&psc=1), so you can measure it yourself and know what’s going on. I keep one in every kit that has a solar panel, as it is worth the weight to be able to measure and optimize your charge. You can also use this to visualize the effect of panel tilt on charging (re: #4). The sun is so far away that from our perspective, the rays are essentially parallel (that’s how Eratosthenes was able to estimate the radius of the Earth in ~200 BC). If your panel is perpendicular to the sun’s rays, the solar intensity and charging rate are highest. If you are off by maybe 10 or 15 degrees, you probably won’t even be able to measure the loss. However, as you get further off from perpendicular, charging power drops off rapidly (it’s a curve function, not linear).

      • 8

        Thanks for clarifications. About #4, I’m still a bit unclear. The rays are parallel to what? Do you mean to one another? How does that help me determine what is perpendicular to the sun’s rays? Don’t the sun’s rays go in all directions? I mean, wherever I go in a sunny environment, the sun is shining on me. So, how can I tell at what angle the rays are coming in so I can determine what’s perpendicular to that angle?

      • 9

        I’m assuming this is an honest question for an honest answer – but I think you’re making this more complicated than it needs to be.  Don’t worry about the “parallel/perpendicular” mathematics of all this, if your goal is to charge a phone, you don’t need that level of detail to meet your goal.

        Your question: “how can I tell at what angle the rays are coming in” – is relatively simple:  Arrange your solar panel so it is as flat as possible, then angle it with the cells pointed towards the sun, the optimal angle is that which casts the largest shadow (assuming you cells are flat…).  This will change throughout the day as the shadow moves.  The optimal angle in the morning and evening are different.

        for more information, read this: https://www.solarreviews.com/blog/best-solar-panel-angle

      • 9

        Yes, the sun’s rays are essentially parallel to each other. They are emitted in all directions, but only the ones that are pointed at Earth hit us, and that’s such a tiny arc of the sun’s overall emissions, that they are essentially parallel. That’s why you cast a crisp shadow in direct sunlight. Imagine a dotted line from the sun to your solar panel; make the panel perpendicular to that line and you are getting max power. So when the sun is directly overhead, your panel should be flat. When the sun is down at the horizon, your panel will be close to 90 degrees to the ground in order to get max power.

        In practice though, the sunlight is most intense and useful when straight overhead (shortest path through the atmosphere to you), so if you leave the panel flat (parallel to the ground) in the late morning and pick it back up again in the early afternoon, you’ve gotten the majority of available energy with the least fuss. If you are in a situation where you are desperate for electricity and you want every last scrap you can wring out of the panel, then go out and adjust the tilt every hour from dawn to dusk. You might get on the order of 20 or 30% more power that way.

      • 5

        For all practical purposes, the sun is a point source.  The rays striking your locality are on a plane of zero degrees.  orient the panel at 90 degrees.  Or place an object on the ground, say a cardboard box (or the upright paneel itself).  It will cast a shadow,  The line from the top of the box to the back of the shadow is a zero degree plane.  Orient the panel at 90 degrees, more or less.  Change as the sun moves.  best done around noon so less need for adjustment..

    • 8

      Others have weighed in with some good answers, but I’ll take a stab as well:

      1. My thinking on this, is that your battery pack is much more replaceable/expendable than your phone. If I have to pick between baking my phone battery and baking an external battery pack, I pick the latter.
      2. Yes, I would always try to do this if possible. It’s much better to place the battery beneath the panel or in some other setup that keeps it cool, than to just leave it out in direct sun.
      3. How well a panel handles multiple devices depends on the panel and on the load. There is a lot of variance in this, and we didn’t really try to test it in the review. But I do want to do some more experimenting with this.
      4. Regarding “Perpendicular to the sun”, the comments from user Rich DC are correct and are the right way to think about this.
    • 6

      I was researching this on my own this very moment, decided to take a break and come over to The Prepared. 🙂 Since several of my questions have now been answered, I’d like to ask another two:  1. “What battery packs do people recommend?”, and, 2. “Will a battery pack charge a laptop?” (Forgive my extreme ignorance, but this is how we learn!)