Multi-solar panel interconnections: Mind the electrons’ directions

The concluding post in a series; here’s part 1 (covering the Energizer PowerSource Pro Battery Generator) and part 2 (discussing its companion 200W portable solar panel)…

Eagle-eyed readers may have already noticed, within the listing for the Energizer Solar Bundle I bought at the beginning of September (and returned shortly thereafter), the following prose:

Pick up another 200-watt solar panel on SideDeal to max out the solar capacity of 400W (but do take note of these instructions)

That I did, for an incremental $179.99. It was (past tense usage because I ended up returning it, too) a “Duracell Heavy Duty 200-Watt Briefcase Solar Panel”, and it also came from Battery Biz. Here are some “stock” images:

Along with a bullet list of specs:

Brand: Duracell
Material: ‎Monocrystalline Silicon [editor note: claimed conversion efficiency of 22%]
Dimensions (unfolded): ‎55.2 x 36.5 x 1.3 in
Item Weight: ‎36.2 lbs
Maximum Power Point Voltage: ‎18.6 V
Maximum Power Point Current: 10.8 V
Open Circuit Voltage: 22.8 V
Short Circuit Current: 11.2 A
Maximum System Voltage (Vmax): 1000VDC
Normal Operating Cell Temperature (NOCT): 45±2°C
Temperature Range (°C): -40°C~+85°C
Power Tolerance: ±5%
Application Class: Class A
Included Components: ‎Panel
Maximum Power: ‎200 Watts
Manufacturer: ‎Duracell

When it arrived, the outer packaging was…err…already breached:

Thankfully, the box inside was still intact:

although, as you may have already noticed, the product image on the front didn’t match the “stock” shots, which left me a bit disconcerted before I even opened it.

Here’s the box backside:

Take the “unfolded” dimensions listed in the earlier bullet list, divide the largest by two, and you end up with an approximation of the data shown in this closeup image, which was reassuring:

Open the box, pull out the hefty contents (the promotional prose claims that you can “Experience the convenience of home no matter where you are with our lightweight and foldable solar panel” (foldable = yes, lightweight = not), and the “Zippered Carrying Case with Handle” conceptualized in the previous photo’s graphic is the first thing you’ll come across:

Chug a can of spinach, unzip the case and huff, puff and pull the bulky panel out:

Now undo the clips on the “handle” edge and unfold it:

Like I said, it looks nothing like the “stock” photos, although it ended up being fully functional.

I only realized after returning it that I’d neglected to snap a photo of the backside, so another closeup of the box will have to do:

The cable coming out of the back of the panel terminates in a pair of MC4 connectors:

The also-included adapter cable converts the MC4s into an Anderson PowerPole PP15-45 connector for use with the Energizer PowerSource Pro Battery Generator (along with other Anderson-compatible battery-based products, of course, including Duracell’s own):

Its measured open-circuit output voltage closely approximated the earlier-listed max spec:

and it did charge up the PowerSource Pro reasonably speedily in direct sunlight:

Had I been able to as-intended combine it with the Energizer solar panel, of course, the combo likely would have been speedier still from a charging-rate standpoint. But from my earlier writeup, you already know about the connector woes that prevented such an arrangement and resultant experiment. And since the other gear I’d bought ended up being subpar and sent back to the retailer, I had no other use for this panel, either, so it got returned for full refund, too.

That all said, for the remainder of this writeup I’d like to delve a bit deeper into the suggested dual-panel configuration linked to within the initial Meh teaser and replicated here:

To a first conceptual approximation, you can think of two (or more) solar panels as batteries. Connect them in series and you boost the effective output voltage. Tether them in parallel, conversely, and the aggregate output current goes up. That said, my research has enlightened me, they also exhibit important differences from batteries in both possible configurations. These variances derive from two fundamental keywords: batteries (ironically) and shade (i.e., “dark”).

Rarely if ever is the solar illumination of a single panel uniform, far from across multiple panels. This inconsistency is due to various factors, such as imperfect orientation versus the sun’s position of the moment in the sky, and the aforementioned shading caused by clouds, trees, and other partially-to-fully obscuring intermediate objects. For series-interconnected panels, illumination inconsistency means that the effective current you’ll be able to squeeze out of a multi-panel configuration is constrained by the least-illuminated panel in the chain. And you’ll of course also want to make sure that the aggregate voltage generated by the multi-panel series arrangement in full illumination doesn’t exceed the max input voltage of whatever it’s driving.

What about multi-panel parallel hookups such as the one recommended in the Energizer documentation? To tether them together requires a combiner cable such as this one I’d bought:

Minimally, you’ll want to ensure that the output voltages of both panels match (for reasons you’ll soon understand) and, this time, that the aggregate current generated by the multi-panel parallel arrangement in full illumination won’t exceed the max input current of whatever it’s driving. “What it’s driving” in my particular case was the Energizer PowerSource Pro Battery Generator, supposedly. Which is where the words “shade” and “battery” fully come to the fore.

Assume first that the combiner cable simply merges the panels’ respective positive and negative feeds, with no added intermediary electronics between them and the electrons’ destination. What happens, first, if all the parallel-connected panels are in shade (or to my earlier “dark” wording surrogate, it’s nighttime)? If the generator is already charged up, its battery pack’s voltage potential will be higher than that of the panels themselves, resulting in possible reverse current flow from the generator to the panels. Further, what happens if there’s an illumination discrepancy between the panels? Here again there’ll be a voltage potential differential, this time between them. And so, in this case, even if they’re still charging up the generator’s batteries as intended, there’ll also be charging-rate-inefficient (not to mention potentially damaging; keep reading) current flow from one panel to the other.

The result, described in this crowded diagram from the same combiner-cable listing on Amazon:

is what’s commonly referred to as a “hotspot” on one or all panels. Whether or not it negatively impacts panel operating lifetime is, judging from the online discussions I’ve auditioned, a topic of no shortage of debate, although I suspect that at least some folks who are skeptical are also naïve…which leads to my next point: how do you prevent (or at least minimize) reverse current flow back to one or both panels? With high power-tolerant diodes, I’d like to postulate.

Those folks who think you can direct-connect multiple panels in parallel with nothing but wire? What I suspect they don’t realize is that there are probably reverse current-suppressing diodes already in the panels, minimally one per but often also multiple (since each panel, particularly for large-area models, is comprised of multiple sub-panels stitched together within the common frame). The perhaps-already-obvious downside of this approach is that there’s a forward-bias voltage drop across each diode, which runs counter to the aspiration of pushing as much charge power as possible to the destination battery pack. To that point, I suspect this is precisely what this Amazon reviewer is experiencing (also check out the video at the source link):

So I can draw about 86 to 92 watts to my Jackery with the cable from the solar panel but as soon as I plug in the splitter, it drops down to about 60-70 watts. When I plug in both 100 watt solar panels, I get about 120-130 watts. So the splitter somehow loses power when hooked up.

If you look closely at the earlier “crowded diagram” you can see a blurry image of what the combiner cable’s circuitry supposedly looks like inside:

Prior to starting this writeup, I’d returned the original combiner cable I bought, since due to my in-parallel return of the Duracell and Energizer devices, I no longer needed the cable, either. But I’ve just re-bought one, to satisfy my own “what’s inside” research-induced curiosity, which I’ll share with you in a teardown to come. Until then, I welcome your thoughts in the comments!

—Brian Dipert is the Editor-in-Chief of the Edge AI and Vision Alliance, and a Senior Analyst at BDTI and Editor-in-Chief of InsideDSP, the company’s online newsletter.

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Multi-solar panel interconnections: Mind the electrons’ directions

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