Charging a Storm 2 with the Renogy E.FLEX 50 Solar Panel

So, I picked up a Storm 2 battery pack, for the capacity, the screen showing charging / discharge details, and because it can be charged via a DC port. I figured it would be a good companion for the Renogy E.Flex 50 solar panel. The panel puts out 21.6V and 3.04A to an open circuit, and has a nominal output of 18V at 2.77A (49.86W). In reality, of course, I usually see the panel top out in the low 30W range, almost always around 17V with the available current dictating the watts transmitted:

17.23V 1.76A 30.3W (12/15 9:25 a.m. direct sun)
13.96V 0.03A 00.4W (12/14 12:13 p.m. shade)
17.12V 1.00A 17.1W (12/14 10:11 a.m. direct sun)
12.12V 1.02A 17.4W (12/17 10:45 a.m. thin cirrus cloud cover)
17.06V 0.62A 10.5W (12/17 10:01 a.m. thin cirrus cloud cover)

Into a full battery pack, I was seeing higher voltages but minimal current, with correspondingly reduced wattage, e.g.: 19.51V 0.04A 00.7W (12/20 9:52 a.m. direct sun).

This should work with the Storm 2 pack, which purportedly supports DC charging from 5-24V ⎓ 3A (up to 72W). (This teardown goes into some detail on the internal buck-boost charging circuit, built around the SC8821A.)

The Storm 2 has a 5.5x2.5mm DC input/output jack. The Renogy panel has direct connectors to the panel that terminate in MC4 connectors, and comes with both an MC4 to 5.5x2.1mm adapter cable, and a set of adapters for other DC plugs. I hooked the Renogy up to the Storm 2 via the Renogy bundled cable and plug adapter, and it immediately started showing a charge.

Just, not a great one. Hooked up to the Renogy RPB-72000PD battery pack, I see about 3x the wattage being delivered, and the voltage is high enough to activate the DC monitor I have inline on the Renogy outputs (which requires 6.5V to work). Connected to the Storm 2, however, it only seems to ever deliver about 4.4V. In the same sun, off the same panel:

RPB-72000PD: 19.69V 0.46A 9.00W | 17.55V 1.27A 22.5W 
Storm 2    :  4.40V 0.68A 2.99W |  4.35V 1.75A  7.6W

The E.FLEX 50 also has what Renogy calls a “smart junction box” with USB-A (12W), non-PD USB-C (15W), and DC (16V ⎓ 2.8A or 19V ⎓ 2.4A, about 45W) outputs. My next attempt will be trying the DC output using the bundled 5.5x2.1mm x2 cable and the 5.5x2.5mm plug adapter. When I tried that during this initial attempt, despite the panel providing in excess of 16V using the direct PV connectors, the Storm 2 did not report any charging activity (I tried with the Renogy panel switched to both 16V (one blue LED lit) and 19V (two blue LEDs lit). I didn’t have a multimeter with me, though, so I couldn’t do much troubleshooting.

Update: I pinged Shargeek and Renogy and r/SolarDIY for help figuring this out. So far Shargeek has responded with:

You should not connect the SolarPanel directly to STORM2 power bank via MC4 to DC5525 connector. Instead, you should use the "Smart Junction Box" which equips a DC output. Though the maximum output of the Solar Panel seems OK to charge STORM2 directly, but the fact is, every Solar Panel needs a converter (including MPPT module) to get a stable DC output. The output resistance of Solar Panel and corresponding MPPT converter are different from each other. The MPPT circuit is the reason why the battery pack you use supports direct solar panel charging. STORM2 has no MPPT module. You should use the DC output of the "Smart Junction Box" to charge the STORM2.

Reddit I think has me on the right track:

Power (volt X amps) of the panel depends on voltage, but not in the way Ohm described it. The power will be 0W at Voc. It's open circuit, so 0A, hence 0W. Power will go up as you go down in voltage till you hit Vmp. At Vmp you will get Imp and that's the wattage on the sticker (if you have perfect conditions which never happen). And power will go down to 0 as you go further down in voltage from Vmp to 0V (you are on the right track if you end up with a triangle). Isc will happen, but at 0V you end up with 0W. Voc depends on temperature, age of the panel,... etc. so this is all dynamic.

Good solar chargers know this and try to keep the panel at Vmp as much as possible. These good chargers will convert whatever the current Vmp is into whatever volts the battery/load needs. Those are called MPPT charger.

Not so good chargers will switch very rapidly between load and open circuit. Those are way was efficient (bad) and will also turn the DC coming from the panel into something closer to AC (confuses some DC measurement equipment). Those are called PWM charger.

My guess: ... [The Storm 2] has a cheap PWM not really hitting the right spot for that panel (or it's not a solar charger at all, just a wide range input).

Update: I hooked up my trusty Fluke on a bright but cloudy day, and sure enough, saw ~16V with the Renogy switched to 16V, and 19V when switched to 19V.

 

I hooked the Storm 2 up to the DC output on the Renogy panel and occasionally saw ~16V (with the panel set for 16V), but not much charging (not enough light); more often than not I was seeing ~4V or no input whatsoever. I’ll try it again on a sunny day, but at least now I know it does work.