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I have been experiencing inconsistent results with my Nextbit Robin. I bought the power brick from Nextbit and it works as it should with their cable. I also have an Aukey QC 2.0 power brick with an Anker A to C cable and that works fine. I'm just having weird issues with battery packs. I have a Mophie Juice Pack, Aukey QC 2.0 and an Anker QC 2.0 battery pack. The Mophie doesn't charge right. Ampere says it charges at 20 mAh. The Aukey jumps all over the place but the Anker charges great. However, I have a USB digital multimeter. When i have that plugged in the Mophie will charge at about 1.7 amps. It's not QC 2.0 but does have an output of 2.1 amps i believe. Idk much about electricity so not sure what's going on....
I bought both the 15 Watt and 21 Watt versions and found the 21 Watt to perform far better. It would charge at the full 2 amp rate with ease on that battery pack which I also have. The main problem I found is that if the light is diminished the charge rate won't return to full even when sunlight is restored. I'd recommend picking up a Drok, I bought the one below and it's only .4 oz. I think it's essential for seeing how your device is charging. I found Mophie battery packs will automatically return to full charge with sunlight changes. The Astro Anker packs also seem to work better for whatever reason than the powercore.
for USB-A, this one, but it looks like it's no longer available; I think they have a newer version if you search:
You can measure via standard meters or something like: [link]
For a more precise measurement, try a USB power meter like this one here