After fooling around with https://backpackinglight.com/forums/topic/myog-usb-power-bank/
I noticed that when I charged my android from my anker 13,000 usb power bank, sometimes it’s very slow. It doesn’t really matter that much because I have hours available and it eventually charges.
I have these short USB A to USB C cables that came with some product I bought. When I plug in a USB power meter I see that sometimes it delivers 1.7A which would charge the phone in a couple hours which is acceptable, but sometimes it only delivers 0.5A or 0.3A which would take forever to charge phone.
I read this https://learn.adafruit.com/minty-boost/icharging
They talk about how a charger communicates with a phone (or other device) what current that charger can deliver. If the phone tries to draw too much current, it can damage the charger.
One way is for the phone to send a digital message to the charger and the charger sends a digital message back with the capacity information, but I think this is unusual. Especially with cheap chargers.
Another way is to bias the data lines.
There are four lines in the USB – +5 volts, ground, D+, and D-. The charger will have pullup and pulldown resistors. If, for example, there’s a 50K pullup from D+ to +5, and a 50K pulldown from D+ to ground, then it will have a voltage of 2.5 volts – a resistor divider.
I noticed with the Anker that the D+ line was 0.6 volts, and the D- line was 2.5 volts. If the Android sees those voltages, then it knows it can draw up to 2 amps.
But, the cheap USB A to USB C cables I used don’t have any wires for D+ and D-. That’s fine for the cheap device they came with – it draws less than 0.5A so it can just do it without looking at the data lines.
My android does look at those lines. With my cheap cables without data lines, the android doesn’t see the 0.6V on the D+ and 2.5V on the D- out of the Anker. So the android is confused about what current it can draw – sometimes it thinks 0.3A, sometimes it thinks 0.5A, sometimes it thinks 1.5A.
So, I bought a short USB A to USB C from amazon. 6 inches. They advertised the speed data can be transferred so I know it includes the data lines. 0.3 ounces. The cable without the data lines weighed 0.2 ounces.
When I use that to charge my android from Anker, it now draws 1.7A. The android is sort of flaky though, occasionally it gets confused and goes back to drawing a low current.
Apple devices are much more regular and consistent, they’re all designed by Apple so they’re the same. Adafruit only talks about Apple phones – it’s as though androids don’t exist. I don’t think Apples have these problems. Unless you tried to use a usb cable without data lines : )
