Old thread but I piked up an Slate in the recent sale an had the same problem where the phone was slowly discharging while tethered.

I resolved the issue by using an Amazon Basics 3.0 USB Hub with 2.5A external power attached the the Slate USB2.0 port, and connecting the phone to one of the hub’s USB ports. Charging is slow but it actually charges rather than discharging so all good.

I tried powering Slate from the hub as well as the phone but that causes the phone to continuously start/stop charging. Not sure what was happening but powering the Slate separately seems to be the way to go.

-PG

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