Joined: Aug 2010
Posts: 12,760
Likes: 2,117
From: Madison, WI
Bikes: 1961 Ideor, 1966 Perfekt 3 Speed AB Hub, 1994 Bridgestone MB-6, 2006 Airnimal Joey, 2009 Thorn Sherpa, 2013 Thorn Nomad MkII, 2015 VO Pass Hunter, 2017 Lynskey Backroad, 2017 Raleigh Gran Prix, 1980s Bianchi Mixte on a trainer. Others are now gone.
When you say it stops working at higher speeds, how do you know that? I recall reading a review a few years ago of a dyno powered USB charger that when the dynohub put out too much power (from going down a hill or something) the USB charger shut down to avoid overloading the circuits. Could the device you have hooked up to the hub be the problem and not the hub itself?
On my hub with nothing attached to it other than a volt meter (one that measured AC), I measured over 30 volts going down a hill at 25 mph. That was why that USB charger was designed to cut out at higher speeds.