I still have one of those devices to charge the ports in a CAU (or was it MAU - it's been a while. One was unpowered, the other plugged into mains) in my "Interesting things I've used in IT" box. Took a 9V battery and charged up the capacitors so when you plugged in a cable it would be detected and the client system would connect. If a port on the CAU wasn't active for too long the capacitors would lose their charge and you'd have to dig one of these out to get it back online.