I did a fresh install from the latest ISO and completely wiped the hard drive (backed up data first of course). Once installation was complete everything ran fine, USB worked either on battery or mains power until I did a system update via Synaptic. The power management options are set as the system defaults, I have made no changes.
I have the original power adapter and a spare non-genuine adapter as a spare in case the original fails. I know what you mean about Dell and their exorbitant charges for replacement batteries and chargers. But this problem occurs not when the power adapter is connected, only when running on battery since the last system update. I have two batteries, both are good, one genuine and one copy. When I go into the BIOS settings there is an option to check the battery status and both batteries show they are operating normally. So if I use the laptop with the genuine charger and battery I have the same problem. It's weird, I have never known this happen before, it has to have something to do with the last update, that's the only logical conclusion I can come to.