Since the one I have is just a USB connector and a battery box with a Joye style battery
device on the end, I don't think that there's any circuitry to control overcharging of the battery. I'd unplug it when it's not in use. You can, however,
vape to your heart's content on the charged battery when it's unplugged from the computer. It makes a good longer lasting power source for those times when you need a little longer battery life than the standard 180mah 510
batteries.
The battery in the USB PT appears to me to be a standard battery made up from NiCad cells. Nicads need to be drained periodically and recharged to prevent the memory effect from occurring. If NiCads are recharged without draining fully they tend to drain faster and run out of power halfway through the full life of a charge. Draining and recharging extends the useful life of a NiCad battery.