it is my understanding that passthroughs are designed to be used while plugged in!! And can be unplugged and used like a regular battery. i bought a passthrough because I work at a desk all day, so it just makes sense. I use it plugged in all week, and on weekends I unplug and bring home and use it as a batt, only plugging it in when it dies, just like any other batt.
I do notice better performance on my eGo PT while it's plugged in.