I'm glad I read this! I just received my hpf series 3 a few days ago and plugged just the output in my rack thinking leaving the input unplugged would save the battery. Guess not!
I think the Fdeck HPF was originally designed for upright players, and the battery switch was placed on the output jack with the idea the HPF would mount on the bass with the lead from the pup left plugged in. Makes sense, that way, but it's opposite from what electric players are used to.
When I use my HPF I put it between my electric bass and the amp input, sitting on top of the amp, so I had to learn about unplugging the patch cord from the HPF to the amp, too!

