Behaviour was consistent regardless of which battery I swapped in. So, internally there must be some form of voltage regulation/conversion to achieve a constant voltage output. I'm guessing the batteries are just not filtered well, so if you combine them with on-board electronics or pedals that are equally not filtered well, it introduces noise where there would be none with a normal battery or a well-filtered power supply. Cool tech for sure but maybe it needs another couple of years to get "there". Or maybe it is already "there" and I haven't found the right product yet. This was literally just a random Amazon find that I wanted to give it a try.In the bass that had no noise, did you take THAT battery and stick it in your other bass?? Or same for pedals? Maybe it's the specific battery and you need to sort through them like you do with JFETs?