newbie here… have had pioneer-fbx for a year or two, mostly unused… tried it again past couple-months and have (like you) been having weird issues with the battery… specifically, it “used to” just work fine plugged in - but recently (days?/weeks?) it has been causing issues if i plug it in…
namely, i unscrew/unplug-battery so i can do hard-resets more-easily while i am changing things… once everything is working-as-i-want-or-need, then i go back and plug the battery in to call-it-done… power-it-up and now the fbx wont work (specifically, it doesnt seem to come back online correctly)…
at this point, like you, i would appreciate knowing what log-files are available to help me sort out what is going on… i have been heavy-duty tinkering, however, so ive just been leaving the battery unplugged for now while i keep doing my tinkering… your message has made me feel better about my tinkering - in that it might not be completely my own fault (about the battery-issue)… 
gl, h.
ps - like you, also, there might be a hard-reset button to push but i dont know what/where it is yet… also, my tinkering was to add a sata-drive so i was wondering if maybe the current-draws were causing issues - but im still not even using the ssd yet… i have so many different issues going on right now i would start several different topics - but just, again, wanted to say thanx for posting your notes… sigh…
pps - since ive been doing manual cmds of “sudo apt update; upgrade; autoremove” im fairly sure im on the latest/greatest - but maybe i should start over with a clean-slate again (sd-card-wise)…