Coming from the world of electric RC. I know a good amount about lipo batteries. I'm pretty sure thats what these are. If they behave the same way as my RC batteries do, once fully charged, you can take it off the charger and it will loose very little charge over time.
One problem I see with leaving the battery in the charger, is that if it keeps re-peaking the battery. If you plan to store the battery, basically not use it for a while, it is best to keep it at about 75-80% charge. Then re-peak right before using.
TBH taking it to 100% and letting it sit for a long time will not do much damage but making it stay at 100& without going down at all COULD be, I just don't have much experience with that issue because when my batteries are 100%, I'm mobbin my RC!
The real killer with lipo batteries is letting them drop too low I believe 1.7V is the danger zone(per cell). My biggest question is, do these devices that use lipo batteries have some sort of cut off to stop the battery from going too low? I've always wondered but never looked into it.
If these are not lipo batteries, disregard my post, lol