Opening the old K800’s battery cover revealed two regular AA batteries (Eneloop’s) that you could swap out if you wanted, instead of plugging the keyboard in to charge it. In that way it was 100% full-time wireless. Removing the battery cover on the new K8800 reveals that the battery is sealed inside and unremoveable.
https://www.amazon.com/Logitech-K800-Wireless-Illuminated-Keyboard/dp/B003VAGXWK