With USB CHG turned off. The E07K will use battery power until it drains to 0% and turn off. At that moment, it will reset its settings (volatile memory loses power and so it goes to default state). Notice a USB cable is plugged in and start charging and will turn on in a minute or two after its gotten enough charge.
With USB CHG turned on. It stops charging once it hits 100%, but once it drops from that, it charges more.
This is all the functions for the battery in the ANDES.
There is no need to worry about battery. Theoretically, through info known about li-ion batteries, the time it takes to completely deplete a E07K battery (if using battery without charge) and to recharge gives us a number of around 2 years of consecutive charging and depletion before the battery will theoretically be at 50% of its original battery capacity.
You using it as a sole desktop unit is of your choice, and so past 2 years approx(theorteically) if the battery does become more dead. A battery replacement you can buy from FiiO and DIY yourself(or send it back) isn't going to be an issue. Hell, at over two years, most already bought a new device.