No, battery is not implemented properly. Android gives a out two informations: percentage of battery level (which iBasso did not configure correctly) and battery voltage. Rockbox takes the percentage information at the moment. But OF seems to take the information from battery voltage and calculates percentage from a curve.
There are some tasks left to do on this port and battery meter is one. It is not my highest priority but I won't be happy until it is done.
For the moment: observe on what percentage the battery dies and remember the value.
In my DX50 i have a original Samsung S3 battery. With this battery, percentage is linear 100%-70% then falls quickly to 0% but stays there for ~20minutes, so I'm alterted with my battery. The original battery was suddenly flat at maybe 75%.