If you already know that Rockbox doesn't charge properly on these iPods, then what is the surprise? The power drawn from the host is limited to 100mA which may indeed not be enough to prevent the battery from discharging, it depends what you are doing with it. It should be able to charge while doing nothing (slowly) but if you are accessing it over USB (causing the disk to need to spin) then yes, it will discharge. The workaround is to use the original firmware or emergency disk mode to access the player over USB (release builds do this, but current builds do not).