"The only time I see the battery icon is when the subpack is connected and the charger plug is plugged into the subpack--with or without AC power."
This is a curious statement. I assume that what you are saying is that the unit is up and running at the user interface and when you plug in the subpack and then the AC charger the battery icon comes on. There is nothing in the subpack between the where the AC plugs in and the connector on the X5. It just passes voltage into the player. What that is telling me is that there is probably voltage going out to the charger from the player. This points to the isolation between these systems being compromised. Voltage and current should only flow from the charger into the unit and not feed back to the charger from the battery. There is probably a diode there that is shorted. This is probably the reason that the unit is having a hard time telling when the battery is used or not and could also be the reason why the battery is not charging. The other issue regarding the charging of the battery is whether or not the protection circuit (PC) is being used. The original battery came with a PC that prevents the battery from damage and overcharging. If the battery was replaced without the PC, with a damaged PC or with an incorrect one then there may be some issues regarding the charging of battery and the safety of using it. There are some threads on the iaudio forums regarding the replacement of the battery and some of these issues. Without schematics, though, there is probably not much you can do except to disassembly the device and look for damaged components on the board. Then there is how do you fix the device without proper equipment -microscopes, smd soldering equipment, etc. If it was mine this is what I would do but this is what I was trained to do. Even then I might not have much luck without schematics and the possibility of not being able to identify the component remains an obstacle. I assume that you are using it but in a diminished capacity. If so, keep using it and look for something to replace it.