Onyx Boox Firefly - Battery doesn't hold charge

[Doranwen]

I ordered a Firefly several years ago, from the German store, so it came with a charger only for European electrical outlets. I tried using an outlet adapter but it didn't seem to really charge the device at all, so I ended up using the power adapter for my previous e-book reader (Astak EZ Reader Pocket Pro) which was for US electrical outlets. That seemed to work, though it took a really long time to charge the Firefly, it felt like (hours - I usually left it overnight because it took so long).

Of late, the device seems to be low battery every time I turn around, even when it's been turned off, so now it's virtually unusable unless I charged it the night before. Should I recharge it, it sometimes comes up with 3 bars out of 5 max, on the first start. (This last time it actually surprised me with 4 bars.)

Should I be trying to find a US power adapter specifically for the Firefly? Replacing a battery? Getting a new device? Any combination of the above? I haven't the faintest idea. Would welcome your advice!