iPod Classic strange charging/battery behavior - hardware or software issue?

[oelimtrila7]

A while back my 6g Classic suddenly stopped charging to full capacity. I had been running RockBox for a couple months without any issues and it really seemed to just happen overnight. I flashed it back to stock to see if that would work; it didn't, and I was too busy with other things at the time to worry much about it.

Now I do have some free time, and this is what I've found so far: after sitting on a shelf for a couple months, the battery was naturally quite dead, so the first step was to charge it. I eventually found a charger that worked with it (between my wife's collection of iPhones and our shared collection of iPods we have an assortment of chargers of various vintages, most of which don't work with my Classic), and let it sit for several hours.

The iPod at this point was in recovery mode, and had most recently been running stock Apple firmware. The charge indicator didn't really seem to move, or maybe a little bit, but I was able to restore it. The battery, despite having been charged for at least half a day, was dropping almost instantly down to nothing. I assumed the battery is bad and ordered a replacement. I also flashed RockBox on it, because I can only run iTunes via virtual machine on my computer.

However, and the reason I wrote this long-winded post, is that the charging behavior itself is a bit strange. When charging, the percentage stops going up at around 15-16%. It topped off once at 20%, but that seems to have been an aberration. BUT, when I pull the charger out, it then instantly jumps up to 55-60%, which, if the display is left open, drops a percent every 5-10 seconds, playing or not. If I let the display idle out and then reopen it, it usually has jumped back up at least a few percent, generally hovering around 45-48%. I've now let it charge overnight (reported charge when I woke up was 16%) and I'm running it to see how long it actually lasts.

I guess I'm still assuming that the battery needs to be replaced (I've owned the machine for I guess seven or eight years now, and it's certainly been through a lot in that time), but because of the weird behavior I've described I thought I'd at least ask people smarter than me if it could be something else. Nothing I've found would indicate that it's possible to really "calibrate" an iPods battery, but does this seem to indicate that I'm still going to have issues even if I do replace the battery? And, now that I'm testing it, what is a reasonable amount of time to expect it to run between charges? I guess I'm a bit worried about always running the battery completely dead since the gauge can't seem to be trusted.

Anyway, thanks for any insights or opinions, I apologize for the length of this post, I'm up way earlier than is usual on a day off and much coffee has been drunk!