iPod Classic 5th gen charging weird

Hello,

I've noticed something odd when charging my iPod classic 5th gen 30GB (software version 1.3). When it runs out of a charge, I will plug it in and let it charge overnight. But, I've noticed that it discharges extremely quickly. I usually only listen to it on my 30 minute commute (60 minutes round trip). After about 3 days of that, it will die and I need a charge.

This hasn't been a normal thing though. And it happened fairly recently. I found out that it only happens when the iPod is charged on a "fast charger" with multiple ports. The specific one I use is the Anker PowerPort 6 60W. It's not just that charger though. Any sort of charger that says it can charge devices faster does it. However, if I charge with an older charger that does a low speed charging, the battery can last weeks. I'm currently using an old Samsung wall charger I got with a phone.

I've tested this multiple times with multiple different fast and "slow" chargers. Same results each time. Fast charge gives a fast discharge and slow charge gives a slow discharge. Also, I replaced the battery on this iPod 2 years ago with a brand new one. It was the correct battery for the 5th gen 30GB as well. I'm also using the same cord for all charging tests, an Apple 32 pin to USB cable. One final note is that if I leave it on the fast charger for a few days instead of just over night, the charge holds for a much longer time. It's almost as if the charger is very very slowly giving it a charge when it has reached max and that eventually charges it. Or maybe after it reaches "full", it quickly drops to slightly below "full". So, the charger takes it to "full" again, and repeats. Maybe that helps it get a better charge.

Anyways, is this a regular thing? Is it expected? I'm not really looking for a fix necessarily. Seeing as how I charge at night, the slow charging doesn't really even affect me. But, I am very curious to know why it's happening. And, if there's anything I can do, it would also be good to know. Please feel free to ask any further questions.

Also, one final thing to know. This happened with an iPhone 4S as well (work phone). I believe they came out around the same time. And I know that the 4S had the exact same charging habits associated with it.