Droid A855 Battery Issue

[DarthWoo]

I'm not much of a smartphone user, but I recently bought a used A855 with the intention of using it as a portable media player at work. I installed a custom rom called Simply Stunning, and as far as the hardware goes, it is fully capable of doing everything I'm asking of it, which is pretty much just running Winamp for playing MP3s for a few hours a day through a powered portable speaker. I was fully aware that the batteries that came with it were used and would probably not be in peak condition, so I bought a supposedly OEM battery from an Amazon seller. I wasn't thrilled with the performance of that first battery, so they sent me a replacement for free. The thing is, all of the batteries I've gotten have acted in a rather peculiar way. If I leave the phone on constantly, but allow it to go into sleep mode as per usual, the battery's reported charge can remain at a comfortably slow decreasing line through several days, with the only significantly faster drops occurring for the time I have it playing music. I was off the past couple days, so I left the phone in sleep, and it seemed to have only dropped from 89 to 75 over those two days. However, since I had noticed some sudden drops in the past, I decided to test it tonight, and shut it down completely for a couple hours. Upon powering the phone back on, it was abruptly at 50%. As I mentioned, I'd seen this in the past couple weeks with different batteries, most often if I turned off the phone for any amount of time. I could charge the battery to full in the evening, turn it off, and then find it at 20% in the morning when I turned it back on. A day later I'd charge it, leave it in sleep, and find it working at what seemed like a normal discharge rate for days if I left it on the whole time. Is there potentially something wrong with more than just the batteries?