Alright, I think I figured it out:
For the past weeks (or maybe even months) I've been charging my BB 8800 via USB on my Laptop, all the way up to 100% charge. Never used the regular charger.
Then I upgraded to OS 4.5 and afterwards still used the USB cable for charging it. Ever since that upgrade the battery emptied after just 36 hours of (barely) using it.
A few days ago, over night, I used the charger to charge it and from the next morning on, after about 6 hours of charging, the battery lasted for more than just a 36 hours (I left it in there even after it was on 100% after just 1.5 hours).
It looks like charging it via USB for a long time brings it up to 100%, that's what it says under "Status", but it doesn't really charge it well. Sounds like an electrical or chemical issue.
I will keep using the charger from now on and we'll see what happens. The more I use it the better the battery life seems to get.
Anyways... problem solved... for now... hopefully
I'll keep you updated on how this goes!