esdel, what are you charging with? Mains or USB?
I only use the wall charger (mains?). I have a USB charger but I have found that it doesn't work nearly as well. I gotta tell ya, maybe I jinxed myself with this thread, but today my Mega only lasted 8 hours -- yeah, still great, but quite a drop from 12.
You know, when I charge a batt in the USB or if I only charge it for a couple of hours (which I've done a few times with the regular batts), I notice a weaker hit as well as reduced time before it needs to be recharged. Why don't you try charging your Megas overnight and see if you notice an improvement? I'd be interested to know how it goes.