Originally Posted by pshute
And if I do have problems when I swap the batteries from now on, can I fix it by swapping my original battery with a friend's?
Yes, your BlackBerry probably thinks the new battery is the old one. It will eventually learn the new capacity but if you change it back it will become confused again. The real issue here is that each time you deplete a Li battery to the point where voltage drop clearly indicates it is empty you have caused about twice as much wear as you would have if you started recharging at about 20% capacity. So you don't want to swap the batteries back and forth too much. On the other hand if you want to use the original battery in the future you can't just ignore it. If you do it will eventually discharge below a critical level and become inert.
The battery management advice that has come with my BlackBerry Smartphones has been quite good in that it strikes a good balance between getting the longest life out of the battery while still providing adequate time between recharges. Li battery chemistry is quite complex and difficult to manage, but the phone power management system and OEM batteries are quite intelligent. Together they provide a much better user experience than previous battery technologies. After market batteries may not be able to meet the same standards. This could be why you are seeing a constant 25C battery temperature. The battery pack should have a thermal sensor built in, but may not.