They do charge when they are in the truck, but like all rechargeable batteries they don't last forever.
I've replaced a number of the batteries. I agree that if you are going to put a new case on it is worth it to go ahead and swap the battery - if you put a new case on and then 6 months later have to destroy it to swap the battery you'll be kicking yourself.
Swapping the battery isn't difficult, but you do need to be able to wield a soldering iron. You have to desolder the old one, and solder in the new one.
This isn't my video (I do a little differently), but illustrates the general concept.