My MiniST developed a bad rattle, which turned out to be the battery in its mount. I now use two of those little bungee cords (the little ones with metal ends) and the rattle is fixed. Because most of my miles are on mountain bike trails, this is a good fix.
I put them around the battery and controller, just above the hoop in the frame for the battery.
Did one of the holding tabs on the bracket break or something? I have a MiniST as well and the battery in its mount is very solid, almost feeling like a single unit. I have to put a bit of oomph into the bottom to get the battery out on the rare occasions I do that (I charge it in place on the bike).
If something did break from bouncing around on MTB trails, it would be a good idea to look closely to see if any other part on that mount is about to fail as well - one failure would probably increase the dynamic stress on the remaining ones. If most of your miles are still on those trails, I'd treat the bungee fix as a get-home temporary arrangement. You don't want a catastrophic failure and complete loss of power up on the side of a mountain. Sorry, the recovering-engineer in me tends to worry about other people's problems sometimes.
If the battery itself is undamaged, you should be able to get another mount from Rad, I assume for a nominal charge since all Minis are out of warranty by now I think.