Be aware that the older backup battery may be thicker than the new ones, although the battery part # is the same. I had to put a little ferrule on the retaining arm/contact thingy, to get it to make contact with the battery. (Because the new battery was thinner than the old one, and it didn't make contact!)
I'm not saying you'll have the same problem, but I did, at least with that one battery manufacturer.
