I think the voltage difference is the answer here. I did the coin battery hack a couple times. I never totally lost time or date, but it did slow down an dI had to reset it now and then if it bothered me.
I've taken to instead put a pin header somewhere and hook up a 3.6V rechargable telephone battery. Things seem to do better with these, and the plastic wrapping should help hold in leaking acids, plus you can put it somewhere less damaging if it does manage to leak, such as stuck to a solid metal piece of framing.
