Do you mean the CMOS battery, which requires taking the machine apart, an internal power battery, or one of the easily-removable power batteries that lets you keep a charged spare handy for a quick swap? In order, my answer would be
1) It's pointless, and the risk of busting something is too great.
2) Would probably work, but since they are the same what good will it do you?
3) Works just fine.
Can you explain the situation a little bit more?