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.
I'm pretty sure, so long as you're capable of disassembly, in which case you wouldn't be asking this question, you'd be fine either way with either type of battery. If you're talking about the CMOS battery connected to the motherboard, that would really depend on the laptop model. Some models have the CMOS battery easily accessible by removing just one or two screws and taking a panel off, while others require extensive disassembly to gain access. If you are talking about the regular battery on two identical models, it will be fine so long as there is not another issue on the unit that has caused the battery to fail.