There was a thread about these 7.2V batteries, my theory is they keep the DRAM powered in sleep mode. Another one (not mutualy exclusive) was the ability to swap the main battery in sleep or standby mode - if done quickly (30s or so).

As for the green stuff, yes that is corrosion. If you open one of these packs you'll see why, those things leak corrosive electrolyte. Even just the fumes are bad. Something acidic like vinegar, then water rinse, and then perhaps alcohol to help it dry can remove it but there's always a risk of using too much vinegar and causing more corrosion. So a good rinse and a few days of drying (because of all those nooks and crannies) is very much recommended.

I've had a laptop in my hands in which these connectors were pretty much done - somebody tried to pry off the battery plugs and caused damage on top of significant corrosion that almost ate the contact points completly. Frankly not much can be done in such case other than replacement. And that is not easy - multilayer board densly populated with various parts.

Thanks, I'll clean it up and replace just the 2.4v cmos battery. Luckily doesn't seem like really spread beyond the connector. It's interesting to see it actually followed the wires down into the board. I'll just leave the 7.2v battery off