I tried using the battery from my 7100t on the 8100. While it fits into the 8100, it is too thick in height, but only slightly. So the cover will not snap into place.
Additionally, when the battery is inserted I get a battery graphic with a red slash through it. Guess that is telling me the battery is a no go.
BEWARE of eBay auctions. I have seen sellers selling a battery that says it will work on the 7100 and 8100. This appears not to be true.
Far as I can tell, the charger for the 7100 does work on the 8100. They are both rated at 5 volt output. I do not have a land line at home, use my cell exclusively. So I have chargers set up near the bed and at my den office computer area. The one by the bed charged the 8100 fine.
As to a phone case, since there are no really decent ones on the market yet, I modified my 7100 Krusell case to be used on the 8100, that is, until Krusell comes out with a case for the 8100 [IMHO Krusell cases are the best. Good quality and fit].
I simply cut the plastic window so the trackball and keys would be exposed.
I did have to put a folded paper towel behind the phone, inside the case, otherwise the phone was too loose.
I know, it's lame, but it's not a bad alternative to not having a case or using one that is not a Krusell. It was odd walking home with the 8100 and not having a case, so I brought the 7100 case and others to try them all out, I mean, a phone has to have a case. I felt like I was walking naked until I had the Pearl safely tucked inside the Krusell.
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lol very creative work there bud!