the C-S1 battery is discontinued...
The C-S2 battery is fully backward compatible with the older (7100t/g/r/x/v and 7105t) units.
The new C-S2 and C-H2 (high cap) are "chipped". If the newer phone doesn't see that chip it will not startup - like a 7100i for example.
If anyone is interested we have 1 new C-S1 left in our old store for 33% our normal price (67% off list) http://store.bizshopr.com/rimacc-07494-001.html