All rechargable batteries are not created equal. They all have different characteristics. Here's a general description:

1) Nicad- cheap, but can build up a "memory" over time. The put out a lot of power.
2) NiMHs - better than Nicad. They also put out a lot of power for high current applications.
3) Lithium - lasts longer than NiMHs, but they don't put out as much power.

For example. If you bought two identical radio controlled cars and put NiMHs batteries in one and Lithium in the other. The NiMHs car would run much faster but run dead quicker. The Lithium would run slower, but last longer. It's like the NiMHs has a larger "pipe" to put out power than the Lithium.

I don't have any idea how much current draw the G2 uses, but I'm guessing that the better choice in this case would be the Lithium rechargeable batteries.

I use a Letus 35mm spinning ground-glass adaptor. If I put Lithium batteries in it, it runs a long time, but sometimes, when I press the power button, the ground-glass won't start spinning. I have to "help it out a bit" (don't ask). But when I run standard alkaline or NiMH batteries, it always starts right up, but it doesn't last as long.

Does that make sense? It probably doesn't help much, but I thought it would be good information to add to this discussion. :)

No - the only choice is the BA 2015 rechargeable pack which was designed specifically for the unit and "talks" to it via the extra contact.

Hopefully Sennheiser isn't still using nicads for their battery packs. If so, you'd be better off with rechargeable lithium's (although they wouldn't "talk" to the unit)

They are NiMh as far as I'm aware and have been for years.