I'm not talking about the main battery power . . . thats lasting AGES. I can drain through the entire expansion pack battery and still have 80% left on the main battery, depending on what I'm doing.
When I play MP3's, I'll turn it on, and then use a hot button to turn off the display . . .
As far as I've noticed, the expansion battery and main battery function COMPLETELY independently of each other. I haven't done testing with the PCMCIA hard drive removed yet, as I just got the darn thing . . . but if my internal battery is quite full and the expansion dies . . . I lose the hard drive functionality. It just shuts off. I can use the rest of the iPaq just fine, but at this moment, thats not what I'm trying to do.
I guess this is just the way it goes . . . a real shame if you ask me. Sharing batteries may not get me much further, but it'll get me a hellova lot further than nowhere.