The Fuji NP80 - equivalents are similar batteries but not necessarily the same as the Epson. They are sometimes 3.7 volts (and sometimes 3.6) but often not 1500 mHa (often 1350). That can be problematic for two reasons:
1. Battery life is obviously shorter.
2. Epson claims that using other batteries may cause long-term problems with the Epson shutter and/or other aspects of the camera. It's hard to know if this is just the party line or valid but caution might lead one to assume the latter. I bought two third-party batteries that are (nominally) 1500 mHa *and* 3.7 volts but I'm simply not sure if they work identically to the Epson batteries or not. Since the cameras are so expensive and since I rely on them for professional assignments, I prefer the cautious approach of using only Epson batteries. The extra cost is worthwhile to me for the peace of mind it provides.
If one is going to use alternate batteries, I'd recommend sticking only with those rated at 3.7 volts and 1500 mHa. Then of course, cross fingers and hope the ratings are accurate. These digital cameras are computers and that 1/10 volt may be important to their proper functioning. Again, fussing with all this is one reason I prefer to just use the official batteries.
I haven't done exact measurements yet (too busy) but I am seeing reduced battery life with the new firmware. I'm trying various things to see if I can find the operative variable, including (just yesterday) resetting my LCD brightness to the next lower setting. I can live with increased battery consumption but it does mean that I need more batteries. I may shoot 700 frames a day with the Epson bodies when covering a wedding and it's very important that I not run out of juice.
Cheers,
Sean