More on the EN-EL4 Dissection
Bertho just put up a nice writeup on the battery here:
Battery capacity?
Although today there are 3rd party brands that advertise a little higher capacity than the original 1900mAr cells used in the Nikon batteries, Nikon’s latest version, the EN-EL4a uses the latest technology 2500mAr cells so there is no longer a capacity advantage with 3rd party batteries. Regardless, the the battery life in the cameras is so good that it is usually not a significant issue, reliability and safety are more important.

His comment about safety is no joke. I wouldn’t use a third-party battery in my camera if it was free and given to me by Duracell.
January 4th, 2008 at 08:15
[...] when the EN-EL4 was dissected? We found it had a bloody RISC microprocessor in it with way more power than you’d think a [...]