Here is some initial information about the Nikon's EN-EL4 battery. Measurements are being done and detailed information will gradually be added here. Most of this temporary information is from my responses to questions on the different news and mail lists.

The cells in the Nikon battery are not unusual. There are three 18 x 65 mm cells in the battery, this cell size is called "G8". Basically, they look like oversized AA cell.
As the saying goes "Do not attempt this at home". Cutting open batteries in general is a bad idea and especially lithium batteries. There are good reasons for those recalls with burning and exploding batteries. It is very easy to get hurt so just look at the pictures instead of trying it yourself.

The battery contains a lot of fancy electronics as can be seen from the pictures below. There is one microprocessor and five other integrated circuits, plus extra transistors, diodes, and other components. To give some feeling for the complexity, let me mention that the specification document for just the microprocessor is 308 pages long and it is not bed time reading material.

The corrosion on the lower left corner is the result of water that leaked into the battery.

Why so much electronics?
This is to ensure a safe and proper operation both when the battery is charged and when used in the camera. There is also circuitry to protect both the battery, the charger and the camera in the case of catastrophic failures.
Besides those functions, the circuitry is also a fuel gage by measuring energy going in and out of the battery and keeping track of ambient temperature to calculate selfdischarge and from all that data, it then tells the camera the state of charge and probable remaining life.

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.

Quality and safety of 3rd party batteries?
I have not tested any non-Nikon batteries so I can not really give any specific answers but here are a few thoughts:

It is easy for some company to copy the plastic housing, (although there have been problems with the end caps falling off some 3rd party batteries), and pick and chose from different brands of current cells that are the same standard size. They can also copy the ICs used and with a significant effort recreate the schematic. Typically though, the software to run the microprocessor is locked away securely inside the chip and without that, they have to do a lot of work to provide all the functions and security checks. I do not know how well it is done since neither do I have Nikon's software nor the third parties' software. I do not even have the 3rd party batteries. With all the smarts in camera, battery and charger, it would be very easy for Nikon to keep track of serial numbers of the batteries used in the camera, and for the batteries to keep track of the cameras, but I do not know if that is done. This could be used to counter warranty claims if a camera or charger is damaged by a battery.
In products that I design I do keep track of accessories' serial numbers when it applies.

Is it worth the risks buying 3rd party batteries?
The current system works very well and to me it is not worth the risk and having to worry about it. The batteries last a very long time so the cost per shoot or per picture is insignificant and it is likely false economy.
No, I do not work for Nikon and I do not buy 3rd party batteries.

Bertho Boman
Vinland Corporation
11600 NW 20^th Street
Fort Lauderdale
FL 33323
(954) 475-9093
Email: boman01 at vinland dot com
www.vinland.com