Can someone explain why this battery on ebay will NOT work in the D3100??? I've very confused. Are there different EN-EL14 batteries. On the link supplied above it shows that battery working for both the P7000 and D3100. Thanks!!
Nikon "chips" their most recent batteries which is read by the camera. A 3rd party generic or counterfeit battery, which is what the battery in the link probably is, will give you an error message and be unusable in your camera. These batteries also will not work in the P7000 Coolpix if it has the latest firmware installed.
Look at where the battery is being sold from and the very low price. Think about it.
The internet marketplace is flooded with counterfeit merchandise. Given the difference in price, it would be a safe bet that the battery is counterfeit. As always: Buyer Be Ware! Good Luck and Enjoy your Nikons!