The LP-E6 Lithium Ion Battery Pack was designed specifically for the new Canon 21 megapixel 5D Mark II camera. The battery is rechargable and takes approximately two and a half hours to recharge on the first initial charging (battery charger sold separately). This battery is 7.2 volts and is 1800mha. It is a longer life battery than its predecessor (the BP-511A which is 1600mha). This means that the battery has more power and can hold a charge a little longer. This battery also has no memory.
One of the new special features of this battery is that it can communicate with the camera. The way it does that is by being able to show in the LCD monitor of the camera how many pictures you have taken on that one battery charge. It can also show you the exact percentage of charge that is remaining in the battery. There are also three little green squares which will come up on the LCD monitor which indicate the condition of the battery. Three bright green squares means the battery condition is excellent. As the battery condition degrades over a period of time with usage, the green squares will start to fade and lose their brightness, for example, instead of having three bright green squares, you may have two bright green squares and one green square starting to fade. As the battery continues to degrade over time, a second square will start to begin fading. When the third square begins to fade as well, it is time to buy a new battery. This way, you always know what condition your battery is in.
Overall, this appears to be a major improvement over previous Canon DSLR batteries. So far, it appears to be an excellent battery and well worth the price.