Battery life on laptops is better than it has ever been, and that's in response to customers' demands that they get all-day usage out of them. The common approach to this objective is to seal the biggest battery possible inside thin laptops.
Lenovo has taken a different approach with new ThinkPads (details from ZDNet's Larry Dignan here) that uses multiple batteries to let the customer decide how long the laptops should last on a charge.
Dubbed Power Bridge by Lenovo, the new system uses both a small sealed internal battery with external batteries to provide a combination of portability/battery life to please customers.
The Power Bridge technology is available with the ThinkPad X240, T440, and T440s. These models have an internal 3-cell battery (23.5 Wh) which should provide around 3-4 hours of battery life. This can be augmented by inserting a second battery in a typical battery compartment on the bottom of the laptops.
Lenovo offers two sizes of the external battery, a 3-cell (23.5 Wh) and a 6-cell (72 Wh). The 3-cell keeps the laptops as light as possible by only adding 0.4lbs to the weight, and fits flush with the bottom of the laptop. This battery should double the time away from an outlet for an estimated life of 6-8 hours.
The 6-cell battery adds 0.74lbs to the laptops but that weight penalty should provide 10+ hours of battery life along with the internal battery according to Lenovo. This battery extends out of the laptop bottom slightly (half an inch).
The two battery options let customers decide how much battery life they desire and how much extra weight they are willing to carry on trips. The presence of a sealed battery means the second batteries are hot-swappable. That means the battery can be removed or inserted while the laptop is still running.
This means a road warrior could head out with only the internal battery active to keep the laptop as light as possible. When the battery starts getting low a second battery can be popped in to keep going. Those who need the longest battery life possible, e.g. international business travelers, can have multiple batteries in the bag to keep going.
Lenovo has not indicated the prices of the 3 or 6-cell batteries for supported ThinkPads.