Mobile phone companies often sell more batteries than phones to consumers. The phones sold to users include a rechargeable battery so that the device is immediately useful, but a certain number of consumers are expected to own more than one battery during the life of their phone. The same can probably be said for laptops and camcorders. Yet, there is little incentive for consumers to buy new batteries except for when they fail or when the consumer feels the need for a larger battery.
Unlike other areas of mobile computing that beneﬁt from exponential improvements in performance, battery energy density (as measured by joules per kilogramor joules per cubic centimeter)changes slowly so that there is little pressure for consumers to upgrade.