In the simplest terms, the demands on batteries in current hybrids are tightly restrained. Unlike the batteries in a flashlight, which run until they are thoroughly depleted, the cells in a hybrid vehicle — whether nickel metal hydride or lithium ion — operate in a very narrow range. To promote extended battery life, auto-makers may engineer them to use as little of 10% of their rated power before demanding they be recharged either by regenerative braking or by the gasoline engine. That's why a conventional hybrid's range of electric power alone is minuscule despite the most frugal driving behaviour. It's also why engineering a plug-in hybrid involves more than adding a wall socket.
The goal of the plug-in is to allow a longer electric-only range and minimize operation of the gasoline portion of the drivetrain. To accomplish this, the existing batteries have to operate through a far greater range of energy dissipation — i.e., allowing their energy levels to reach as low as 30% of reserves.
Experts contend these deeper discharge rates will reduce battery life from an expected eight years to just three. The other alternative is to bump up the battery's size, increasing lithium usage.
Although there are numerous alternative battery technologies being tested, the study of electrons is a relatively mature science. While computer advancements still manage to follow Moore's Law, which states that the number of integrated transistors per integrated circuit (a measure of computing power) would double every two years, battery performance seems to improve at a much more modest pace.
Despite many claims of huge nanotechnology developments, the mass production of a miracle battery may still be some distance in the future.