When Dick Dell opens the back of his Prius to show off its A123-model battery pack, he’s displaying the state-of-the-art in car batteries today. The A123, made in Massachusetts by a company called Hymotion, is a 5-kilowatt-hour (kwh) lithium-ion battery that fills the wheel-well compartment where a spare tire would otherwise be.

For its size, the A123 is a much more powerful battery than any that existed even a few years ago, and it’s a big step forward from the standard Prius battery. In truth, though, the new battery is not even halfway to the power level that would cause most car owners, or even hybrid owners, to consider getting rid of their gas tanks and running on pure electricity.

But it’s so much closer than anything before it that it brings into view a future dominated by all-electric vehicles, with gasoline reserved for long-haul truckers and NASCAR drivers.

Consider: A stock Prius is equipped with a 1.8-kwh nickel-metal hydride (NiMH) battery, as well as a gas tank, and it’s rated at about 50 mpg of gas. Running on the battery alonebefore the gas kicks inthe car goes from a cold start to a speed of about 8 mph, but no faster, and even at that speed, the battery would run out of juice after the car traveled just eight or nine miles.

Adding the A123 battery gives the Prius another 5 kwh of battery powerfor a total of 6.8 kwhallowing it to run on batteries alone from 0–35 mph, a four-fold boost in speed from the stock battery. The range is also improved four-fold: A plug-in Prius can travel about 35 miles on battery power.

Only at speeds over 35 mph, or when traveling more than 35 miles at low speeds, does the plug-in Prius need any gasoline at all, which is how it achieves its 100-plus mpg of gas, Dell says. That’s an average rating. In theory, a plug-in Prius driven at low speeds for short distances would never need gas.

In the stock Prius, the gasoline engine recharges the batteryno plug-in needed. The lithium-ion battery is a plug-in, however, and on a standard 120-volt household outlet, it will fully charge in about 5 1/2 hours, Dell says. On a 240-volt outletthe kind that runs your clothes dryerit takes about half that time. On one of the brand-new 420-volt charging stations, it takes just 20 minutes.

The 5-kwh battery pack sells for $10,395 installed, Dell says. At that price, which he acknowledges isn’t cheap, the battery will pay for itself under ordinary use in seven years, not counting what it might add to the car’s resale value.

The batteries have a three-year warranty, but testing shows they last about 10 years at full strength, he says. After that, they retain most of their power and, if not still in the car, can be used instead of generators as back-up power for a house or office.