A main problem was the longevity of the battery, which needed to be between 7 to 10 years. The solution the engineers came up with was to keep the battery pack between 60% and 40% charged, proving to be the "sweet spot" for extending the battery life to roughly that of the other car components.
The car was a modest success in Japan and after gathering several years of test data from the Japanese early adopters, the company felt it was safe to put it on sale in the U.S. market. Sales were small at first but picked up substantially with a complete redesign in 2004. The Prius is considered to be a successful car for Toyota from both technical and marketing perspectives. The break-even date is uncertain, but according to the head of the Prius team, the company initially lost money on the Prius. The word prius is a Latin word meaning "to go before." A Toyota spokesman stated that "Toyota chose this name because the Prius vehicle is the predecessor of cars to come."
The computer program used in the Prius shuts off the engine when the car is stopped, is reversing, or is descending hills, thereby dramatically decreasing fuel consumption in city driving . The on-board computer program determines when and how to use the engine, motor, or both to power the car and recharge the battery so as to maximize efficiency. Typically, a gasoline engine runs inefficiently at half-throttle, creating a choking condition called pumping loss, a major reason for the inefficiency of gasoline engines compared to diesels. The Prius minimizes pumping loss by using a high torque range as much as possible with the throttle fully open. Drive-by-wire throttle control technology and Toyota's Hybrid Synergy Drive (a torque combiner, electric drive, and computer control) are essential to this engine control. In addition to the immediate benefit of reducing fuel consumption and emissions, stopping the gasoline engine also improves the performance of the catalytic converter, as the exhaust gases from an idling engine tend to cool the catalyst below its optimal temperature.Source: Wikipedia.