A lithium-ion battery manufacturer, A123Systems, will begin marketing a kit in 2008 that will convert a regular hybrid car into a plug-in hybrid electric vehicle, as known as a PHEV. Currently priced at $10,000 (with a $3,500 tax credit), these modules are still expensive for the average car owner. For example, a commuter driving 11,000 miles per year with gas at $3 a gallon, the payback time would be 5.5 years. The payback for a fleet owners is quicker at 2.5 years, against an expected life of 10 or more years. About the conversion kits themselves, A123Systems says:
The Hymotion PHEV module requires minimal modification to the stock vehicle. All necessary components and safety features are integrated and contained within the module, including: batteries, power electronics, crash sensors, power electronics, charger, battery management system, safety sensors and manual-electric interlock. The system does not require removal of the OEM battery pack and can be installed in less than 2 hours, according to the companies.
See this article for more details: Green Car Congress