For Prius owners in California who want to take fuel economy to a new level, a Bay Area company called OEMtek may have the answer. The Milpitas-based company is one of the growing number of companies offering plug-in conversion services. OEMtek will outfit a Prius with a bigger battery pack and potentially double its mileage—to more than 100 miles per gallon—for a cost of $12,500. The company claims the enhancement will give the Prius up to 30 miles of all-electric driving,. “There are people who want this right now, no matter what,” said Cindi Choi, vice president of business development of the six-employee company.

OEMtek’s conversion process is dubbed “BREEZ” (or battery-range extender EZ). The change involves installing a 200-pound pack of lithium-phosphate batteries with six times greater energy storage than the Prius’s original battery pack. That original battery system remains intact and unaltered, along with the vehicle’s existing software system. The batteries come from Texas-based Valence Technology. After the conversion, Toyota will not honor the original buyer agreement and void its factory warranty.

With more than one million Priuses already on American roadways, and an additional million annual global sales expected in the next few years, the plug-in conversion market could grow from a cottage industry to substantial proportions. OEMtek is planning to do about 100 conversions per month by the end of 2008.