AUBURN HILLS, Mich. -- Several companies will soon offer services and products for converting hybrid cars into plug-in hybrids that rely more heavily on electrical power and can get about 100 miles per gallon. Plug-in hybrids generally have larger battery packs than standard hybrids and smaller gas motors. California’s EDrive Systems will charge nearly $10,000 to $12,000 to install the extra lithium batteries needed to turn a standard Prius into a plug-in hybrid when its service begins later this summer. At that price, and with gas at $3 a gallon, it would take nearly 160,000 to 200,000 miles of driving to break even. DaimlerChrysler will produce 40 plug-in versions of its Sprinter minivan for testing. No commitment has been made to turn it into a product.