Mountain View, Calif.-based Google (Nasdaq: GOOG) announced today that it's accepting proposals, offering to invest $10 million in companies that are developing plug-in hybrid electric vehicles.

The company, which started its RechargeIT initiative in June to promote plug-in cars, said proposals for the cash are due Oct. 22.

The search giant said it plans to invest $500,000 to $2 million in selected for-profit companies working on plug-in hybrids, as well as companies developing electric vehicles, vehicle to grid solutions, and components or technology that can be used in electric vehicles.

Google said the request for proposals is open to companies anywhere in the world.

The company said it may fund additional proposals in support of policy, research and development, and other related work by non-profits in the future, but that this proposal is limited to for-profits.

The investments will take the form of a convertible note, according to Google, which said it does not plan on taking any spots on the boards of the companies it invests in.

In June, the company handed out over $1 million in grants to groups including the Brookings Institution, CalCars and the Electrical Power Research Institute.

The Brookings Institution in Washington, D.C., took in $200,000 to support a spring 2008 conference on federal policy to promote plug-ins.

CalCars, a California initiative, received $200,000 for its work to educate the public about plug-ins.

Another $200,000 went to California's Electrical Power Research Institute, which has a plug-in research and development program.

The market for electric vehicles in the U.S. is slowly growing, with two more companies joining the sector last week, Irvine, Calif.-based Fisker Automotive and New York's Visionary Vehicles (see Attack of the plug-in hybrids!).