US Congress is considering implementing a tax break for owners of hybrid cars who convert them into rechargeable plug-ins despite the fact that such a move would void any manufacturer’s warranty. The proposal was put forward by the head of the House global warming committee, Ed Markey, and if passed would give hybrid owners a 35% tax credit to pay for the costs of converting their cars into plug-ins.

Unlike a conventional hybrid that uses an internal combustion engine to charge the car’s batteries, plug-ins have the advantage of being able to use a household electrical outlet for charging and can run on electric power alone for short distances. Under the deal, consumers could receive up to $7,500 in tax credits for the conversion, reports The Detroit News. There are several battery companies that sell the kits for around $10,000.

The carmakers aren’t too pleased about the proposal because such conversions not only void warranties but can also be dangerous as the extra batteries required can sometimes catch fire or forces the removal of the spare tire. Further, tests of a plug-in converted Toyota Prius showed that the vehicle actually emitted higher emissions of harmful nitrogen oxide.