Run Your Car On Water?

As I understand this technology,You're not actually fuelling your car with water..Using your car's battery,a container of water in the engine compartment is electrolysed to produce a mixture of Hydrogen and oxygen gas (Known as Brown's Gas) which is then fed into the engine to be mixed with air and fuel..I'm hearing conflicting reports on the claimed results,the people producing the kits say it works,while others say don't waste your money..What I'd like to know is,how much energy is required from the battery for the electolytic reaction,and how much is produced by the engine combustion,and is more produced by the engine than is used in the creation of the gas?