SMG said...I've heard the science behind these devices is correct..... They use the same method on large bridges - Sydney Harbour Bridge for example!!

Did a bit of googling, found out it's also called cathodic protection, because like a cathode ray tube it involves electrons being emitted from the surface. If the surface is chock full of electons the chemical reaction causing rust is blocked. But electrons are are lost from the surface as they block the reaction and more have to be continuously pumped into the structure.
A battery is an electron pump, it is not a reservoir of electrons. So to pump electrons it needs a supply, which is why they need an array of spikes into the surrounding earth when using the system to protect a bridge.
It's a bit like a swimming pool and a water pump. If you want to keep that pool full for some reason you need to supply the pump from an external reservoir. Pumping it from one end of the pool to the other won't work, evaporation will eventually empty it.
Same with a battery in your car - on rubber tyres - pumping electrons from one end to the other doesn't ensure that the metal surfaces are saturated with electrons. Maybe a separate battery, the positive terminal connected to a ground spike might help?