[Al Skierkiewicz]

Jose,
Doc's suggestion is the simple approach and requires no additional circuitry. You don't need to add something to turn it off since it will reset when you remove the main power. What you need is a small relay with SPST contacts. You should be able to get this at Radio Shack in both 5 volt and 12 volt versions. You wire one of the relays contacts and one side of the relay coil together with the positive side of the power supply on the vehicle. Then wire this connection to one side of the start button. Attach the other side of the relay coil to the negative terminal of the power supply. Then connect the positive side of the power supply to the second contact on the relay and the second contact on the start switch. When you press the start button, the relay pulls in and supplies current to the vehicle and the relay coil which holds it closed until the power is removed. The relay needs to be on the vehicle but you can wire the start button through the charging stand and the same kind of connector you are using to supply charge current to the vehicle.