The "Charging device" you need is an automatic charge relay (ACR) Blue Sea makes a nice one but there are others. This devices monitors the voltage of the vehicle electrical system so it knows when the alternator is running. When this happens it connects your 2nd battery so it will charge. When the engine is off the voltage will drop and the 2nd battery will be isolated.
If you use a relay hooked to your key switch and then leave the key on, like to listen to the radio, it will still drain both batteries.
If you use an old fashioned diode type isolator you will not get a proper charge on your batteries.
