Battery isolators provide a positive electrical disconnection between the battery and the rest of the car's electrical system, with applications ranging from security (the car cannot be started without re-engaging the isolator) through storage protection (the battery does not slowly discharge through residual electrical loads) to workshop safety (positive isolation prevents accidental short circuits during loom or component work). For any classic MG that sits for periods between drives, a battery isolator is one of the simplest and most effective protective accessories available.
Discarnect and Rotary Isolators
The headline MGOC product is the Discarnect battery isolator, a compact unit that mounts directly to the battery posts and cuts off power at the turn of a knob, with a by-pass fuse arrangement that maintains supply to the clock and other low-current items even with the main isolation engaged. Beyond the Discarnect, rotary isolators are the traditional pattern, a quarter-turn rotary switch with a removable key or handle that engages or disengages the battery connection. The switch is typically a heavy-duty design rated for the full starting current of the engine, with appropriate brass or copper contacts inside a moulded plastic or metal housing. The rotary action is positive and the on/off positions are clearly indicated visually.
Removing the key after engaging the isolator (where the design supports a removable key) provides an additional security feature, without the key, the car cannot be started even if the keys to the ignition lock are available. The isolator is typically mounted near the battery itself, with the switch position arranged for convenient access, typically in the boot for boot-mounted battery applications or in the engine bay for engine-bay-mounted batteries.
Push-Button and Electronic Isolators
Push-button isolators offer a more refined operation than the traditional rotary pattern. The switch is a momentary push-button that triggers an electronic latching relay, pressing the button once switches the relay one way, pressing it again switches it the other way, with appropriate indication of the current state through an LED or other visual signal. The electronic latching design allows the isolator to be remote from the battery, the button can be mounted in a discreet location accessible to the driver, with the heavy-current contact being a dedicated relay mounted close to the battery. Some electronic isolators also include automatic features, disconnection after a set period of vehicle inactivity, automatic reconnection when the ignition is turned on, or remote operation through a key-fob system.
Installation and Application Notes
Battery isolator installation involves routing the main positive cable from the battery to the isolator, and then from the isolator to the starter motor and the rest of the car's electrical system. The cable cross-section must be appropriate to the starting current of the engine, typically a heavy-gauge welding cable or specialist battery cable of 35 to 50 square millimetres for most classic-MG applications. The connections at the battery, the isolator and the starter motor should be made with appropriate ring terminals and the connections protected against vibration and moisture. The negative side of the battery is typically left connected directly to chassis earth, the isolator is fitted only in the positive line, providing the necessary electrical isolation when engaged.
The technical team is available to advise on the right isolator specification and installation approach for a specific car.