If you inspect the electrical circuits of a truck, 4×4, agricultural machine, or other vehicle, you are likely to find at least one battery isolator. The principal function of this device (also called a battery main switch, battery cutout switch, or kill switch) is to prevent current drain and ensure that the starting battery is sufficiently charged to start the engine.
Depending on its complexity, a vehicle may have a battery for each circuit: one battery starts and runs the engine and another operates accessories such as the compressor and lights. In this case, each circuit will have its own battery isolator.
The isolator is a heavy-duty switch that interrupts the power to an entire circuit. It is operated by a simple key inserted into a slot and turned clockwise, locking it into place and closing the circuit. Turning the key in the opposite direction opens the circuit and allows the key to be removed if required.
The isolator is usually fitted or retrofitted into the firewall. The terminals are on the engine side of the firewall and bridge the live (+) lead from the battery. The key is inserted and operated from the passenger compartment side of the firewall.
Match it up!
Isolators vary in capacity: an isolator designed for a small car may not be able to handle the power of, say, a combine harvester’s starting battery. Thus, when fitting an isolator, match its specifications with those of the battery and circuit of your vehicle.
Safety and security
The battery isolator allows one to interrupt the power before working on the vehicle’s wiring. Earthing a live contact will produce sparks that may ignite petrol fumes and cause a fire. Additionally, opening the circuit and removing the key makes it impossible for anyone to start the engine and steal the car.
The Merit-Malta battery main switch unit shown here is an example of a good quality isolator. The company manufactures a number of items for the agricultural industry. In this isolator (code 328602), the housing and base are made of 30% glass fibre, reinforced plastic for high-impact strength, rigidity and heat resistance.
The bolts, terminals and contact bridges are silver-plated, and the switch has a unique wiping contact system. The isolator is rated at 200A continuous, 400A instantaneous (30s) with a 25mm² cable and 600A instantaneous (30s) load with a 70mm² cable.
Two screw-in M8 studs and bolts secure the switch to the firewall. It is available at R350 (incl. VAT) from Autozone branches.
Fitting it to a modern vehicle is, unfortunately, not a DIY project. This is best left to a specialised service centre.
For more information, visit www.autozone.co.za.