How to Use the Right Battery Isolator

by David McGuffin
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Installing a battery isolator in your vehicle allows you to use two or more car batteries for different purposes. One battery should always be dedicated to holding a charge for starting up your vehicle, while the other battery can have multiple uses ranging from car audio equipment or powering portable devices through a power inverter, such as a laptop, printer or other powered equipment. It is important to size your battery isolator correctly so that it properly interacts with your motor vehicle's alternator system.

Step 1

Size your battery isolator appropriately in order to guarantee that it is right for your vehicle. Open the hood on your vehicle to inspect the alternator that you currently have installed on your engine. Look for a small plate with printing on your alternator indicating the maximum amperage (measured in Amperes) for your car system. The battery isolator that you select should have an amperage rating "equal to or greater than the peak current output" of your alternator.

Step 2

Consult the battery isolator sizing and vehicle application charts at Don Rowe. Charts show manufacturers and manufacturing years for which specific models of battery isolators can be applied. The chart available through Multi-battery Isolator Application and Installation Instructions also shows recommended wire sizes for specific distances connecting the the alternator and battery isolator according to the amperage of the alternator. Most charts available online only show typical applications for battery isolators. If you have a specific need for the sizing of your isolator, are utilizing a higher powered alternator or modifying the output amperage of your alternator in any way, then you should consult an automotive electrical professional.

Step 3

Install your battery isolator by disconnecting the negative battery cable from the car battery. Disconnect the BAT terminal from the vehicle's alternator. Rewire the terminals from the alternator to the battery isolator. Wire the battery isolator to both car batteries. Be sure to follow isolator specific directions for your application.

Step 4

Mount the isolator in a dry location, such as in the cab of your vehicle and connect it using the appropriate sized wire, as determined by the amperage of your alternator and the distance from the alternator to the isolator. For example, an alternator rated at 95 Amps should use a #8 gauge cable for a length of 15 feet or less.

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