How to Wire a Battery Isolator

Connecting multiple batteries can intimidate the do it your self installer, however the process is fairly simple when broken down. No matter how many batteries your adding each one of them needs to be isolated. What exactly does isolated mean? We will discuss the process and some terms below that you will need to know when looking at battery isolators (relays).

The first thing you will notice when looking at relays is an amperage rating. This refers to how much current (amps) that the relay is able to handle internally. This means if looking at the top of the relay it refers to how much current can transfer from one post to the opposite post. This number is important since it will tell you how much energy can flow between the two batteries.

Second, most relay isolators will specify how many posts it has. This will determine how many batteries can be connected the isolator. For instance if a relay isolator features 3 posts then it is possible to connect up to 4 separate batteries to the single relay isolator.

So why do you need a relay isolator to run two or more batteries? Isolators serve two purposes. First they allow a set amount of current to travel between the positive terminals of two or more batteries. So for instance if a system was set up with a relay isolator that is rated to handle 200 amps this means up to 200 amps of current is able to flow from one battery to another.  Second, all batteries have a different potential voltage so this means if two batteries are connected together without using a relay isolator the batteries would actually drain each other until they are both completely discharged (two completely dead batteries). Since each battery would have a different potential voltage they would push and pull on each until there is no energy left. This is where the relay comes into play; the relay will ONLY connect the two batteries together when triggered to the "ON" position (while the car is running). So when the relay is switched "OFF" the batteries are disconnected and no energy will be flowing between them (when the car is off).

As far as the connections that will be found on a relay they all share similar inputs regardless of the specific model. The first connections are the actual posts, depending on the relay there will be two or more of these. These connection points are used to hook up each positive battery terminal to the relay. Second is the ignition wire connection. This will either turn the relay "ON" or "OFF" depending if 12 volts are being run though the wire or not. This wire needs to be connected to a 12 volt source that shows 12 volts both while the car is cranking and when in the run position (a true 12 volt ignition wire can be found in the main power harness, under the steering column). The last connection is a ground. This allows for the relay to make a complete circuit. Below is a diagram that shows the process. 

Next we will look at the actual process of how a relay works. This is a typical mechanical relay that is used for battery isolation and this model is able to handle 200 amps and will isolate two batteries. This particular model is the Stinger SR200 or SGP32. We have labeled the connection points in each picture to make things clearer.

This is a more detailed side view the outer waterproof case removed.

This is an internal view in which you can see where the actual connections are made. The plunger in the center lifts up whenever 12 volts is applied to relay and makes the connection between the two batteries.

This is a more detailed image of the of the relay plunger moving up and down depending if 12 volts is applied or if the relay is at rest.

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Car Audio Batteries - Battery Isolators