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Wiring diagram battery isolator relay3/30/2023 Our 12v auxiliary power circuit is protected by a 20 amp fuse (fuse size should be based on the wire gauge and length of the circuit). This is extremely overkill for the application but will make for a very reliable system. We've used a 12 volt, 120 amp marine grade relay. Split charge circuit wired to 12v+ alternator output terminal An example circuit would then be represented in the following figure: This is recommended if you are running a new circuit to the trailer plug or are replacing an existing circuit. Alternatively, the auxiliary charge circuit may be wired to the 12v+ output terminal of the alternator (this is common). Keep in mind that on a dual battery system (all diesel vehicles) the 12v+ recharge circuit only needs to be connected to ONE positive battery terminal since the batteries are connected in parallel. ![]() Split charge circuit wired directly to the vehicle battery A typical circuit looks something like this: All vehicles have empty circuits in their fuse box and thus is not particularly difficult to find a power source with a multimeter or probe light. The switched terminals of the relay are spliced into the 12v auxiliary power circuit of the tow vehicle and the coil of the relay is wired to a 12v source on the tow vehicle that is hot only when the key is in the "RUN" position. I've tried to illustrate this in the chicken scratch drawing attached to the post - the upper battery being the main battery, the lower battery the auxiliary, the left relay being the heavy-duty isolator, the left switch being a manual override, the right switch representing the ignition-controlled circuits that also provide power to other circuit devices, and the heavily scribbled line showing the effect of closing the manual override switch energizing the trigger post of the heavy-duty isolator, and also energizing the line connecting the isolator to one of the secondary posts of the smaller relay, but not continuing on to the 'other circuits' connected to the ignition-switched circuit.All that is required to build this circuit is a SPST (single pole, single throw) 12 volt relay, some automotive wire (14 AWG will suffice), and an assortment of terminals. This would allow me to wire a second manual switch in parallel to the smaller relay, which when closed would not energize the other devices on the 12v vehicle running circuit. What I thought would solve this problem was to use a second, smaller relay which would use the 12v vehicle running line as a trigger, and would have the secondary side connecting the main battery to the 12v trigger post of the isolator. Then I realized that doing so would also energize the 12v line that's normally on when the vehicle is running, possibly providing power to other devices on the same circuit. At first I thought I could simply wire a switch in parallel to the existing trigger, that would connect the main battery to the 12v trigger post when the switch is closed. The 500 amp isolator that I'm looking at is simply a large relay - the trigger posts are connected to ground and to a 12v signal that's hot when the vehicle is running, and the secondary side of the relay connects the positive terminals of the two batteries. Many smaller battery isolators have such a reset switch or terminal built-in, but larger 500 amp isolators don't appear to have this function. I would also, however, like to be able to override the behavior of the isolator with a manual switch (e.g., to connect the batteries when necessary to use the auxiliary battery to assist in starting). ![]() So far so good, there are plenty of battery isolators to choose from. ![]() So I'd like to add a second battery to my car and have it isolated from the main battery when the alternator is not running.
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