Right way to add override switch to battery isolator relay?

Thread Starter

josh.himself

Joined Jun 12, 2019
1
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. So far so good, there are plenty of battery isolators to choose from. 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). 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.

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. 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. 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.

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. 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.

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.

Does this sound accurate to those of you who actually know what's up?Photo on 6-24-19 at 11.03 PM #2.jpg
 

Nkoch1985

Joined May 3, 2017
1
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. So far so good, there are plenty of battery isolators to choose from. 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). 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.

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. 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. 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.

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. 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.

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.

Does this sound accurate to those of you who actually know what's up?View attachment 180352
Hey there….im sure u have figured something out already but when i did this i got a 300 amp isolater

i put a blocking diode on each wire for the trigger wires for the accessory circuit and the override switch because they will only allow current to flow one direction through it…i hope this helps u in the future
 
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