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Discussion Starter · #1 ·
Hey all, first post here. I'm making a few additions to my truck to make it a little more capable for longer expeditions. Currently I'm working on a second battery setup, battery is in and the second battery relay I got is a cheap "manual" relay versus the automatic 14v sensing relay. I was originally planning on running it off the alternator signal wire since that seems to only power up once the engine is running but throwing the meter on it shows it only hits about 5.5v when the engine is running.

Is there a dedicated wire somewhere that powers up to ~12v only when the engine is running and drops when off? I could run a second relay off the alternator wire but would prefer to avoid that if possible.
 

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You can take your power off the main stud on the back of the alternator. Your relay should only come on when the igniton is on if it's wired correctly.
 

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Discussion Starter · #3 ·
Doesn't the main stud on the alternator sit directly attached to the battery? If so that would sit at 12v even when the car is off and still power the relay. I was originally just going to go off an ignition wire for it but that would mean that it would turn on as soon as I started turning my key I risk the starter pulling power through the relay and burning it up since it's only rated for 80 amps.
 

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Connecting to the alternator for the large gauge charge wire would be a good idea. For the signal wire to turn your relay on however, you obviously want it to come on after the engine has started running. I don't think there's any circuit that comes up after the engine is running, it's generally if the key is in the ON position, the circuits are on, some may momentarily disconnect during cranking, such as the stereo/hvac but otherwise that's it.

You can maybe add a switch to control it manually?

Edit: Just struck me that maybe you can use a circuit you activate yourself. For example the DRLs on my truck come on after releasing the parking brake, so that could control your relay, or maybe the parking brake switch itself.

If it's automatic you can maybe use the Neutral Safety Switch, have it connected to activate your relay anytime it's in Drive for example.

With all the messing around it might be worth it to drop the extra coins on an automatic 14v sensing relay. It would simplify things a lot.
 

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Discussion Starter · #6 ·
...some may momentarily disconnect during cranking, such as the stereo/hvac but otherwise that's it.
Hadn't thought of the items that disconnect. That could potentially work since the real goal is just to make sure it's off when cranking, and that would also work for when the ignition is off as well. Just a question of whether it would have time to disconnect before getting fried by the starter.


You can maybe add a switch to control it manually?
That's the only other option, which I would prefer be a last resort so I don't accidentally leave it on when starting.
 

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Another idea that just struck me, Use a 2nd relay to cut the signal to the charging relay during cranking.

At any rate some kind of protection in the form of a fuse or breaker between the new battery and alternator would be a really good idea. It would pop before anything fries, if for some reason it did try to crank off the second battery.
 

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Discussion Starter · #8 ·
Another idea that just struck me, Use a 2nd relay to cut the signal to the charging relay during cranking.

At any rate some kind of protection in the form of a fuse or breaker between the new battery and alternator would be a really good idea. It would pop before anything fries, if for some reason it did try to crank off the second battery.
I do have relays laying around, I think in the case of going with a second relay I would go off the alternator signal wire since that seems to be the safest route. The kit I got did come with fuses and I've already got one in line before it reaches the charging relay.
 
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