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Discussion Starter #1
I currently have an aux battery where my spare used to be. I'm about to hook up a fuse panel to it to run lights, inverter and whatever else I can think of. To ensure that everything gets shut off, I was thinking of hooking up a relay between the battery and the fuse panel spliced into something ignition powered.

Opinions? Worthwhile or just being paranoid about leaving stuff on?

Thanks all
 

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If you have an aux battery why are you paranoid about things draining it? Do you have a proper isolator and battery selection switch setup?

If you want things to shut off when ignition is off then sure, go for it. It's user preference. I'd suggest doing it proper with a large solenoid or contactor capable of breaking current more than your fuse box is fused for.
 

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Here is a little thought.
The factory 7-pin trailer plug has a switched, high amp, power feed for the trailer. It is there to charge the trailer battery while driving down the road. It is already on a relay so it is only connected when they key is on (so if you have a camper plugged into the truck and leave the lights and fridge on overnight it will only drain the trailer battery). As you drive it charges the trailer with the vehicles electrical system. All of which is already in the vehicle and already factory engineered.
 

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Here is a little thought.
The factory 7-pin trailer plug has a switched, high amp, power feed for the trailer. It is there to charge the trailer battery while driving down the road. It is already on a relay so it is only connected when they key is on (so if you have a camper plugged into the truck and leave the lights and fridge on overnight it will only drain the trailer battery). As you drive it charges the trailer with the vehicles electrical system. All of which is already in the vehicle and already factory engineered.
Is it current limited on the vehicle side or does it just supply 12V fused straight from the battery and expect the camper to limit charging current? If you discharge your trailer battery all the way, then apply 12V from it, it will basically act as a short circuit and draw as much current as it can. Just mentioning this so that if the charging circuitry is typically built into the trailer (which is my guess), you can't just expect to use it as a aux battery setup.
Either way, nice idea, I didn't think of that for a power source.
 

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No charge controller on trailers.
I can't tell you all the physics that are involved, but it works. Has worked for years, used by countless trailer builders and vehicle manufacturers. I am not inventing it, just stating that it is there and it is used all the time with success. Trying to save you the effort to re-invent the wheel. You want to charge a battery in the back of your vehicle, they build a charge wire for a spare battery that is already routed to the back of the vehicle. Use it if you want. If you think you can engineer a better system then on of the worlds largest car makers then go for it. But I see it as a huge expense to parallel something that already exists.

I top off my boat and motorcycle batteries with this charge wire. Has been working great for years. My plug in battery charger is a POS that won't charge completely. But a couple wires off the 7-pin to the battery and drive around for a weekend with the battery in the back. Battery is nice and fully charged.
 

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Discussion Starter #6
If you have an aux battery why are you paranoid about things draining it? Do you have a proper isolator and battery selection switch setup?

If you want things to shut off when ignition is off then sure, go for it. It's user preference. I'd suggest doing it proper with a large solenoid or contactor capable of breaking current more than your fuse box is fused for.
Just hate draining the battery. It is completely isolated and set up properly.
 

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Draining a battery completely is NOT good for it. Especially a wet cell. I would hook up the relay or as suggested use the circuit that is already there. I would fuse the circuit separtely regardless of which way you go. How many amps you plan on pulling through this aux fuse panel?
 

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Discussion Starter #8
Draining a battery completely is NOT good for it. Especially a wet cell. I would hook up the relay or as suggested use the circuit that is already there. I would fuse the circuit separtely regardless of which way you go. How many amps you plan on pulling through this aux fuse panel?
Unsure on the amps right now. Planning on a scanner, set of forward/rear/side lights ( unsure on W as that's down the road) and any other accessories I end up getting added. Plus there will be a winch added eventually, but that will be connected and fused independently
 

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Trailer Tow Relay #2 is the relay that feeds power to pin 5 of the tow harness when the key is on. This circuit is fused at 30 amps. This is typical fusing for a trailer charging circuit. You would have to have a really large battery on the camper (or your aux battery) that was fully discharged to pull that kind of current.

But this means if you use that circuit to power your aux fuse panel DIRECTLY you would be limited to 30 amps. If you hook that lead to your aux battery(basically hooking the aux battery in parallel with the truck battery), then feed your aux fuse panel through a fuse from that battery, you could pull more than 30 amps without blowing the charge fuse.

However... You now have the problem of two dissimilar batteries in parallel with each other and the charging issues that sometimes produces. I have towed a camper with two Trojan T105 6V golf cart batteries in series for 12V for years and have never had a charging issue. I just make sure to put the batteries on a 6V, three stage charger, after every trip to charge them completely. You could do the same with your aux battery once a month or so.

You would need to be aware of the state of charge of the aux battery at all times. If the battery gets really discharged AND you tried to turn on say all the exterior lights on the aux fuse panel at the same time, you could easily pull more than 30 amps through the charge fuse as the truck battery will now try to make up for the extra current being pulled. Just fuse everything correctly, stay aware of the state of charge on the aux battery and you should be fine. Oh and get some 30 amp fuses just in case. :iconbiggrin:
 

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Yorkdiver, did you mention which kind of battery you are using for the AUX circuit (lead acid, AGL, LiFePo4)? From what I have read, this is a very important variable.

I'm really curious how you mounted your battery in the spare tire space. Did you fabricate yourself? Any chance you can share a picture?
 
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