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Discussion Starter #1
I have a spare battery (U1L from lawn mower) hanging around on the trickle charger. We like to use a 12v small cooler on rides to keep drinks and food cold without ice in a cooler. First problem is the rear aux power outlet on our crew is key power only. We might stop for an hour or more somewhere and the 12v cooler will get warm. The tiny cooler/fridge uses about 4.5 amps. I thought about adding a mount and box for the small auxiliary battery under the back seat. All I really need it for is to power a dual USB/cig lighter power panel. I could keep it completely isolated from the main system but thought it might be handier to connect it to the main system in parallel to charge it while driving. I have a new battery disconnect I could use to "isolate" it manually. Otherwise it would be a total loss system until I return home and hook up the trickle charger to it.

I really don't want all the expense and complication of the nice automatic isolator systems. Just a simple twist to join the battery to the primary system while driving and twist to isolate after charging. Make sense? Is it best to install the disconnect in the pos or neg cable circuit of the aux battery?
 

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I'm thinking of doing the same thing only with a full size battery. I want to install a starter solenoid and switch on the cluster to tie it into the system when I need it. My 07 has a really bad history of going through batteries. I travel a lot for work so it sits sometimes for weeks without being run thus the batteries don't hold up as they should.
 

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Oh, I will likely tie the pos and neg through the solenoid but I'm thinking the pos is all that needs to be switched.
 

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Discussion Starter #4
Everything I have been told is that you should always disconnect the negative terminal from the battery when disconnecting them to minimize sparks. Not sure that applies to the disconnect switches but it might be prudent to do so. I would mostly keep the aux battery separate from the primary system just for the aux power outlet. But thought it could be convenient to allow the primary charging system to charge the aux battery when desired. For letting batteries sit for long periods I keep a tender on them. Most all of my power equipment with its own battery has a tender quick hookup so I can rotate a few tenders around to everything. Mower, tractor, side by side, pickup (I have a company car), etc.
 

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I was thinking of putting in a small motorcycle LiFEPO4 battery as the main starting battery for my rig, keeping the existing AGM to power the winch, with a simple isolator.

I've always put battery isolators on the positive side.

Disconnect negative first is a thing when removing a battery so that you don't accidentally have your wrench go from the bolt on the positive to the frame (which is connected to negative). It doesn't apply to switches.
 

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Discussion Starter #6
Good to know vilord. It makes good logic. If the switch is up to the voltage/amperage I suppose there shouldn't be an issue with breaking current on the positive side.

I understand the benefits of the "isolator systems" but seems to be more foolproofing than real necessity. I still prefer manual headlight switches in my pickup.
 

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Voltage/amperage through the switch will be the same whether disconnecting positive or negative :)

One advantage of a battery isolator is that many of them have a current limiter. If your back battery is at 11.5 volts and you connect that switch, then the main battery (or the reg/rec) is going to try to charge the back battery as fast as it can. This could end up pushing too much power across the wires. If you have a fuse it'll blow the fuse. If you don't have a fuse, it could melt things.

Also, if you forget and leave the switch on when you turn the key to start it, and the back battery happens to have higher state of charge than the front one, it might try to pull full starter current out of the back battery. Ouch.

You'll want something to limit the current.
 

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Discussion Starter #8
That explanation is beyond my education on the subject. Thanks for pointing out the over current potential on a low battery in the system. In all reality, I intended to keep them separated most or all of the time. Pairing them up was only for charging purposes away from home. A trickle charger will be set up to charge the auxiliary battery under normal circumstances.
 

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One thing you could do is just put a brake light bulb in the circuit with the switch, so ranger +12v -> switch -> light bulb -> aux battery. Then it'll at maximum only put 2 or 3 amps in or out :)
 

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Discussion Starter #10
That has me curious. Is the bulb going to do the same thing as a fuse? How does the bulb limit amperage transfer? I was assuming I would be installing small battery cable type connections rather than 10ga primary wire to connect the auxiliary battery into the system. Maybe I need to know a bit more.
 

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The bulb would be similar to a fuse, but it won't blow.
If the batteries are the same voltage, then the bulb would be off. If one is more charged than the other, it would glow, and a max of however many watts the bulb is could go through it, so if you put a 27 watt / 12 volt bulb, it'll only let through 27 watts.
At least that's how I understand it. A friend used to do this for charging up a backup battery for his ham radio from the constant power supply
 

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There are any number of ways to do a backup battery. My main objective is to have starting power when my main battery craps the bed. If the battery is low or dead I can flip the rocker switch to my starter solenoid and be able to start the machine without lifting the windshield and hood and jumping it off.
 

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I'm thinking of doing the same thing only with a full size battery. I want to install a starter solenoid and switch on the cluster to tie it into the system when I need it. My 07 has a really bad history of going through batteries. I travel a lot for work so it sits sometimes for weeks without being run thus the batteries don't hold up as they should.
Don't use a starter solenoid , they're not rated for 100 % duty , you will burn the coil out , use a 12 volt contactor designed for continuos duty .
 

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Yea, I said starter solenoid out of habit. There are continuous duty solenoids. I use one that is rated for 150 amps and continuous duty. 300 amp end rush. 13 bucks.
 
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