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After having a dead battery the other night, I decided to quit playing around. I replaced the old Exide battery with a Deka Intimidator AGM. I have a second battery coming to go in the second tray as a jump start battery.
The intent is that the main battery can be completely dead and the flick of a switch on the dash will energize a solenoid and connect the back up batt. to the main, jumping it. the solenoid will get it's power from the back up battery, so it will always have juice. The ignition switch will also energize the solenoid, so the back up battery will charge when the vehicle is running.
I have made a diagram that I hope makes sense.
My question is about the connection from the solenoid to the main battery. That would be the red wire with the ? at the end of it. The way I see it, I have three places to hook it up. One is to go directly to the main battery, which is no problem, as it is dual terminal. The second is to connect to the fuse next to the main battery, on the side that goes to the battery, and the third is to connect to the stud at the relay center. The first would be the best, but would require another cable along the core support and using one top post of the battery. The second, is very similar to the first but a little cleaner by going to the fuse stud instead of the battery. The third is the cleanest and can be made to look almost factory, but I am concerned about feedback etc. to the relay center. Would any of these work just as good as the other, or should I avoid one? FWIW, it will be 4GA cable.

 

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get yourself a 600amp starter solenoid and a double throw switch. ground both batteries, connect both + to each side of the solenoid (leaving original battery wiring in place, so add an extra cable from the original to the solenoid).

ground the one side of the solenoid trigger

run the other side of the trigger to the center of the switch.

one side of the switch gets power from 12v run/start/on power (needs to be powered during start)

other side of the switch gets power directly from the second battery.

with the switch off, its single battery like normal

with switch in one mode, power comes from on/start/run and lets you start with double amps and charge both while running

with switch in other mode, power comes from second battery and is always on, ,regardless of running/charging so you can self-jump. this is on when truck is off so you need to be careful or you drain you rback up battery

i used a double pole double throw switch so I can see what mode its in.


edit: btw this is a setup from 12voltguy, inexpensive and easy to setup
 

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Discussion Starter #3
get yourself a 600amp starter solenoid and a double throw switch. ground both batteries, connect both + to each side of the solenoid (leaving original battery wiring in place, so add an extra cable from the original to the solenoid).

ground the one side of the solenoid trigger

run the other side of the trigger to the center of the switch.

one side of the switch gets power from 12v run/start/on power (needs to be powered during start)

other side of the switch gets power directly from the second battery.

with the switch off, its single battery like normal

with switch in one mode, power comes from on/start/run and lets you start with double amps and charge both while running

with switch in other mode, power comes from second battery and is always on, ,regardless of running/charging so you can self-jump. this is on when truck is off so you need to be careful or you drain you rback up battery

i used a double pole double throw switch so I can see what mode its in.


edit: btw this is a setup from 12voltguy, inexpensive and easy to setup
I see what you are saying. It's basically what I already have, just with a different switch to control it manually.
I have the 600A solenoid in place already.
I don't need the extra amps at startup, that Deka cranks it over faster than I have ever heard a 7.4 crank.
Unless I am misunderstanding, I don't think that way is what I am looking for. The way my diagram shows, it will automatically connect the two batteries when the vehicle is running, keeping them both charged at all times. Or, if the main is dead, I can manually connect them with a switch to start, turn the switch off and it will charge both batteries. With the double throw switch, wouldn't I have to manually switch it to keep the aux. battery charged, or keep it on the start/run position all the time? My diagram basically does that, but goes directly to the solenoid, bypassing the switch.
Keep in mind, my wife usually drives this rig, so automatic is better.
 

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i just keep the switch in the start/run position and when i turn the key it enabled dual batteries for charge/start.

If you can get a relay to handle start/run duty automatically, then go for it. I prefer to be able to disconnect the second via a switch though
 

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Keep in mind, you are going to wear your good battery down if you are expecting the altenator to charge the dead battery.

The isolator is going to want to send most of the juice over to the battery with "no" voltage.

If you are not running an isolator, and that box is a solenoid of some sort, then the dead battery is going to drag down the good and the altenator is going to overcharge the good.

I propose 2 possible solutions:

1) You could learn to turn your lights off, or figure out what is killing your battery.

2) You could put a battery isolator in on the charging circuit and have a starting battery and an accessory battery. Just run the starter's power from the other battery.
 

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Keep in mind, you are going to wear your good battery down if you are expecting the altenator to charge the dead battery.

The isolator is going to want to send most of the juice over to the battery with "no" voltage.

If you are not running an isolator, and that box is a solenoid of some sort, then the dead battery is going to drag down the good and the altenator is going to overcharge the good.

I propose 2 possible solutions:

1) You could learn to turn your lights off, or figure out what is killing your battery.

2) You could put a battery isolator in on the charging circuit and have a starting battery and an accessory battery. Just run the starter's power from the other battery.
I ran #2 for year -- just a simple isolator - no switches, no thinking, no nothing. Yes it required jumper cables to jump start myself -- but really how cool is that :D
 
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