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Old 09-12-2014, 01:45 PM   #1
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Default Second Battery Isolation System made easy?

Maybe it's just wishful thinking.

My 02 supercrew is, as I'm sure many if not most other f150s are, wired to charge a battery on a trailer through the larger of the two trailer wiring harnesses. So what is to stop me from rerouting those wires to a second battery in my truck?

There's a fuse in the fuse box under the hood for this explicit purpose. I assume it's just a protection for the overall system as there would have to be a circuit breaker wired somewhere to prevent the second battery from overcharging and prevent excessive draw from the primary battery right?

Or is it maybe ran off of the alternator, separate from the primary battery and only charging while the vehicle is running?

Some insight into that system would be greatly appreciated.
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Old 09-13-2014, 10:57 AM   #2
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Thanks for all the help guys
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Old 09-13-2014, 02:05 PM   #3
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Sorry Mike, but automotive electric is not my bag. I pay my RV shop the big bucks to take care of my 12-volt wiring needs. But I'm surprised that others have not weighed in on your question.
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Old 09-13-2014, 05:05 PM   #4
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Default 2nd Battery Tips

I can't Reverse Engineer what you have, so that will remain an unknown to me. No worries, because 2nd Battery 'stuff' is easily found on the Web. Also, RVers do this sort of thing all the time. The 2nd Battery [or Bank of Batteries] is in the Trailer. Charging that Battery is what existing Truck Wiring back to the Rear Bumper 7 Pin Connector is typically all about.

Charging a Trailer 'House' Battery is nice because then you have maximum Boondock time when you set up Camp. The Truck Alternator won't overcharge a House Battery. For those who plug in at RV Parks, the built-in Trailer Charger keeps the Battery charged up.

Here's a bunch of Schematics to choose from. At RV Stores, there's 2 Battery Switches readily available that can handle the high current. Some of us automate this function and charge while driving. A hefty Relay to direct Alternator Current to the 2nd Battery [in the Trailer or in the Truck] can be wired to disconnect once the Truck is turned off. A key benefit is to keep from discharging the Truck Battery accidentally after Boondocking in remote areas. The Relay Coil on such a hefty Relay is energized from some 'Ignition On' Wire that's live only when the Motor is running. Some Folks manually override this function. They charge their 2nd or House Battery for 'x' driving time, and then switch back to charging the primary Truck Starting Battery.

A very common mistake is to under-size both Battery Charging Wiring and Trailer Brake Wiring. I'll likely upsize both these Wires in my new Truck. Factory Wiring is 'ok', but is not ideal.

Dual Battery Schematics & Search Results

Dual Battery Switches & Search Results


P.S. An important detail is to NOT switch a running Alternator between 2 Batteries. This can cause damage. It's all dependent on exact Vehicle Alternator details, and the nature of the Solid State Control used.

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Last edited by Engineer Guy; 09-13-2014 at 06:55 PM.
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Old 09-13-2014, 07:25 PM   #5
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Thanks for the replies guys.

Sounds like wiring a new set up is the way to go.
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Old 09-13-2014, 10:47 PM   #6
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What you need is a constant run solenoid. Looks just like the starter solenoid on my 95 and works the same way except it is designed to run constantly (whereas a starter solenoid would burn out if you used one. You then use a battery cable to connect the battery ground to the block and another 2 battery cables to join the positive posts of both batteries with the solenoid connected between. There will be a place to connect a small wire which when it has power closes the solenoid and connects both batteries so they both charge when the truck is running and disconnect when the truck is off. You can set this up so that they connect automatically when the truck is running or you can mount a switch on the dash and control it manually.
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Old 09-13-2014, 10:47 PM
 
 
 
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