Dual battery setup

[Another JR]

Keep in mind that a side effect of connecting any load to the trailer marker light circuit is that the truck detects a trailer. That causes the rear parking assist to be turned off. This will happen even if you simply have a connector with an LED circuit check light on the connector.  A slide in camper sized for a full size truck is typically wide enough that the DOT marker light requirement applies, so it will have marker lights you’ll want to wire to the truck marker light circuit. I would recommend separately wiring your camper to the appropriate truck battery and truck marker light circuit rather than using the trailer circuits to avoid defeat of safety features you may want active. 
 

Also, taking off the tailgate will disable the entire camera system unless you wire in a replacement camera. 
 

Will your camper have its own battery?

 

I installed a Four Wheel Campers Grandby pop-up slide in on my truck with a 200 Ah lithium battery, solar, and a dc to dc charger for charging from the truck. I installed a camera on the back of the camper to replace the tailgate camera. I’m happy to answer any questions about how I set mine up and why. 

Edited September 1, 2021 by Another JR

[03 Cobra]

On 9/1/2021 at 1:26 PM, Another JR said:

Keep in mind that a side effect of connecting any load to the trailer marker light circuit is that the truck detects a trailer. That causes the rear parking assist to be turned off. This will happen even if you simply have a connector with an LED circuit check light on the connector.  A slide in camper sized for a full size truck is typically wide enough that the DOT marker light requirement applies, so it will have marker lights you’ll want to wire to the truck marker light circuit. I would recommend separately wiring your camper to the appropriate truck battery and truck marker light circuit rather than using the trailer circuits to avoid defeat of safety features you may want active. 
 

Also, taking off the tailgate will disable the entire camera system unless you wire in a replacement camera. 
 

Will your camper have its own battery?

 

I installed a Four Wheel Campers Grandby pop-up slide in on my truck with a 200 Ah lithium battery, solar, and a dc to dc charger for charging from the truck. I installed a camera on the back of the camper to replace the tailgate camera. I’m happy to answer any questions about how I set mine up and why. 

 

I am interested to know how you tied in the DC-DC charger for your lithium battery.  Did you integrate it into the factory isolator on your aux battery?  Or wire it direct to the alternator?  I was toying with the idea of replacing my aux battery with a lithium and hooking a DC-DC charger up to the wiring that's already there for the aux set up.  First I would check the under hood temp as they don't like to live above 135*F.  If it passes the temp test that's what I thought to do for camping in the back of my truck under my camper shell.  Then I would use the 7 pin plug that's already in the bed that came with the 5th wheel package.  There's probably a smarter way but that's what I came up with.  Thoughts?!

[Another JR]

The dc to dc charger for camper battery charging normally is only intended to charge the camper battery when the engine is running, and will have some means to detect engine running. Because it only draws current when the engine is running there is no need to further isolate it from the main battery to prevent discharge. 
 

I connected 6awg wire to the main truck battery fuse block unused 80 amp fuse for the positive, and to one if the large body ground studs for the negative. I actually used the ground stud the aux battery negative is connected to because it was most accessible, but it doesn’t matter which one you use as long as it’s good for the total current. The dc to dc charger is located next to the camper battery in the camper. 
 

I wouldn’t think that the environment under the hood is acceptable for a lithium battery, especially on hot days. Also, dc to dc chargers generate quite a bit of heat and at least the 30 and 50 amp ones are typically passively cooled with fins, and they need cooler environmental conditions to stay within acceptable operating temperatures to avoid output cutback or failure.  There would be concerns with subfreezing conditions as well. 

 

I ordered my second battery for truck mounted accessories like a compressor and in-cab portable refrigerator, not for supporting a camper electrical system. I briefly considered a lithium battery and dc to dc charger setup under the hood for these accessories, but ruled it out because of the temperature issue.  The second battery option was so cheap I just went with it and expect it will be fine for my planned accessory use. 
 

For camping in a shell I’d probably use an easily removeable battery installation inside the shell. If you already have the fifth wheel connector you can use the power from that as you said to run a small dc to dc charger that is at your camper battery.  

[03 Cobra]

On 9/3/2021 at 11:54 AM, Another JR said:

The dc to dc charger for camper battery charging normally is only intended to charge the camper battery when the engine is running, and will have some means to detect engine running. Because it only draws current when the engine is running there is no need to further isolate it from the main battery to prevent discharge. 
 

I connected 6awg wire to the main truck battery fuse block unused 80 amp fuse for the positive, and to one if the large body ground studs for the negative. I actually used the ground stud the aux battery negative is connected to because it was most accessible, but it doesn’t matter which one you use as long as it’s good for the total current. The dc to dc charger is located next to the camper battery in the camper. 
 

I wouldn’t think that the environment under the hood is acceptable for a lithium battery, especially on hot days. Also, dc to dc chargers generate quite a bit of heat and at least the 30 and 50 amp ones are typically passively cooled with fins, and they need cooler environmental conditions to stay within acceptable operating temperatures to avoid output cutback or failure.  There would be concerns with subfreezing conditions as well. 

 

I ordered my second battery for truck mounted accessories like a compressor and in-cab portable refrigerator, not for supporting a camper electrical system. I briefly considered a lithium battery and dc to dc charger setup under the hood for these accessories, but ruled it out because of the temperature issue.  The second battery option was so cheap I just went with it and expect it will be fine for my planned accessory use. 
 

For camping in a shell I’d probably use an easily removeable battery installation inside the shell. If you already have the fifth wheel connector you can use the power from that as you said to run a small dc to dc charger that is at your camper battery.  

 

JR, your points resonate with what was already in the back of my head....the lithium battery and dc to dc charger will not live as long under the hood.  I think you helped convince me to tuck them away in the bed somewhere instead.  I hadn't thought of using the 7 pin that's in the bed to run the dc to dc charger.  Do you think the wiring for the 7 pin will be heavy enough to run the dc to dc?  I might just run a new positive and negative wire up to the engine bay and hook mine up similar to what you did.  Then, if I am not putting the lithium battery under the hood I wonder it if would be worth it to replace the factory aux battery with a good AGM battery.  Just to get more out of it and avoid the scenario where I discharge the factory aux battery too far (I think 50% is the accepted limit) and cut it's life short.  Anyway...fun stuff to think about.

[Another JR]

You can use pin 4 of the trailer connector, which comes from the aux battery fuse block, as long as you ensure your dc to dc charger engine running detection logic is set properly to shut off the charger when the engine is shut down. You don’t want to discharge the aux battery with the dc to dc charger. Also, that wire is protected by a 30 amp fuse. A 20 amp dc to dc charger should work fine. A 30 amp one probably would blow the fuse.  If you do it this way, use the trailer connector ground wire - don’t use the truck body as the current return. 
 

If I was doing lights, electronics charging, and a fridge in a canopy without solar, I’d get a 70 to 100 amp hour battery and a 30 amp charger, and run new 8 awg positive and negative wires to the main battery fuse block and a large engine compartment ground stud.  That way you’d have at least two days of energy storage and you could recharge a nearly discharged battery in two to three hours of driving.  

 

[03 Cobra]

17 hours ago, Another JR said:

You can use pin 4 of the trailer connector, which comes from the aux battery fuse block, as long as you ensure your dc to dc charger engine running detection logic is set properly to shut off the charger when the engine is shut down. You don’t want to discharge the aux battery with the dc to dc charger. Also, that wire is protected by a 30 amp fuse. A 20 amp dc to dc charger should work fine. A 30 amp one probably would blow the fuse.  If you do it this way, use the trailer connector ground wire - don’t use the truck body as the current return. 
 

If I was doing lights, electronics charging, and a fridge in a canopy without solar, I’d get a 70 to 100 amp hour battery and a 30 amp charger, and run new 8 awg positive and negative wires to the main battery fuse block and a large engine compartment ground stud.  That way you’d have at least two days of energy storage and you could recharge a nearly discharged battery in two to three hours of driving.  

 

 

I think my plan is starting to come together.  Everything will be in the truck bed except for the wires that run up to the engine bay. I'm looking at a Redarc BCDC 25amp charger so I can handle solar and have ultimate flexibility with the type of battery.  As for the batteries I will go with 2-100 Ah Battle Born lithium batteries that both have the heating element to  keep them warm in the frigid Michigan temps.  I need to do more research on the pure sine inverter I want to get.  Fun, and expensive, stuff!