Charging Battery with Tow Vehicle

[chiefvintage]

OK folks, I'm in the process of setting up my 12 VDC house battery system, which will basically provide power for just some interior LED lighting, a 3-speed Fantastic vent fan, and USB ports for charging phones, etc. I may eventually add a pure sinewave inverter for my fridge to keep cold while traveling. I have purchased a VMAX Tanks 100aH solar battery. I have a smart charger that i can use with shore power or when at home to recharge the AGM battery, was wondering if I can tie the battery via the trailer harness wiring from TV to charge/top off when traveling. My TV has factory tow package and the tech spec i have show the aux 12V circuit fused at 30A through the 7-pin harness, so I'm assuming the OEM wiring is sized accordingly. My CTC came from the factory with interior LED lights tied to the 12VDC AUX on the 7-pin via a wall switch. I just need to tie in the fan and USB ports along with the AGM house battery. Thoughts? Thanks for your help.

[tony.latham]

My TV has factory tow package...

In too installed a VMAX AGM in this last build.

I talk about charging an AGM via tow vehicle briefly in this video starting at about 7:30.



Keep in mind, I don't consider myself an expert with this issue so don't take this to the bank. There are several on this forum that actually know what they are talking about when it comes to the magic pixies that run through the wires.

Tony

[dirtsailor2003]

You can set up your TV to charge the trailer battery. It can be done from the 7 pin, but you won't be able to fully charge the trailer battery. The run is typically too long from the alternator and lose of power becomes an issue. So if your battery is in good shape, the power being supplied keeps the battery near the level it is. If you go this route and if your vehicle is pre-wired make sure that you have an isolation solenoid so that when your TV is off no power is going to the trailer. Most new rigs are set up this way. It's not fun waking up in the morning to a dead TV battery. Of course a good habit is to always unplug the TV from the trailer when you're parked.

Now if you want to get a full charge you'll need to look at getting a dc to dc charger or combination charger (dc/ac/solar). If you go to eTrailer.com and search dc-dc charger there are some good options.

There is one more route, solar. Having had my cheap Harbor Freight system for 3 years I am at some point going to go full blown. Currently though it does everything I need it to do. Keeps me fully charged running led lights, stereo, and soon to added fan.

[chiefvintage]

My main concern is starting the TV when the 7-pin is connected, won't this cause the TV to draw "starting" current from both batteries?

[dirtsailor2003]

My main concern is starting the TV when the 7-pin is connected, won't this cause the TV to draw "starting" current from both batteries?

It shouldn't if your TV is rigged with an isolation solenoid.My last several large TV's that came factory wired this wasn't an issue.

[chiefvintage]

Based on vehicle schematics (2020 Sierra AT4) the 7-pin 12V AUX is wire straight from battery via 250A main fuse and 30A sub fuse, no isolation that i can see.

[dirtsailor2003]

Based on vehicle schematics (2020 Sierra AT4) the 7-pin 12V AUX is wire straight from battery via 250A main fuse and 30A sub fuse, no isolation that i can see.

If there isn't one you can add one. They are pretty simple to install and eTrailer.com has some good tutorials and wiring diagrams. I haven't installed one in 20 years. All of my TV's that were set up to tow had one factory installed. I think the last one I installed was on my charter fishing boat back in the 90's.

The other option if you are worried is to not plug your trailer in until after you start the rig. Unplug it when you park, pug it back in after you start again.

[John61CT]

You really need a more robust connector like Andersons, and a fatter gauge pair of wires to get decent amps over that distance without too much voltage drop.

Put in a DCDC charger at the back bank to take care of the proper charge profile,

and you won't need **as** fat a connection.

Blue Sea Circuit Wizard is good for sizing wire once your amps draw is known, you really want to maximize that.

It will take with lead at least 6-7 hours to get it back to full, so best to combine with solar, high amps early AM for a few hours, then a full day for sun to finish the job.

LFP will charge much faster, no need to get to Full anyway.

[tony.latham]

You really need a more robust connector like Andersons, and a fatter gauge pair of wires to get decent amps...

John:

I didn't have that problem with my previous teardrop. It charged just fine. There are thousands of RVs that do fine too.

Is your statement from personal experience? If so, perhaps you can share a photo of how your plugs are rigged on your trailer?

Tony

[chiefvintage]

I’m really not concerned with charging a discharged battery with my TV, just mainly to keep it charged if I use an inverter for my standard 110V fridge while driving. I would use shore power with smart charger to recharge it from 40-50%. I always disconnect my 7-pin once set up.


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[RJ Howell]

I agree completely with setting up a DC/DC unit. I bought the Renogy 20a https://www.renogy.com/12v-dc-to-dc-on-board-battery-charger/ on sale for just under $100 shipped. I do plan on charging while driving as my main source, so topping off should work very well.

[flboy]

DC to DC is the best way to go. I am going straight off my alternator through a continuous duty relay and it does well when the battery is below 80%, but as the battery charges and the resistance goes up, the line drop and etc. keep the battery from ever fully charging. Of course this is a backup for me as I have solar and a generator, but if it was my primary means to charge the batteries, I'd really need the DC to DC charger to ensure a good charge. Good luck!

[John61CT]

I’m really not concerned with charging a discharged battery with my TV, just mainly to keep it charged if I use an inverter for my standard 110V fridge while driving. I would use shore power with smart charger to recharge it from 40-50%. I always disconnect my 7-pin once set up.


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Yes, a maintenance "trickle" is a different story.

It's when you want 20/40/60A charging over a long distance, and want to get max longevity for an expensive bank

that more stringent measures are required.

[RJ Howell]

I’m really not concerned with charging a discharged battery with my TV, just mainly to keep it charged if I use an inverter for my standard 110V fridge while driving. I would use shore power with smart charger to recharge it from 40-50%. I always disconnect my 7-pin once set up.


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I have to ask... a 110v frig, run from DC, through a inverter... Have you ever measured the actual power usage? I'll telll ya, it took some convincing, from a few, to get me to buy a DC chest frig. Well, a good sale really helped. That and being advised of compressors used. They work awesome! Would love to hear the power usage of this setup...

Also would like to understand the setup (wiring).

[chiefvintage]

I have a very small Whirlpool 3.1 cu. ft. fridge with freezer section, rated at 47W @120 VAC, not sure what the peak draw is yet when compressor is running, need to do some testing; if needed I’ll go the DC/DC route with an inverter.


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