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Old 10-02-2022, 02:32 PM   #1
2 Rivet Member
 
2020 20' Bambi
Dearborn , Michigan
Join Date: Oct 2019
Posts: 63
TV Battery "vs" Lithium

Hello All! Looking for thoughts/input.

I have run across a number of posts regarding a compatibility issue between the charging profiles of the TV and RV/Lithium batteries, where the TV battery charges from the RV. I have also done a few searches to understand this topic.

There seems to be three solutions... a DC to DC charger, disconnect the charge wire coming from TV, or to pull the charge fuse from the TV.

Questions:

How do I know if I even have an issue? I put a voltmeter on the TV and RV when disconnected and connected and there was no difference in readings.

I did a search of the TV (2015 F150 w/tow package) manual and of the internet and can't see that I have a charge fuse to remove if wanted. Some years have one, some years don't.

Under the back seat was/is a "Jumper Harness" FL3T-14A348-A which seems to serve no purpose as the tow harness and built in brake controller connect the TV and RV.

How can I test if the tow harness "Trailer Battery Feed" (hot with key in run only) is hot?

I have traced the RV feed into the trailer and to the buss bar where I can disconnect, but do I need to.

And to cap it off, the JC tech said that this whole topic is a "myth."

How do I tell if I have an issue? I will be driving 12 hours to Williamsburg this coming weekend and can monitor voltage if need be.

If there is no issue, great. If there is, how should I deal with it?

I really appreciate your thoughts/input. Thanks.

peace
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Old 10-02-2022, 02:55 PM   #2
4 Rivet Member
 
1949 22' Liner
1969 27' Overlander
1969 27' Overlander
Walnut Creek , California
Join Date: Apr 2015
Posts: 323
You need a DC to DC charger between the tow vehicle and your lithium batteries. This isolates the two systems because the lithium system will operate at a higher voltage than the lead acid system. The DC to DC charger acts like a voltage pump. It takes the much lower voltage produced by the tow vehicle and pumps it up to a voltage that is usable by the lithium BMS. Most good DC to DC chargers also have a trigger terminal to turn them on or off. I normally use the running lights power as the trigger. The DC to DC charger should be mounted fairly close to the lithium’s in order to minimize the voltage drop on the charging wires.
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Old 10-02-2022, 03:00 PM   #3
4 Rivet Member
 
1949 22' Liner
1969 27' Overlander
1969 27' Overlander
Walnut Creek , California
Join Date: Apr 2015
Posts: 323
The DC to DC charger also will limit the charge current. This can prevent blown fuses on your tow vehicle charging circuit. A dead lithium connected to a charging vehicle can pull a lot of current. Lithium’s have low internal resistance.
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Old 10-02-2022, 03:04 PM   #4
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2017 30' Classic
2022 Interstate 24X
Carlisle , Pennsylvania
Join Date: May 2017
Posts: 14,854
Hi

Unfortunately this is just the basic parameters of a lithium battery. It needs > 14V to properly charge. That's just how they work. You can look at a *lot* of battery spec sheets for RV lithiums and they all say this.

Lead acids want a much lower voltage ( down around 13.x V ) for normal charging. Your TV has a lead acid battery in it. Again, there are lots of data sheets out there. You can also hook a voltmeter to your TV and see what it does. If you take a lead acid up above 14V for any length of time and you will create significant issues ( = boil the battery / dry it out / fry it ).

As long as you don't mind arriving at your destination with the lithiums in your trailer at a lower state of charge than when you left, there is no problem. Nothing gets fired, nothing is destroyed. You simply have discharged batteries in your trailer. In this respect the JC tech is correct, it is not an end of the world issue. You just have flat batteries. Not a big deal.

If all you want to do is to prevent discharge then any approach to disconnecting the charge wire works. Disconnect it inside the trailer. Disconnect it at the wiring harness on the TV. Pull a fuse in the TV. Anything that breaks the circuit will do the trick.

On some TV's the discharge process *might* continue while things are parked and the 7 pin is still plugged in. Unplugging the 7 pin each time you stop will take care of this issue. The problem ( if it exists ) relates to the lithium charger putting a lot of voltage on the truck battery. On most modern trucks, the truck disconnects things and it's a non-issue. Who knows what happens with a car or a "hand wired" 7 pin connector.

Properly done ( and there are fiddly details ....) a DC/DC isolates things. It eliminates the truck damage from the trailer charger issue. It also fixes things so that the truck *does* charge the lithiums as you go down the road. The only thing it adds over pulling the wire is the charge while driving part. Is it worth it? That is very dependent on your use case.

So no, this is in no way black magic. You can find YouTube videos to support just about any weird idea you might come up with. You can also find ones that are quite accurate. Sorting them out is not easy.

Fun

Bob
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Old 10-02-2022, 03:10 PM   #5
4 Rivet Member
 
1949 22' Liner
1969 27' Overlander
1969 27' Overlander
Walnut Creek , California
Join Date: Apr 2015
Posts: 323
Spending all that money on lithium batteries and then not charging them correctly is silly. Lead acid batteries also benefit from a DC to DC charger.
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Old 10-02-2022, 04:30 PM   #6
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2020 20' Bambi
Dearborn , Michigan
Join Date: Oct 2019
Posts: 63
Thank you both!

Is there a way I can test if the battery charge wire at the TV (at the 7pin receptacle) is even "hot"? There is no fuse to pull that I can find to disable things in the interim.

peace
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Old 10-03-2022, 10:08 AM   #7
4 Rivet Member
 
1949 22' Liner
1969 27' Overlander
1969 27' Overlander
Walnut Creek , California
Join Date: Apr 2015
Posts: 323
Quote:
Originally Posted by harphunt View Post
Thank you both!

Is there a way I can test if the battery charge wire at the TV (at the 7pin receptacle) is even "hot"? There is no fuse to pull that I can find to disable things in the interim.

peace
you probably should buy a test plug. Most modern vehicles detect a trailer connected and then supply power through the control module or relay. A test plug is great to have in the future.
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Old 10-03-2022, 10:59 AM   #8
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2002 19' Bambi
Auburn , Maine
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Quote:
Originally Posted by uncle_bob View Post
Hi

You can find YouTube videos to support just about any weird idea you might come up with. You can also find ones that are quite accurate. Sorting them out is not easy.

Fun

Bob
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Old 10-03-2022, 11:34 AM   #9
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1988 32' Excella
Robbinsville , New Jersey
Join Date: May 2012
Posts: 2,374
Quote:
Originally Posted by harphunt View Post
Thank you both!

Is there a way I can test if the battery charge wire at the TV (at the 7pin receptacle) is even "hot"? There is no fuse to pull that I can find to disable things in the interim.

peace
Testing can be tricky, some vehicles the charge wire is hot once you put the key in accessory or run position, others you need to start the engine, others you need to have it in drive and some you need to get up to 5 or 10 mph and hit the brakes or some other set of conditions. You need to find out when your vehicle is supposed to be hot, mine is always hot so easy to test.
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