Lithium battery, TV battery and 7-way plug charging

[hovr]

Hi folks,
I just did the winterization for the first time and now it's trailer upgrade season
My next project is Lithium battery upgrade, but after much research I still don't know enough to design the wiring with confidence. My main confusion is the interaction between TV and Trailer 12v systems(mostly battery).

Basically I have 4 options for wiring

1. (No isolation, Airstream factory wiring) - charging wire directly connects to positive busbar, no isolation in either direction. This works well when batteries of TV and trailer are both lead acid, but if the trailer is lithium, there are some concerns: a) Lithium battery will back feed the TV battery as lithium has higher voltage when in similar SOC (except when it's close to completely drained). b) when Lithium is at very low SOC, it can draw large current from TV, which might trip the breaker/fuse? c) If I camp hitched and forget to disconnect the 7-way, I could drain both trailer and TV battery and won't be able to start TV.

2. (Complete isolation) - leave 7-way plug charging wire disconnected.
This way TV and trailer 12v system including battery act independently so no unexpected charge/discharge. The downside is if trailer battery is in very low SOC, and (unlikely event) the trailer breaks away from TV, trailer would not have necessary juice to activate brake. If TV can charge the trailer battery then hopefully before this happens TV already give enough charge to the trailer battery.

3 (One way flow from TV to trailer) - Use a high current diode ($12) that only allow current flow from TV to trailer. This still have the problem #1.b, #1.c

4 (One way flow from trailer to TV) - I include this just for completeness, doesn't seems to make a lot of sense, still suffer from the same issue as #2

I am leaning towards #3, as to problem 1.b (there might be some current limiting feature on TV so this might not be an issue, or when it happens it basically becomes #2. for 1.c It's avoidable (unhitch) and even the awkward situation happens it's still far better than trailer unable to brake and kill someone.

Would appreciate if you can share your thoughts/experience on this, thanks in advance!

-Hovr

[waninae39]

this is what i use

Victron Cyric-CT & ( ensure 1 way flow)
Victron Orion Dc-DC converter
( boosts voltage from TV to ensure it can charge the Lion)

this design ensures 1 way power and that the TV can charge the As LiON battery while driving

[hovr]

Originally Posted by waninae39

this is what i use

Victron Cyric-CT & ( ensure 1 way flow)
Victron Orion Dc-DC converter
( boosts voltage from TV to ensure it can charge the Lion)

this design ensures 1 way power and that the TV can charge the As LiON battery while driving

Thanks! Are you worried that your trailer would drain your TV battery if you forget to disconnect at campsite?

[wulfraat]

waninae39 has it if you want to charge from the TV whilst going down the road.

I didn't bother in my setup with so much Solar, so I chose #2 and removed the 30A fuse in my TV 12v DC fuse box for the 12v constant / charge line.

[hissilver]

It’s all a mystery to me.

[drbrick]

Did the same as Wulfraat above - pulled the 30a fuse in my TV simply because I run 200w solar on the AS which is a continuous charge. No need to charge from TV while on the road.

[uncle_bob]

Originally Posted by hovr

Hi folks,
I just did the winterization for the first time and now it's trailer upgrade season
My next project is Lithium battery upgrade, but after much research I still don't know enough to design the wiring with confidence. My main confusion is the interaction between TV and Trailer 12v systems(mostly battery).

Basically I have 4 options for wiring

1. (No isolation, Airstream factory wiring) - charging wire directly connects to positive busbar, no isolation in either direction. This works well when batteries of TV and trailer are both lead acid, but if the trailer is lithium, there are some concerns: a) Lithium battery will back feed the TV battery as lithium has higher voltage when in similar SOC (except when it's close to completely drained). b) when Lithium is at very low SOC, it can draw large current from TV, which might trip the breaker/fuse? c) If I camp hitched and forget to disconnect the 7-way, I could drain both trailer and TV battery and won't be able to start TV.

2. (Complete isolation) - leave 7-way plug charging wire disconnected.
This way TV and trailer 12v system including battery act independently so no unexpected charge/discharge. The downside is if trailer battery is in very low SOC, and (unlikely event) the trailer breaks away from TV, trailer would not have necessary juice to activate brake. If TV can charge the trailer battery then hopefully before this happens TV already give enough charge to the trailer battery.

3 (One way flow from TV to trailer) - Use a high current diode ($12) that only allow current flow from TV to trailer. This still have the problem #1.b, #1.c

4 (One way flow from trailer to TV) - I include this just for completeness, doesn't seems to make a lot of sense, still suffer from the same issue as #2

I am leaning towards #3, as to problem 1.b (there might be some current limiting feature on TV so this might not be an issue, or when it happens it basically becomes #2. for 1.c It's avoidable (unhitch) and even the awkward situation happens it's still far better than trailer unable to brake and kill someone.

Would appreciate if you can share your thoughts/experience on this, thanks in advance!

-Hovr

Hi

1 above will not work with the normal Lithium's we use. The voltages are not compatible. Most of the time you would simply discharge your Lithiums into your TV's electrical system. ( = your trailer batteries are more dead after a drive than before the drive). There is no reason to wire it this way.

2 above is the easy answer. Break away does not use much "juice" and it is highly unlikely you would run your batteries (of any sort) that far down.

3 sounds like fun, however diodes have a forward voltage drop. That plus the wire loss means next to nothing gets into the trailer batteries in most cases. In the rare ( = very brief) case that current does flow, get a diode that is rated for around 500A *average* current and heatsink it accordingly. ( = it will dissipate around 200 to 400W .... yikes ....).

4 has no value, there is no way to start the TV from the trailer batteries.

The only real answer other than 2 is a DC/DC converter.

Bob

[waninae39]

solar and TV charge the lion while on the road

Up north, i cant always get enough solar, especially in spring and fall camping.

thus the TV with the victron dc/dc booster help to maintain the battery.

IMHO, if we were in Texas, CA or Florida maybe solar alone would be fine. But that is not my case

[WP53]

Following. Trying to figure this out myself.

[daleyocum]

I had the 10a version of the Victron DC-DC converter on my system with a good deal of solar. With my 2019 F150 the charge wire from the TV was so underpowered that I just disconnected it. That was because I wanted to use what little current the TV was providing to drive the backup camera and tire pressure monitor repeater. If you don’t have those then a DC-DC converter like the Victron will do the job. Depending on your TV you’d get the size that makes sense.

Renogy makes a fancier one. I’m not sure what it does if the TV doesn’t supply its full amount of current. If it adapts to that it seems ideal. https://www.renogy.com/12v-dc-to-dc-...ttery-charger/

There no need for another insolator with either of these. They are one-way by design.

[hovr]

Originally Posted by uncle_bob

Hi

1 above will not work with the normal Lithium's we use. The voltages are not compatible. Most of the time you would simply discharge your Lithiums into your TV's electrical system. ( = your trailer batteries are more dead after a drive than before the drive). There is no reason to wire it this way.

2 above is the easy answer. Break away does not use much "juice" and it is highly unlikely you would run your batteries (of any sort) that far down.

3 sounds like fun, however diodes have a forward voltage drop. That plus the wire loss means next to nothing gets into the trailer batteries in most cases. In the rare ( = very brief) case that current does flow, get a diode that is rated for around 500A *average* current and heatsink it accordingly. ( = it will dissipate around 200 to 400W .... yikes ....).

4 has no value, there is no way to start the TV from the trailer batteries.

The only real answer other than 2 is a DC/DC converter.

Bob

Thank you Bob!
I didn't know voltage drop issue, the one I plan to buy is this one:
https://www.amazon.com/Roadmaster-69...ct_top?ie=UTF8

it does have a heat sink - I don't actually expect this to fully charge the battery or even close to that. if my Lithium battery dropped to 11.4-12v, I hope TV can give it a bit more juice, that's it. It could be a trip to home at night when no solar available. Do you think that would work?

[daleyocum]

Originally Posted by hovr

Thank you Bob!
I didn't know voltage drop issue, the one I plan to buy is this one:
https://www.amazon.com/Roadmaster-69...ct_top?ie=UTF8

it does have a heat sink - I don't actually expect this to fully charge the battery or even close to that. if my Lithium battery dropped to 11.4-12v, I hope TV can give it a bit more juice, that's it. It could be a trip to home at night when no solar available. Do you think that would work?

For lithium you really want something that will produce a constant 14.3 volts, (thereabouts depending on your battery brand.) A diode wouldn't do the job. These are expensive batteries that deserve to be treated well.

Depending on your tow vehicle's charge current this may or may not be worth the trouble. I'd ask the Renogy folks what happens if the TV can't produce the 20amps their unit is rated for.

[Wazbro]

One other option, disconnect charge wire, then hook up inverter under hood of TV and run 120v extension cord to back bumper of TV and cord from trailer converter to plug in at back bumper of TV.
EDIT would be best to have inverter on relay so it is only on while TV is running.

[alevinman]

I have two lithiums, 400W of Renogy panels and Renogy controller - i noted that the lithiums keep my TV battery from losing charge if i run my TV onboard air compressor with the engine off. So i stay connected and my TV battery stays maintained via the 7-pin connection. Of course the lithiums have plenty to offer and the solar is keeping them topped off - might as well use/send the power to the TV.

[GMFL]

Best way to do this is with a modern DC/DC Charger. Renogy, Victron, Redac, MasterVolt and several others maKe them. If they are not voltage triggered then “I” now use the (Green) wire from the Airstream 7-Pin (it’s the running light wire) as my “on” trigger. This means that with the running lights no the charger is on and with the running lights off the charger is off.

[uncle_bob]

Originally Posted by hovr

Thank you Bob!
I didn't know voltage drop issue, the one I plan to buy is this one:
https://www.amazon.com/Roadmaster-69...ct_top?ie=UTF8

it does have a heat sink - I don't actually expect this to fully charge the battery or even close to that. if my Lithium battery dropped to 11.4-12v, I hope TV can give it a bit more juice, that's it. It could be a trip to home at night when no solar available. Do you think that would work?

Hi

Nowhere near capable enough. What you would need is something roughly the size of one of your batteries. The heat sinking issue is a really big deal. I say that as somebody with a EE degree and >40 years spent designing electronics ....

=====

You do *not* want to run your batteries to zero. That's true of any sort of battery. There should never be a case where you are headed home with dead batteries. If that *is* a situation you get into, buy a small generator and use that to top things off.

====

Rolling down the road, you should have about 1A of drain on the batteries ( = the fridge might pull that). If you drive for 8 hours in the dark, that's 8 AH. If you set up a low voltage cutout on the fridge, set at 12V, the fridge dies but there is plenty in the battery to do the breakaway.

The battery voltage *only* runs the breakaway. It does not power the brakes normally or any of the signals / lights while driving. Except for a hitch failure, you would be fine driving down the road with no battery. It's a really rare thing to need.

====

If indeed you feel that all isn't good enough, just get a small 10AH sealed lead acid battery. Hook it up to the charge lead and feed the breakaway switch with it. You now have a dedicated system that does everything you need to do. Total cost would be < $20. Those batteries are small and work ok indoors. Not at all hard to stash here or there in the trailer .....

Bob

[hovr]

Originally Posted by GMFL

Best way to do this is with a modern DC/DC Charger. Renogy, Victron, Redac, MasterVolt and several others maKe them. If they are not voltage triggered then “I” now use the (Green) wire from the Airstream 7-Pin (it’s the running light wire) as my “on” trigger. This means that with the running lights no the charger is on and with the running lights off the charger is off.

Thanks @GMFL! I have seen DC/DC charger pops up in different posts but had no idea what they were for. Now I have some idea of this, still wondering though - why would people spend several hundred dollars on them (and they are fairly bulky as well) when they already have sizable solar panels on their roof?

Since the space is so limited for my 16RB under the dinette seat, I think what I will do is to connect the charging wire with a 20-30A breaker with manual reset, I will leave the breaker open/disconnected most of time, if I found myself drain the trailer battery too low, I will close/reset the breaker and start the TV engine to give it a bit charge for the unlikely breakaway event. This assumes my TV's alternator circuit has some current regulator so it won't burn the fuse or trip the breaker.

[PB_NB]

Hovr, I have disconnected the charge line from my truck to the 7 pin connector. I didn't want to pull voltage to my big Lithium battery from my trucks little charging system.

I think this comes from my past experience when I ran a small diesel tow vehicle and had 4 x 6V deep cycle batteries on the trailer. I really didn't want to stress the charging system.

I like the idea of one way charging like waninae's setup. I am still working out the solar aspects of my system and will dive that when our business slows down for the winter.

With your Caravel, you will be limited to the amount of roof space for panels so you probably will be okay with the factory prewire gauge. Additional portable panels might be a good choice to get enough amps to keep the battery charged.

While travelling, roof solar can be a good means of trickle charging the lithium batteries.

If you are doing some winter camping/boondocking, the generator option could be a good one to deal with the reduced sun light or the infamous PNW rain.

[hovr]

Originally Posted by PB_NB

Hovr, I have disconnected the charge line from my truck to the 7 pin connector. I didn't want to pull voltage to my big Lithium battery from my trucks little charging system.

I think this comes from my past experience when I ran a small diesel tow vehicle and had 4 x 6V deep cycle batteries on the trailer. I really didn't want to stress the charging system.

I like the idea of one way charging like waninae's setup. I am still working out the solar aspects of my system and will dive that when our business slows down for the winter.

With your Caravel, you will be limited to the amount of roof space for panels so you probably will be okay with the factory prewire gauge. Additional portable panels might be a good choice to get enough amps to keep the battery charged.

While travelling, roof solar can be a good means of trickle charging the lithium batteries.

If you are doing some winter camping/boondocking, the generator option could be a good one to deal with the reduced sun light or the infamous PNW rain.

Thanks Peter, I am using the winter season to give my 16RB a major electrical upgrade. after I am done it would have 200ah lithium and 200 or 230w solar. The solar prewire is 8awg so it should be fine. It's going to be a bit challenging to fit 2 batteries and SCC plus breakers, terminal blocks under the bench but this forum has provided very good resources and I think it can be done.

I most likely will leave the car charging wire disconnected. 2 low SOC 100ah lithium batteries in parallel probably will draw too high current that would burn the TV fuse.

[hovr]

Originally Posted by uncle_bob

Hi

Nowhere near capable enough. What you would need is something roughly the size of one of your batteries. The heat sinking issue is a really big deal. I say that as somebody with a EE degree and >40 years spent designing electronics ....

=====

You do *not* want to run your batteries to zero. That's true of any sort of battery. There should never be a case where you are headed home with dead batteries. If that *is* a situation you get into, buy a small generator and use that to top things off.

====

Rolling down the road, you should have about 1A of drain on the batteries ( = the fridge might pull that). If you drive for 8 hours in the dark, that's 8 AH. If you set up a low voltage cutout on the fridge, set at 12V, the fridge dies but there is plenty in the battery to do the breakaway.

The battery voltage *only* runs the breakaway. It does not power the brakes normally or any of the signals / lights while driving. Except for a hitch failure, you would be fine driving down the road with no battery. It's a really rare thing to need.

====

If indeed you feel that all isn't good enough, just get a small 10AH sealed lead acid battery. Hook it up to the charge lead and feed the breakaway switch with it. You now have a dedicated system that does everything you need to do. Total cost would be < $20. Those batteries are small and work ok indoors. Not at all hard to stash here or there in the trailer .....

Bob

Bob, thank you for the detailed explanation! I don't plan to run the battery to zero, It was mostly for unintended situation. however reading your comments made me realize that I do have a scenario I need to be mindful - the electrical only fridge draws 2.4a according to its manual, At the end of a typical 2 nights camping, late afternoon when I leave campsite, if the battery is low I probably need to turn off fridge(hopefully I have consumed most content), otherwise it will continue to draw 2.4a for a typical 3 hours trip.

Also I was not aware there is a fridge cutout voltage. is it a standard thing or I have to add some device for it?

Thanks!