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Old 06-15-2021, 09:13 AM   #1
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TV to TT diode

Having local tech install solar, lithium upgrade to 2014 FC fbt. Question about TV charging TT lithiums. I've read about the very limited capabilities of TV charging. My tech is suggesting a "one way diode" installed on the 7 pin charging wire (TT side). This would allow charging but solving all the problems with staying connected while stationary and not having to remember to disconnect 7 pin. Is this prudent solution?
Thanks all..
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Old 06-15-2021, 11:27 AM   #2
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All diodes conduct in only 1 direction. This sounds like a solution but did he provide a spec on the diode? I found this information. Maybe someone makes something for this type of application.

https://www.tutorialspoint.com/semic...cteristics.htm

Diode Specifications
Like any other selection, selection of a diode for a specific application must be considered. Manufacturer generally provides this type of information. Specifications like maximum voltage and current ratings, usual operating conditions, mechanical facts, lead identification, mounting procedures, etc.

Following are some of the important specifications -

Maximum forward current (IFM) - The absolute maximum repetitive forward current that can pass through a diode.

Maximum reverse voltage (VRM) - The absolute maximum or peak reverse bias voltage that can be applied to a diode.

Reverse breakdown voltage (VBR) - The minimum steady-state reverse voltage at which breakdown will occur.

Maximum forward surge current (IFM-surge) - The maximum current that can be tolerated for a short interval of time. This current value is much greater than IFM.

Maximum reverse current (IR) - The absolute maximum reverse current that can be tolerated at device operating temperature.

Forward voltage (VF) - Maximum forward voltage drop for a given forward current at device operating temperature.

Power dissipation (PD) - The maximum power that the device can safely absorb on a continuous basis in free air at 25į C.

Reverse recovery time (Trr) - The maximum time that it takes the device to switch from on to off stat.
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Old 06-15-2021, 11:37 AM   #3
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Yes, I think the diode will prevent problems from the lithium dumping power into the TV.

I doubt if you will actually get any charging of the trailer batteries though. A better way in my opinion is to install a Victron DC to DC converter in the trailer. That way you will actually get some charge while you are driving. I am putting one in now to charge my lead acid batteries and to be ready for a possible lithium switch. My voltage is too low to even put any appreciable charge on the lead acids and lithium takes a higher voltage. And the converter isolates the tv and the trailer unless the alternator is running.
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Old 06-15-2021, 12:03 PM   #4
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Hi

As somebody who spent their working life designing electronics .... no it's not a good idea.

A very common thing you hear is "the diode gave up it's life to protect the poor little fuse". Basically diodes burn out from overcurrent *much* more quickly than a normal fuse can react.

You might say, "the diode is rated for 140A, that's a lot !!". Indeed it is. It's rated for that amount of current with a heatsink coming in around 0.5 C/W thermal resistance. Sitting in free air, it might be rated for 4A.

When can you see a lot of current? Run your trailer batteries down and fire up the truck. If your truck electrical is up around 14V fast charging it's battery and the lithiums are down around 11V, you will see a good chunk of current flowing ....

Next up is that all diodes have a forward voltage drop. Your truck's electrical system is "tuned" to lead acid batteries. They charge at a lower voltage than lithiums. Your charge wire has a drop in it due to current flow, that makes things worse *before* the diode goes in. Add the diode drop and you are into "yikes" territory. Again, look at the drop when it's carrying 20A of current, not at 20ma.

You are better off simply disconnecting the charge wire than playing with a diode.

Bob
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Old 06-15-2021, 12:06 PM   #5
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DC to DC

Bill M. That is my understanding also, that TV voltage drop due to long runs/wire guage issue allows for little useful charging capabilities to TT lithiums. I am still puzzled by how that low voltage into a DC to DC charger mounted in the TT can be transformed into a usable charge. I am thinking like a water hose can easily have a restriction (lowering voltage) but impossible to increase flow from its max capacity. But, who am I? If i knew what I was talking about I probably wouldn't have asked the question. Thanks, for the reply.
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Old 06-15-2021, 12:40 PM   #6
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The DC to DC converter is a regulated power supply that supplies the set voltage output, maybe 14.6 for lithiums, regardless of the input voltage. So it will take in from 8 to 17 volts or so input and put out the set 14.6 or whatever it is set to. Think of it the same as you power converter but with a dc supply. Someone posted the numbers from his installation at idle. His was set for AGM's. It was reporting 13.1 input (not enough to charge a battery) and 13.6 output (good for AGM's) with about a 8 amp charge occurring, I think. Of course it draws more current that it outputs to make up for the setup in voltage. And if you get the isolated one it also has diodes to prevent the tv from drawing from the trailer. I do not have mine in yet. But I am sold enough on the idea to order one and I know I am going to have a huge hassle installing it. Might be easier on a new trailer.

I appreciate Bobs comments. Unless the "diode" is a package especially designed and tested for that application I expect you will end up just disconnected anyway. And I am fairly sure you will get no charge while driving. The tech might know if a diode that actually works. I would still prefer the Victron unit.

My unit just came in the mail. Be a few days or weeks until I get to it. After I check bearings and re do the brake connections and go to work some. The one I got is 18 amps, bluetooth, and $165 from Amazon. I think there is are a couple of $70 ones from Victron that will work also. Maybe your tech can install it as cheaply as he could do the diode. I got to cut some aluminum interior skins behind the sofa, I think. I think it would be a good supplement to a solar system if you got some charge while driving. I do not know your TV. Mine has a 30 amp fuse on the 7 pin so I know I have big enough wire to handle the 18 amps. You might check yours. The units come in several sizes.
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Old 06-15-2021, 12:47 PM   #7
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Quote:
Originally Posted by Gen0 View Post
Bill M. That is my understanding also, that TV voltage drop due to long runs/wire guage issue allows for little useful charging capabilities to TT lithiums. I am still puzzled by how that low voltage into a DC to DC charger mounted in the TT can be transformed into a usable charge. I am thinking like a water hose can easily have a restriction (lowering voltage) but impossible to increase flow from its max capacity. But, who am I? If i knew what I was talking about I probably wouldn't have asked the question. Thanks, for the reply.
Hi

In order to charge a lithium battery, the voltage needs to be up around 13.5 or so. Even with little or no drop in the system, that condition is often not met by your truck's electrical system. Yes, temperature, engine speed, options, electrical load all figure it, but still, not going to hit the target.

The DC/DC is quite happy with a varying voltage. They often run down to 8 or 10V. It's designed to convert that into the correct target voltage for your batteries. As a side benefit, it also limits the maximum current pulled from the truck's electrical system. This keeps things like fuses happy

Given that a very adequate DC/DC is maybe $70, it's not that big a deal cost wise. You can get a very fancy one with all sorts of bells and whistles for around $170.

https://shop.pkys.com/Victron-Energy...er_p_6996.html

https://shop.pkys.com/Victron-Energy...er_p_8042.html

Bob
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Old 06-15-2021, 03:18 PM   #8
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Thanks Bob and Bill M.
I think I will tell the tech to not install the diode. For now I'll just pull the charging line fuse in the TV to prevent any problems.
I understand a little better the DC to DC charger and will probably go in that direction once I test my usage needs while I'm on the road. I carry a 2500kw Genset, Solar and don't anticipate a lot of boondocking. I don't think it is a pressing issue for now. I think the 18A Victron would do the job keeping things topped off while on the road.
Interesting class... thanks for the info.
Gene
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Old 06-15-2021, 10:10 PM   #9
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Bob covered the facts well, I was perhaps the first to detect the reverse charging issue here and installed a DC-DC converter back in 2018. I happen to be replacing a previous non-smart version of the Victron Orion, for my specific setup I need the smart model. I could make you a great deal on it.
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Old 06-16-2021, 09:16 AM   #10
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Silver House Dreams.. thanks for the offer, however I think I will opt for the smart model as well when the time comes.

Additional question: My tech advises me that the existing (2020 Expedition) is not adequate and that a separate 6 AWG must be run from the TV battery to the DC to DC charger with a disconnect at the the TV and the TT. Comments?
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Old 06-16-2021, 09:29 AM   #11
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Originally Posted by Gen0 View Post
Silver House Dreams.. thanks for the offer, however I think I will opt for the smart model as well when the time comes.

Additional question: My tech advises me that the existing (2020 Expedition) is not adequate and that a separate 6 AWG must be run from the TV battery to the DC to DC charger with a disconnect at the the TV and the TT. Comments?
Hi

The trick is to pick a DC/DC that will not "overload" whatever wiring you have. If you go for a 30A unit, it will nuke the 7 pin its self. A 20A will be marginal with some TV setups. Something down around 8A should work with any TV that has a rational wiring harness (current wise).

Indeed some of these modern vehicles have all sorts of smarts in them. Those smarts try to manage this and manage that. They look for something (maybe a voltage), don't see it, and don't supply power. That's different than wires that are not adequate. A simple jumper should solve that kind of problem.

If you *do* take out the smarts, be sure to keep something somewhere in the system that still disables power to the trailer when the ignition is off in the truck. If you don't, the truck battery will be flat in the morning .....

Fun !!

Bob
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Old 06-16-2021, 09:30 AM   #12
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I chose the disconnect method, but only after I tried installing the Victron DC to DC converter and could not get it to work properly automatically with TV and TT electronics. I figure I almost always tow my trailer outdoors so I let the solar panels do their thing while I drive. With 200 ah of Lithium batteries Iíve never dropped below 35% charge and we almost never have hook ups so it just didnít seem necessary to use the converter or tow vehicle at all, except to tow!
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Old 06-16-2021, 09:49 AM   #13
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I chose the disconnect method, but only after I tried installing the Victron DC to DC converter and could not get it to work properly automatically with TV and TT electronics. I figure I almost always tow my trailer outdoors so I let the solar panels do their thing while I drive. With 200 ah of Lithium batteries Iíve never dropped below 35% charge and we almost never have hook ups so it just didnít seem necessary to use the converter or tow vehicle at all, except to tow!
Hi

Troubleshooting that sort of problem (which is not uncommon ...) is one reason the "Smart" converter with Bluetooth might be worth the money.

Bob
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Old 06-16-2021, 12:26 PM   #14
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I chose the disconnect method, but a better solution is to disconnect the charge wire where it connects in the 7-way junction box located at the roadside inside corner of the trailer. Then, if you use a different TV you won't accidentally cycle the Lithiums.
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Old 06-16-2021, 02:06 PM   #15
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We installed a Renogy 20amp DC2DV charger. It acts like an isolator when properly set up. Ours has the control wire connected to the trailer running lights and we pulled the fuse that runs 12volts to the 7 way. Charges our lithium and no need to unplug all the time.
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Old 06-16-2021, 02:44 PM   #16
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All diodes conduct in only 1 direction.
I just invented the two-way diode that is disrupting the industry and taking the internet by storm!
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Old 06-16-2021, 03:12 PM   #17
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Quote:
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Bill M. That is my understanding also, that TV voltage drop due to long runs/wire guage issue allows for little useful charging capabilities to TT lithiums. I am still puzzled by how that low voltage into a DC to DC charger mounted in the TT can be transformed into a usable charge. I am thinking like a water hose can easily have a restriction (lowering voltage) but impossible to increase flow from its max capacity. But, who am I? If i knew what I was talking about I probably wouldn't have asked the question. Thanks, for the reply.
Comparing to a hose, think of voltage as pressure and volume as amps. Even if you have 1000 amps if you don't have enough voltage/pressure you can't charge a battery that requires more voltage. Think drop a balloon in a lake it wont fill up no matter how much water there is. Now while you only have say 20 amps you increase the volts you get a charge. Like a using pressure washer you start out with 50-60 psi in the hose and increase it to 1000-4000 psi (depending on the pressure washer) still about the same amount of volume can go through the hose though.
EDIT Better example might be put a balloon on the fresh tank drain it wont fill much water into the balloon, put the balloon on a faucet and turn on the water pump and you can easily fill the balloon.
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Old 06-16-2021, 03:43 PM   #18
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I just invented the two-way diode that is disrupting the industry and taking the internet by storm!
If you haven't patented it then you'll be robbed. Better get busy on that.
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Old 06-16-2021, 07:23 PM   #19
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Just thinking out loud...

If you have both a charge coming from the TV (no matter minimal or max amps) AND you have solar on the roof do they play well together or will the solar charger "read" the voltage coming from the TV and slow down the solar charge??

How about with a dc2dc...
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Old 06-17-2021, 08:23 AM   #20
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Just thinking out loud...

If you have both a charge coming from the TV (no matter minimal or max amps) AND you have solar on the roof do they play well together or will the solar charger "read" the voltage coming from the TV and slow down the solar charge??

How about with a dc2dc...
Hi

If you have a "normal" solar controller and a "fixed output" DC/DC, the solar may well go into float mode ( = stop supplying current). This is more common with lead acid's than with lithiums.

If both the solar controller and the DC/DC are multi stage beasts, it's anybody's guess which one takes over. Indeed one will do so, there's just no way to know which one.

Is this a big deal?

With lead acid batteries, it can be a big deal. The charge current drops and the batteries sit there at 80% charge for a long time. The gotcha is that you only run between 50% and 100% on lead acid batteries. If the charge process hangs up just past "half full" (in terms of usable capacity), that's a fairly big deal.

With lithiums and chargers with a correct profile programmed into them ( which may or may not be the case ) you will get to a pretty high charge state before things get weird.

Fun !!

Bob
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