Charging Travel Trailer Battery from Tesla

[ohmman]

The Model X has a 7-pin trailer connector, and one of those pins (#4) supplies +12V which, if connected properly, can charge the trailer's battery during travel. In an ICE, this power is supplied by the alternator and vehicle's 12V battery. On the Tesla, I'm assuming it's provided by the 12V battery and that battery's charger from the vehicle battery.

This got me thinking. If I'm boondocking/dry camping with a travel trailer, and I have significant energy available in my vehicle battery, but limited availability in my travel trailer, can I just draw off of that pin 4 as needed to charge the travel trailer battery? My research shows that this is done with ICE vehicles from time to time. One important step is to size up the conductor between that pin and the travel trailer's battery to #10 - this limits resistive losses along the length. But it raises a few questions.

1. Is that pin hot when the Tesla is shut down and locked? That is, does it work like an accessory switched outlet would work on an ICE, or is it always on?
2. Will the Tesla lead-acid 12V battery deep cycle too often and become even more likely to be replaced?
3. Does indeed the 12V system get charged even when the Tesla is "off"?

I can probably answer the first question with my multimeter here shortly. I am curious about this situation, though, because if one can get a high SOC before finding a nice dispersed/dry camping spot, it could really take away any need for solar or other charging methods while camped. I can't imagine the draw from the trailer would impact the SOC very much on the Tesla, considering the size of most trailer batteries. Mine will have a single 80Ah 12V lead-acid battery, which only supplies 50% before needing to be recharged. That's less than half a kWh.

I'd love thoughts on this and explanations why I might be missing something crucial. I recognize limitations in getting to and from dispersed camping sites, and not having a charge available while parked. The question is more about the technical feasibility of this configuration.

 

[ohmman]

Quick follow-up: Trying to answer my question #1 above was more difficult than I expected. Attempting to hold the 7-pin door open while measuring voltage at the pins proved to be impossible. I'll need to measure it from the trailer once it's connected. If anyone else has the info, I'd love to hear it.

 

[vandacca]

While not the best idea, there is also a 12V adapter in the trunk that could be tapped. I also remember reading about a fuse-box hack to keep the 12V adapter alive (not sure if it will work on X too).

 

[BerTX]

So are you able to charge the trailer battery from the 12v in the plug when it's plugged in? I would have thought that circuit would be pretty low amperage. I suspect they are trying to minimize load on the dc-dc converter (since they don't want inverters to be run from the 12v outlets, either).

 

[ecarfan]

The Model X has a 7-pin trailer connector, and one of those pins (#4) supplies +12V which, if connected properly, can charge the trailer's battery during travel.

Like @BerTX I wonder just how many amps that pin would supply.

Is that pin hot when the Tesla is shut down and locked?

I wonder that as well. My guess would be it is not active so as to protect the Tesla 12V battery.

Will the Tesla lead-acid 12V battery deep cycle too often and become even more likely to be replaced?

That seems like a serious concern, but not one I can answer with even a modicum of authority.

 

[ohmman]

So are you able to charge the trailer battery from the 12v in the plug when it's plugged in? I would have thought that circuit would be pretty low amperage. I suspect they are trying to minimize load on the dc-dc converter (since they don't want inverters to be run from the 12v outlets, either).

I think you're asking @vandacca about the 12V outlet. If you're asking about the 7-pin connector, I take delivery of my trailer next week - they were doing a few minor customizations - so I cannot confirm how it works with the Model X. However, Airstreams (and indeed, it seems many/most trailers) are set up to take a trickle charge from that 12V pin. I've seen a range from as low as 6A to as high as 12A coming through that pin to the trailer. It's not much, but if there's a way to keep it on, it's more than enough to keep a trailer charged indefinitely so long as the X is still connected to it.

Another detail I'll need to work out is how the charge is managed in order to maintain the health of the trailer's lead-acid battery. If it's a constant charge, that's detrimental. It'll have to be managed somehow.

I do feel like this concept has some legs, though. I have an enormous battery supply available to me in the towing vehicle. Why upgrade my trailer battery unnecessarily?

 

[BerTX]

I think you're asking @vandacca about the 12V outlet. If you're asking about the 7-pin connector, I take delivery of my trailer next week - they were doing a few minor customizations - so I cannot confirm how it works with the Model X. However, Airstreams (and indeed, it seems many/most trailers) are set up to take a trickle charge from that 12V pin. I've seen a range from as low as 6A to as high as 12A coming through that pin to the trailer. It's not much, but if there's a way to keep it on, it's more than enough to keep a trailer charged indefinitely so long as the X is still connected to it.

Another detail I'll need to work out is how the charge is managed in order to maintain the health of the trailer's lead-acid battery. If it's a constant charge, that's detrimental. It'll have to be managed somehow.

I do feel like this concept has some legs, though. I have an enormous battery supply available to me in the towing vehicle. Why upgrade my trailer battery unnecessarily?

I didn't catch that you didn't have the trailer yet.

I think it's a great idea, definitely don't want to haul around extra lead-acid batteries if you can help it. I'd be a little concerned, again, about the load on the converter and, as above, the 12v battery longevity. I suspect that they may have engineered to closer tolerances than we might be used to in the past -- no longer do they double the capacity of everything because, heck, its only weight and a little more gasoline to power it. These guys want to be in control of every system, and throwing extra, unanticipated load on the system might have unexpected results.

Obviously there are 12v accessories that are designed to be used while the car is stationary. If you have those systems off, maybe there is not much to worry about.

Besides, it's still under warranty...

 

[BerTX]

Also, you can get a 7-pin pigtail plug to do your testing until the trailer arrives. I would monitor the voltage on that pin. It probably needs to be 14+ volts to charge the battery. Even if you don't get that much, what you are trying to do, perhaps, is just to maintain the charge, so start with a charged battery and put a load on it.

 

[ohmman]

Also, you can get a 7-pin pigtail plug to do your testing until the trailer arrives. I would monitor the voltage on that pin. It probably needs to be 14+ volts to charge the battery. Even if you don't get that much, what you are trying to do, perhaps, is just to maintain the charge, so start with a charged battery and put a load on it.

Right, I may do just that. I'm only 6 days away from having the trailer, and we have family in town for the holidays, so I also may not.

I'm aware I'll need 14V+ at the pin. Most vehicles deliver that at traveling RPM. I don't know what it looks like from the Tesla. Maybe @JimVandegriff can chime in on how the X charged his Airstream on the recent trip?

 

[Saghost]

I'm not 100% certain of any of your answers, but my understanding is that the computers monitor the 12V voltage (continuously? at frequent intervals?) and will turn on the DC-DC converter to recharge the 12V when needed even with the car off - one of the big philosophy differences between Tesla and GM EV designs.

GM cars (mainly the Volt, but I'm pretty sure they adopted the same approach across the board) are off when they are off, with the high voltage contactors open unless the TMS needs to run the HVAC - no vampire drain, but they have stranded people because the 12V battery died before (in 2014 they introduced a 12V battery tender mode when the car is plugged in for 2014 and newer cars only.)

Tesla seems to leave the car enabled even when set to sleep to some extent - it's drawing power through the DC-DC converter for TMS and computer functions that causes the vampire drain as I understand it. That should mean that a modest draw for charging the house battery would just increase your vampire drain, as long as it doesn't draw the 12V down too far between voltage check intervals, whatever those are. The 10 gauge will of course be a limit to the power you can draw.

 

[ecarfan]

However, Airstreams (and indeed, it seems many/most trailers) are set up to take a trickle charge from that 12V pin. I've seen a range from as low as 6A to as high as 12A coming through that pin to the trailer. It's not much, but if there's a way to keep it on, it's more than enough to keep a trailer charged indefinitely so long as the X is still connected to it.

That is very interesting. If there is a way to keep that pin live when the X is not on, and if the X 12V battery is not adversely impacted by currently continuously flowing through it over extended periods, and if the Tesla BMS will properly maintain the X 12V battery under those circumstances, it seems like a useful way to keep the trailer 12V battery charged while using it when one is away from an electrical hookup for the trailer.

That is a lot of "ifs", but I applaud your efforts to determine the feasibility of such an approach! It sounds like you are going to have to deal with Tesla directly for the answers to most of your questions.

Since the 12V outlets inside the S and X are turned off when the car is off it seems unlikely that pin #4 in the trailer connector would remain live. However it seems possible that Tesla might consider in the future making it live and managing any 12V car battery issues that might arise from keeping pin #4 live for extended periods.

 

[ecarfan]

Maybe @JimVandegriff can chime in on how the X charged his Airstream on the recent trip?

Are you saying that his X did indeed charge his Airstream battery while driving?

 

[ohmman]

Are you saying that his X did indeed charge his Airstream battery while driving?

No. I'm saying I hope Jim chimes in. I would assume that it did charge while driving, though - I can't see why Tesla's implementation of the 7-pin would be different from other manufacturers'. Truthfully, I just need to get that pigtail @BerTX mentioned.. it's going to drive me nuts for the next few days thinking about this. I have a bit of a problem when it comes to this type of thing, but I think everyone here probably already knows that.

 

[ohmman]

An update. I exchanged messages with an RV energy professional and was encouraged by the response. He directed me away from my initial thought, which is to use the 7-pin. He finds it "woefully inadequate" for the purpose. Instead, he suggests going straight to the 12V source on the Tesla and running the entire trailer from that. It's an interesting concept, and would definitely require some data on the robustness of the DC-DC converter in the Model X. The suggestion is to use 2AWG or 1/0 or 2/0AWG for the connection to the trailer. Connections would be done with something like Anderson Powerpole connectors on the Tesla. He believes one could run the AC or microwave if desired, but I would probably omit those from my plan.

This setup would not charge the 12V battery in the trailer, but it would be charged enough from the 7-pin and driving to cover emergency braking, etc.

It's certainly not as seamless as I'd like, since the 7-pin is already there, but it's an interesting concept. It would truly be the ultimate boondocking setup, and would be a great example of one of the benefits to an EV/trailer combination that helps to offset the range issue.

Tesla would need to chat with me about the converter. Can anyone else think of failure points in this concept?

 

[ecarfan]

he suggests going straight to the 12V source on the Tesla and running the entire trailer from that.

Wow. If that could be safely and reliably implemented, the ICE RV community is going to look at Teslas with new found respect!

Tesla would need to chat with me about the converter.

I suggest tweeting Elon and asking him. We need an authoritative answer!

 

[ohmman]

It's possible @Ingineer may have some input here. Maybe that tag will get him to check out the concept and weigh in...

 

[McRat]

From a person who has towed heavy for hundreds of thousands of miles. Diesel pickup & Motorhomes, Gas Motorhomes, Travel Trailers, Race Trailers, Toy Boxes.

A normal 7-pin electric brake trailer plug has an aux power lead, mine are fused at 30 amp and driven via relay off the ACC circuit. ie - they are not 'hot' unless the ign or ACC is on. 10 ga min wire, waterproof connections.

Another of my vehicles uses a battery isolator and has the AUX Trailer fed right from the battery via fuse. Always hot. Not recommended unless you have a specific need.

Most vehicles made for towing have the 30 amp ACC provision BUT the fuse is not installed from the factory. You must put the fuse in to activate it. See your service manual.

Do not use an EV to replace a trailer generator for running AC/Heater blower/Microwave/Coffee Maker/Fridge/Shower, etc. You WILL be sorry.
Expect up to a 10kW draw so you will be stranded after 8 hours.

Your travel trailer needs a generator unless you are using it as a tent. If you want TV, microwave, AC. running water, lights, etc, you need a dedicated generator and 2 12vdc deep cycle batteries. The tow vehicle is just to support short intervals when you are not allowed to run generators, and to replenish the 2 deep cycles when you wake up.

If you only park at RV hookup locations, nevermind. Don't even bother with the ACC 30a supply. My lifestyle has required remote locations like Bonneville and other isolated areas.

 

[ohmman]

Do not use an EV to replace a trailer generator for running AC/Heater blower/Microwave/Coffee Maker/Fridge/Shower, etc. You WILL be sorry.
Expect up to a 10kW draw so you will be stranded after 8 hours.

Your travel trailer needs a generator unless you are using it as a tent. If you want TV, microwave, AC. running water, lights, etc, you need a dedicated generator and 2 12vdc deep cycle batteries.

Thanks for your participation, @McRat. I have to take issue with your power estimate. 10kW average draw over 8 hours?! My 2100 square foot house plus guesthouse plus swimming pool averages 25kWh over 24 hours. That includes electric oven, microwave, espresso machine that's regularly on, toasters, etc.

For some people, I could possibly imagine a 10kW peak draw. Maybe for a big camper with three AC units, dual ovens, four hair dryers, and a toaster for every slice of bread. But I still can't see averaging more than 20kWh over 24 hours unless one is being tremendously inattentive to energy use. Also, for the type of trailer that the X can haul, 10kW sustained draw would melt the entire thing to the ground.

The Airstream boondockers who participate online have numbers like 10A-30A average draw during waking or active hours. That's a small fraction of time when we're camping, since typically being out and about is our goal. At the upper bound, that's 360W, and a far cry from 10kW. Most can use one or two 100W solar panels to supply all of their needs (they exclude AC and microwave usage because of peak draw).

I think there are probably a few things at work here. First, you likely camp in bigger, more energy intensive rigs, and spend a lot more time inside of the camper. Second, you were thinking about peak consumption instead of average consumption (and in my opinion, vastly overestimating it).

The battery pack on a Tesla has more than enough energy to keep a small trailer boondocked longer than the tanks will support. The concern I have is whether the 12V system gets fried while boondocking, which would render the car temporarily useless. For me, that's the more pressing issue.

 

[McRat]

Thanks for your participation, @McRat. I have to take issue with your power estimate. 10kW average draw over 8 hours?! My 2100 square foot house plus guesthouse plus swimming pool averages 25kWh over 24 hours. That includes electric oven, microwave, espresso machine that's regularly on, toasters, etc.

For some people, I could possibly imagine a 10kW peak draw. Maybe for a big camper with three AC units, dual ovens, four hair dryers, and a toaster for every slice of bread. But I still can't see averaging more than 20kWh over 24 hours unless one is being tremendously inattentive to energy use. Also, for the type of trailer that the X can haul, 10kW sustained draw would melt the entire thing to the ground.

The Airstream boondockers who participate online have numbers like 10A-30A average draw during waking or active hours. That's a small fraction of time when we're camping, since typically being out and about is our goal. At the upper bound, that's 360W, and a far cry from 10kW. Most can use one or two 100W solar panels to supply all of their needs (they exclude AC and microwave usage because of peak draw).

I think there are probably a few things at work here. First, you likely camp in bigger, more energy intensive rigs, and spend a lot more time inside of the camper. Second, you were thinking about peak consumption instead of average consumption (and in my opinion, vastly overestimating it).

The battery pack on a Tesla has more than enough energy to keep a small trailer boondocked longer than the tanks will support. The concern I have is whether the 12V system gets fried while boondocking, which would render the car temporarily useless. For me, that's the more pressing issue.

Yeah, 10kW is the size of generator I like to run. And yes, to keep the voltage above 105vac to avoid damaging things, the generator will run pretty hard if you have AC.

Pessimism on my part, but just wait until the lights dim at 4 am, the heater turns off, the Carbon Monoxide alarm is blaring, and it's 10°F outside, the kids are crying and Dad better do something quick.

When you are in a remote area, the last thing you want is fall short of is fuel, be it diesel. gas, or electric. Plan for the worst, wish for the best.

Nobody I know has ever said, "Crap, I had too much reserve power!" however, the opposite is experienced by most remote campers at some stage in their lives.

In many ways, a tent and matches is really a better solution. It's what many RV people end up returning to in the end. But a real stud, brings a flint, a knife, some rope, a compass, and 3 layers of clothing.

 

[S'toon]

Those folks who traveled across Canada in their X and a trailer did the opposite. Figured out a way to charge their X from the batteries in their trailer. They might have some suggestions. fortytwo is their name in these forums.