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Charging by towing

I know at least one case when an owner successfully charged his S85 by towing it in a Drive mode. The owner reports that in about 20 minutes of dragging the car he recovered about 30 miles, which allowed him to successfully return home. This was an emergency of course. I wonder if this an acceptable way to charge the car from the technical perspective. How does the car distinguish this from, say, going downhill? Another question is regarding warranty. Do you think Tesla would object this kind of stunt?

P.S. I'm aware of a similar thread on TMC. It's pretty old and there was some guessing but nothing concrete.

Tesla definitely recommends against doing this.

@kback Do you have any link that supports this? I searched through the user manual and found nothing against towing in D mode.

I can't see that the S drive train would care, but I doubt it could be done with a safe or legal tow hookup. If the example above actually had tow truck or tow dolly, it would make more sense to tow it to a charger.

I suspect there is nothing in the manual about towing in drive because it is illegal to have people in a towed vehicle. The only exception I know of is off-road where the speeds are very slow.

Hopefully this will void the warranty so that other don't try it.

From the manual: "Use a flatbed trailer only, unless otherwise specified by Tesla. Do not transport Model S with the tires directly on the ground."

I read this as saying do not tow the Tesla with wheels on the ground. That would include towing to charge the battery. I have seen this mentioned in the forum before and Tesla said they do not recommend doing this. I cannot find that reference now.

It seems like towing to charge would work if a driver was in the car and the car is working other than low charge. No idea about the legal issues of trying this.

I can think of a number of reasons why Tesla does not recommend towing while the wheels are on the ground:

1) If a bearing was frozen, running the drive train will likely cause more damage or could cause extreme tire wear.

2) Without the car turned on and a driver in the seat, the HVAC is not running to cool the battery and motor. Forcing the motor to turn could overheat and cause significant damage to the motor.

3) A less likely concern is overcharging the battery. Then again if the car is being towed for some electrical problems, overcharging the battery might occur (which is really bad for the battery life).

Rather than try to explain some situations that are very bad, and others that might be Ok to tow and charge, Tesla has taken the safe route and recommends no towing with the wheels on the ground turning.

In physics terms, towing to charge the battery is no different than driving down a steep hill using regen.

This seems like a inefficient method of charging. Why no just go all the way, or to the closest charge point?

"This seems like a inefficient method of charging. Why no just go all the way, or to the closest charge point?"

Depends on where you're stuck. If you're in the boonies with a tractor and a tank of diesel (and a rope and some friends), but only 115Vac, then I imagine that 30 minutes of towing @30kW regen might buy you 50 badly needed miles of conservative driving.

Thanks everyone for your comments so far. Just a couple additional points.

* Towing with a person inside is legal in the country where the incident took place. But yes, legality is a good point. It would be illegal anywhere in North America AFAIK, at least on public roads.
* Yes, I would definitely not recommend charging this way if the car was having any problems. In this case it was simply out of juice.
* It is a very inefficient way to charge for sure. My question is whether it is acceptable (both technically and warranty-wise) if there is absolutely no other option.

What about towing behind a motorhome to regen.

Once saw a Russian video of a Leaf owner being rope-towed behind a friend's pickup to recharge. He was delighted it worked so well.

I saw one from NL. Also a Leaf.

uh, no. Even if this is possible, those results are not.

Let's (fantastically) say for a moment that for every mile you roll the car, you could get a mile back driving. Assuming flat windless conditions, that is going to be the upper bound, considering the flat torque curve.

In order to obtain 30 miles of range, one would have to pull his car 30 miles. To do that in 20 minutes, one would have to drive his tow truck at 90 mph. If one were to allow for a 20% loss in energy transfer, said tow truck would have to average about 112 miles per hour.

Let's also not forget that the charging car is not free rolling, and thus "consuming" energy at the expense of the tow truck. To regain 30 miles of range, the MS will need about 10kWh of energy. That energy must come from the ICE truck.

A gallon of gas has about 38 kWh of energy, and ICES run at about 20% efficiency (or less due to the wind resistance at the near relativistic speeds required to charge an MS). So, to get 10kWh of energy, it will require about 1.3 gallons of fuel.

Assuming a modern tow truck can achieve 15 MPG while towing a car that is not charging, then in 20 minutes, even at 60 mph, it burns approximately 1.3 gallons of fuel as well.

Combining these facts, the engine output of the tow truck would need to be double that of an ordinary tow, for a "charging tow," compared to a regular tow.

Does anyone believe that a tow truck, traveling at say, 112 miles per hour, with a full load, has an additional 100% power reserve to allow you to turn on charging?

@DonS

"I suspect there is nothing in the manual about towing in drive because it is illegal to have people in a towed vehicle."

How can it be illegal to have a driver in a rope-towed vehicle? It would be illegal NOT to have one.

I have towed several cars that have been in trouble, and I always keep a towing rope in my car, to help or be helped. It is much better to be towed away to the nearest repair point, or gas station, than to leave a car behind on the road.

You wouldn't need to tow it necessarily a single mile to get a mile of charge because the rate of charge (60kW) is greater than the amount of energy required to drive that distance under a typical circumstance. This does not violate law of conservation of energy because it will take more energy to tow the vehicle with full charge going on than it would just to tow it naturally.

Mark

With a fixed gear ratio, you won't be able to do better than 1:1. Imagine hand cranking a motor to store "cranks" in a battery. You won't get more cranks out than you put in.

Assuming flat terrain, the energy you could generate (and the rate at which you could generate it) can not exceed the energy that it take would take to drive that distance. If you could, then you would use just what is needed to drive, store off the rest, and drive forever.

Mark is right, regen would charge at a rate
Of three to one at 60kwh, granted the trucks
Mileage would be horrible!

Does the rate of regen increase when the brake is applied? If so, you could charge at a rate of more than one mile per mile. On the other hand, regen charging is far from 100% efficient. I'm thinking no better than 70%. Anyone know for sure?

Regarding towing for charging, I would not recommend it for others, but I would do it myself.

The breaks have nothing to do with it.

I mean brakes.

Regarding towing behind an RV, there are two problems with it. First, the car would have to be on and in Drive, which implies that there would have to be someone in the driver's seat. Secondly, it's brutally inefficient as mjs's calculations show. Bottom line, charge up when you get to the RV park.

I'm not sure where the idea of a continuous regen of 30kW or 60kW came from, but it doesn't make sense either.

The MS uses about .3 kWh/mile, so a charging rate of 30kW would deliver about 100 miles per hour of range. So, turning this around (and admittedly waving my hands a little, as the regen rate will vary according to the voltage differential between the motor and the charging batteries), you'd have to pull your MS at 100 miles per hour to acquire energy at 30kW. Or, 200 mph to generate 60kW.

And there's still a matter of a 20% (at least) energy loss, since you're doing a round trip to/from the batteries on this experiment, rather than the one-way trip from energy already in the batteries.

Still, the towing vehicle has to pay for that power, as well as the power to move itself.

I'm not buying this, except for at very low speeds, and for negligible gains.

@mjs
Your speeds (100-200 MPH) to achieve regen rates of 30 or 60 kW is suspect. One routinely sees regen rates max out when lifting off the accelerator at much slower speeds. One needn't tow a car 100 miles to get 100 miles worth of charge--I think this is where people are getting confused.

Regen does go higher for brief periods, but that is when the car is decelerating, and dumping all of that kinetic energy back into the battery.

In the towing scenario we're talking about a steady state.

To be honest, I don't know what the charging rate would be, as the battery load can be adjusted for a variety of charging rates, and I don't know how much the car would choose to "bleed off" at that speed.

But, what I am saying is that the additional load on the towing vehicle would need to be so great, that it is not practical for more than a couple of miles of range, at best, before taxing the towing vehicle excessively.

Here towing with rope (3 m - 6 m) is legal with max 60 km/h. No passengers allowed in towed car. Driver is of course necessary. Towed car should not be heavier than towing car. I believe MS can use 300 kW to accelerate at speed of 60 km/h. So charging with 60 kW should be easy. Charging would generate more heat than using 60 kW for driving. If road is not slippery, charging while towed would increase safety.

I agree with klevins: In physics terms, towing to charge the battery is no different than driving down a steep hill using regen.

Cars power-train does not know where force comes from. From gravity or from rope.

@mjs
I agree that the tow vehicle will be investing a lot of energy if in fact countering 30-60 kW of regen, but the speeds would not be jail-inducing.

mjs I don't know where you are getting any of your numbers from.

Lets take the car out of it. Let say I have a generator that creates 60kWh of power when I hand crank the handle at 30 miles per hour (whatever number of revolutions that takes). If I take the crank off the generator and put a tire on it and drag the generator down the road at 30mph (same number of revolutions) then it will still create 60kWh of power.

Let look at it another way. I am driving down the highway at 60mph. I let off the accelerator and regen at 60kWh for 30 seconds. I then put it in neutral and another car pushes me back up to 60mph. Then I put it in drive and let the car regen at 60kWh again. Sooner or later repeating the process I will fully charge my battery at 60kWh plus whatever time it takes the other car to push me up to speed each time.
You can take the whole pushing up to speed thing out of the equation and just have the car push or pull you and regen the 60kWh. It's not that complicated. It would use a lot of torque from the tow vehicle and I would be worried about burning out the clutch or transition on the tow vehicle.

In general I would not recommend this and I am sure Tesla would not warranty any damage caused by it. They are very specific about flatbed towing.

It is no different than driving up a mountain and then regen all the way down the mountain at a constant speed. Anyone can test this theory. Find a steep tall hill. Stop slightly down the hill. Let off the brake and let the car start coasting down the hill. You will see it start regening more and more the faster the car goes. At some speed the car will hit 60kWh of regen. That is no different than someone pushing/pulling you on level ground or up hill... well except for the tremendous amount of gas it would take from the other vehicle.

"I don't know what the charging rate would be"... do you own an MS? It's 60kW... says right there on the dash.

No difference between being towed and rolling down a hill. Cloudcroft-Alamagordo is a ~6000' drop 6% grade for ~12 miles. I regen @~30kW for ~12 minutes. Absolutely no difference between that and being towed. Two main risks for towing to Regen would be a collision or tow line breaking.

@nwdiver - yes I do own one. P85.
We do not know what the charging rate would be when towed at say, 60 mph. The charging rate is not constant at 60kW. It is limited by the energy input, among other things. I have no doubt that you can charge at 30kW at 6% grade. We don't know how aggressively the MS will attempt to bleed off the energy when the towing is at constant speed.

Now all you need is a supercharger at the top :)

@Sudre - Your example is correct, except that on flat terrain, you cannot regen at 60kWh for 30 seconds. I am not arguing that you couldn't do this (if the car in fact doesn't attempt to limit it), but that the power required to make it happen is so much larger than can be practically delivered, that except in the case of charging a couple of miles, it is useless.

Continuing your example, I suppose a tow truck could bring the MS+truck to 60mph in about 15 seconds (pure guess), then the MS could bleed off most of the the energy in about 5-6 seconds. That 1:4 ratio of the duty cycle shows that the tow truck is working 4x the charge level to make this happen.

With regard to my previous numbers, I made up the 15 MPG number, but the rest are real life numbers. Not sure which ones you question, or which equations you think are incorrect.


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