Charging by towing

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.

kback | December 20, 2013

Tesla definitely recommends against doing this.

wolfpet | December 20, 2013

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

DonS | December 20, 2013

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.

mclary | December 20, 2013

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

kback | December 20, 2013

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. | December 20, 2013

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.

Gluaisrothai | December 20, 2013

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

Chuck Lusin | December 20, 2013

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

Gluaisrothai | December 20, 2013

"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.

wolfpet | December 20, 2013

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.

55steve | December 20, 2013

What about towing behind a motorhome to regen.

Brian H | December 21, 2013

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.

Lessmog | December 21, 2013

I saw one from NL. Also a Leaf.

mjs | December 21, 2013

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?

robert | December 21, 2013


"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.

markopenguin | December 21, 2013

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.


mjs | December 21, 2013

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.

chadrchristensen79 | December 21, 2013

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

Out4aDuck | December 21, 2013

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.

chadrchristensen79 | December 21, 2013

The breaks have nothing to do with it.

chadrchristensen79 | December 21, 2013

I mean brakes.

Out4aDuck | December 21, 2013

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.

mjs | December 21, 2013

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.

Bighorn | December 21, 2013

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.

mjs | December 21, 2013

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.

jkn | December 21, 2013

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.

Bighorn | December 21, 2013

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.

Sudre_ | December 21, 2013

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.

nwdiver93 | December 21, 2013

"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.

mjs | December 21, 2013

@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.

AoneOne | December 21, 2013

The extra power required of the tow truck to charge the battery through regen is just the regen power (30 kW) plus generation losses (guess 15%), or 35 kW. That's less than 50 HP. Putting out an extra 50 HP is certainly a strain on the tow truck, but is it really impossible?

mjs | December 21, 2013

@AoneOne - I agree with the horsepower calculation, but that's on top of the towing itself. At low speeds, the additional 50HP, plus maybe another 25HP to lug around another 4600 pounds, might not be an issue. But as the speed increases, the power required to overcome aerodynamic drag goes up with the cube of the velocity.

Those horses are going to run out of steam pretty quickly.

michael1800 | December 21, 2013

In response to the OP, acceptable by whom? Since that method of recharging is not something that was intended nor tested, Tesla most likely would object. Theoretically, it appears sound. In practice, there may be a case (or a few cases) where it worked, but without serious testing, there could be several situations which result in undesired outcomes. Legally, using a product in unintended ways may free the manufacturer from liability, but they would be foolish to suggest, acknowledge, or approve unintended methods without stringent testing lest they risk assuming liability again.

Out4aDuck | December 21, 2013

Next time someone comes down from the Eisenhower tunnel, let us know what your steady state regen rate is.

Gluaisrothai | December 21, 2013

MJS- Don't confuse speed with power. It would take about 6 kW to tow a MS at 30 mph in neutral. Kick in full regen, and the MS can send up to 60kW into the battery. This power has to come from the towing vehicle (or gravity). Can a tow truck crank out 66 kW (~85hp) to continuously pull the MS in addition to whatever it takes to pull it's own weight? Sure, an F450 could handle this. Efficient? Hell no.

Aero drag at 30mph is pretty low, and given that the MS seems to be able to regen 60kW at 30mph there's no reason to tow it any faster.

Carry on.

mjs | December 21, 2013

I mostly agree with you, with one caveat. In one of Tesla's articles, it stated that regenerative efficiency is about 64% (Roadster), so I would up the 85hp number to about 132hp.

However, the factor which is still unknown is how much energy bleed there is during coasting, at say, 30 mph.

Before you beat me up for that and say that it must be 60kW, consider this:

If I pull the MS at 1 mph coasting, I have to supply X energy to maintain that speed. If I then turn on regen, I will have to add Y more power to maintain that speed. At 1 mph, how much power is Y. I would say that it is likely to be less than 60kW.

At 10 mph, I do the same experiment. In this scenario, I expect Y to be higher than Y in the previous run, but will it max out at 30kW or 60kW? or some other number?

At 200 mph, I do it again. Now I fully expect Y to be the maximum the car can accept.

Somewhere in the regen system, there is a throttling mechanism that governs how much energy to bleed off, if the car is coasting. We all know that to be the case, and that this is under software control, as there are user settings for it.

How do we know the value of Y at 30 mph?

Does the Eisenhower approach slope such that an MS will remain at a constant speed? That would be useful data.

Does anyone know of a road sloped such that it would yield a steady speed of 30 mph? And another at 60 mph?

Brian H | December 22, 2013

KISS. What's happening when you are getting 60 kW regen? You are slowing, hard. That's why you do it. You slow hard for a little while and then are almost stopped. That's what it's for. The recharge rate when being towed at some constant speed would be far lower/slower than that, unless it was a honkin' big truck towing you, which could "override" the regen drag. You would get as much charge as the tow truck could afford to "spare" in its output.

In the MS, you could modulate the drag with the goose pedal. Ease off for full drag, press for less. Depending how long you got that draw, you would have some battery charge to work with. You could then drive a little distance fast, or a longer distance slowly (remember the efficiency curve).

aviationfw | December 22, 2013

Nwdriver93 states earlier Cloudcroft-Alamagordo is a ~6000' drop 6% grade for ~12 miles. I regen @~30kW for ~12 minutes. I would be interested to know the difference between displayed range at the start of the twelve minutes and at the end of the twelve minutes of regen. How many extra miles did the car show he had gained in range in this time.
I wonder how much strain the tow rope would put on the MS chasis when towed in regen. As essentially this would be the same as towing an traditional car with the rear brkes partially applied.
On a realistic note my Mercedes ML320 has had in th past its front brakes partially applied due to 2 stuck brake calipers. As the disc heated and expanded more brake was applied not to dissimilair to deceleration by regen on the MS . When I took my foot off the accelerate the car would come to a quick stop due to the stuck brakes. I have driven the Mercedes car at 40 miles per hour to get home like this and it certInly was drivable with no excess overheating but no extra power to get above 40 mph. The deceleration was similar but not as severe as regen but my guess is that with the extra power of a disel tow truck it could certainly manage to pull the MS in regen based on my experience with stuck brakes under my own power. My concern would be the twisting or torque on the MS chasis by pulling by a rope. Anyone with a stick shift would appreciate this twisting load when trying to bump start an ICE with a rope tow in low gear. As the MS is aluminium with the main support comming from the battery pack i presume if there is a tow point on the MS the torque from the tow is linked via the main battery pack to the rear wheels as the physical main battery compartment supplies the strength to the MS along the length of the car. Much as I like the tow regen idea, i would be concerned that i might twist or stretch some metal work either aluminium or within the battery by tuging from the front of the car to turn the wheels at the back of the car. Applying D at tow speed would certainly apply quiet a Tug on the chasis.

muleferg | December 22, 2013

Read the Manual. It say.

Use a Flatbed Only

Gluaisrothai | December 22, 2013

Tensile load on a tow rope @ 30 mph and 60kW is:

30mph= 13.4 m/s

W= fxd
P = fxd/t
60000 = f x 13.4

f = 60000/13
f= 4447 N or approximately 1000lbs force.

jkn | December 22, 2013

I agree with klevins. But constant load is not a problem. Sometimes towing car has to stop because of traffic. Towed car cannot usually stop at the same moment, so it comes closer. When there is break in traffic, driver of the towing car wants to get moving quickly. Result is jerk in the rope. (Done that.) Having regen on reduces this risk. Let the towed car do all braking.

Of course Tesla does not recommend this. Risk of collision is rather high. I have only towed an old car with another old car.

Jewsh | December 22, 2013

This is (as others have mentioned) *very* not recommended.

Thankfully no one was hurt.

jonerickson1 | December 23, 2013

Assuming .3 kwh/mi * 30 mi/h = 9Kw

The best I can think of is the Mythbusters episode @ 55mph the car got 39% better fuel economy. @30 mph there wouldn't be as much to gain but there is a significant gain.

say it needed 44kw @ 68% efficiency plus losses and 9kw of drag from the car (assuming no drafting).

That would get you 30Kw of regen for an additional 71hp from the truck.

I am not a Model S owner so I am wondering why does the driver have to be in the car? Can you just leave the key fob in the car? Is there a weight sensor? Could that be easy fixed by something sitting on the seat?

jkn | December 23, 2013

Max legal towing speed is about 37 mph. So risk is mainly to cars. I have newer damaged cars or rope by towing. Towing is allowed in Sweden, so it is not that dangerous. There are some issues. So don't even try, if you don't know what you are doing.

mjs | December 24, 2013

@klevins -
Who is delivering the 4447 newtons?

The biggest engine I could find on the Ford Super Duty is the 6.7L V8, which delivers 1100NM of torque.
Let's assume this beast has 22" wheels, for a moment arm of about .28 meters.
1100NM / .28 = 3928 newtons.

So even if it has the horsepower, it still doesn't have the torque to deliver enough force to charge at max.

Gluaisrothai | December 24, 2013

@ MJS- does your Ford Super Duty not have a gearbox or differential between the engine and wheels?

Taking your number for torque- 1100NM.

Let's conservatively assume the tow truck is in 4th gear, which according to Ford has a ratio of 1.14:1

- and that the rear end is configured for towing with a ratio of 4.3:1

- and has 30" wheels (including tire height)

F = T x R/r

F= 1100 x (4.3/1.14)/.38

F = 10918 N

Which looks like plenty, even in fourth gear.

Gluaisrothai | December 24, 2013

Correction- should be

F= 1100 x (4.31 x 1.14)/.38

- 14190 N.

I initially thought fourth was an overdrive- but it is not. So there is a significant margin for drawbar pull in this instance.

Also - as the S can modulate regen, in a situation where drawbar force is limited but adequate power is available, it is possibly to allow the S to run faster for the same kW regen. Think about that one.


mjs | December 24, 2013

Klevins - Thanks for that explanation.

It looks like even with a loss factor, a truck of that size could do it. Maybe not so for an average garden variety passenger vehicle, but I see your point.

Car t man | December 24, 2013

You can easily use regen during towing for charging the battery.
The flatbed thing is stated to prevent unexperienced drivers
to case harm. If you know what you are doing and the car
is operational, but empty, it is OK. Start off slow though.
When the battery is completely empty and almost full, it
heats up a bit more under heavy current than in between
those two extremes.

Car t man | December 24, 2013

Oh, and don't do it if the battery is below 0C. Even if below 5C (temperature), only do it very slowly in beginning. For about a mile, only up to about 15 mph, then 30 for another mile and then you can go faster. Don't charge the battery, especially with any meaningful current when cold and at all when under 0C. Then it really is better to flatbed it or really start really slowly and only go really slow for a while.