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Towing a Tesla at 70 MPH replenishes battery at fast charger rates

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Re: Towing a Tesla at 70 MPH replenishes battery at fast charger rates

#371

Post by mint2 »

asdff wrote: post #127
lapetitejort wrote: post #12

Regenerative braking is so neat. I drove up a steep windy road to go on a hike with my plugin hybrid. On the way down I tried to let gravity do all of the acceleration and regenerative braking do all of the deceleration. At the bottom of the hill I had racked up +3 miles in total. Another fringe benefit is that brake pads wear out slower and produce less pollution.

You can get this sort of benefit with a gas car with a manual transmission too. Engine braking uses no gas, its just your wheels turning your pistons and the friction from all of that slows the car down. Most of the time I only hit the brakes to go from 5mph-0mph in a manual car thanks to this.

It’s not quite the same, once you’re down to level ground with stop lights, or stopped engine breaking doesn’t let you drive 3-5 miles for free or get back up to 25mph for “free” if it was just a stop light. Quotation due to the penalty of accelerating a heavier car or lugging it up hill.

Re: Towing a Tesla at 70 MPH replenishes battery at fast charger rates

#372

Post by rootusrootus »

throwaway0a5e wrote: post #366
rootusrootus wrote: post #253

Earlier quoted context omitted.

I'm surprised it was that good. I get 8 MPG from my F250 when towing a travel trailer that doesn't have the brakes engaged.

Your travel trailer makes up for it by punching a very big hole in the air.

Yeah, I liken it to what it would be like towing an open parachute behind you.

I'm disappointed at the terrible efficiency we're getting from this rig, to be honest. Based on experience I was expecting more like 10mpg. It doesn't make a big difference in cost, but Ford only gave the 6.2L F250 a 30 gallon tank. For something that gets such abysmal gas mileage, that is a really questionable decision. Our last truck, an F150, had a bigger tank even with a shorter bed. I have no idea what Ford was thinking here.

Re: Towing a Tesla at 70 MPH replenishes battery at fast charger rates

#373

Post by dahfizz »

throwaway0a5e wrote: post #364
dahfizz wrote: post #361

Earlier quoted context omitted.

The SAE J2807 tow grade test is done on a real highway in Arizona, not some made up crazy scenario. If you can only tow a certain load in ideal circumstances (no hills, no sudden braking, no wind, etc) then you actually cannot safely tow that load. The real world has hills and the occasional need to slam on your brakes.

>The SAE J2807 tow grade test is done on a real highway in Arizona, not some made up crazy scenario. So? That doesn't make it representative of typical conditions. >If you can only tow a certain load in ideal circumstances (no hills, no sudden braking, no wind, etc) then you actually cannot safely tow that load. So someone in Kansas or Florida should be limited by grades that aren't even with a day's drive of them? D…

> So someone in Kansas or Florida should be limited by grades that aren't even with a day's drive of them? Do you seriously believe this?

I never said that. I think the current tow rating system does represent the safety margins needed in real world towing. If a truck can't tow X pounds up an actual hill on an actual US highway, then it shouldn't advertise being able to tow X pounds.

But, if you live in Kansas and you are sure that your brakes can handle a higher load, that's fine. As you said, it's up to you the driver to know what's safe and not. I also think you have it coming if you lose control of an overloaded truck and your insurance company refuses to cover damages, etc.

I view it similar to overclocking a server. Servers are clocked somewhat conservatively to ensure reliability. You are free to overclock a machine you own, but you run the risk of instability and you cannot complain to the manufacturer if you have downtime resulting from running your machine outside of spec.

Re: Towing a Tesla at 70 MPH replenishes battery at fast charger rates

#375

Post by gorkish »

elihu wrote: post #325
matt-attack wrote: post #314

Earlier quoted context omitted.

The weight of the Tesla is moot. The majority of the load experienced by the pulling vehicle is in the incredible drag of the Tesla’s wheels. They’re extremely difficult to spin and this for pace far exceeds any others that would normally be encountered when pulling a traditional load.

Well, at 70 mph the load would be a lot. If the car were towed at 25 mph it wouldn't be as difficult, and I assume most EVs let you configure the regen amount. If you set it to light regen it'll be easier to tow, but charging will take longer. Having at least some regen braking all the time seems like kind of a benefit, since there's less risk of the rear car hitting the front car if the front car stops abruptly and…

The cool thing about EVs is that they have an instantaneous power meter in them, so there really is no need to speculate.

The following numbers roughly correspond to my 2017 Model S: At 30mph the car consumes approximately 200Wh/mi (6kW or 8hp) and at 70mph approximately 300Wh/mi (21kW / 28hp). This is the power required to maintain these velocities against all external factors, and therefore is exactly equal to the power required to tow the vehicle at these speeds.

If you have regen set to normal, the car will supply up to 50kW / 67hp through regen. TBH I don't know exactly what the minimum speed to achieve full 50kW is, but I am reasonably confident that the car will do it at 30mph since the tires have more than 10 times the required grip to do it.

Towing at 30mph = 56kW / 75hp, 89% charging efficiency, equivalent to a ~2000lb trailer Towing at 70mph = 71kW / 95hp, 70% charging efficiency, equivalent to a ~2500lb trailer

The trailer weight estimates are roughly basaed average of engine power / towing capacity figures for medium duty pickups. The tow vehicle to do this safely in my estimation should have a 250hp power plant or larger.

The biggest flaw with the linked article is that it's not necessary to go 70mph to achieve the maximum charge rate from regen, and indeed it is less efficient to do so.

Re: Towing a Tesla at 70 MPH replenishes battery at fast charger rates

#376

Post by recursive »

worldsayshi wrote: post #331
recursive wrote: post #171

Earlier quoted context omitted.

How often will someone be driving slower than the wind? Almost never.

You can sail faster than the wind when you push against a medium though. Veritasium has a very relevant interesting episode on this: https://youtu.be/jyQwgBAaBag Although I agree it would be challenging to get any significant speed from it.

Love that episode, but I don't think this would work at all. Attaching a kite to that vehicle would only slow it down.

Re: Towing a Tesla at 70 MPH replenishes battery at fast charger rates

#377

Post by gorkish »

dahfizz wrote: post #273
elihu wrote: post #120

This seems like an interesting solution to the "what to do if my battery runs out in the middle of nowhere" problem. Normally, you'd either have to have to call a tow truck or have someone with a generator come along and recharge. However, towing for awhile (probably at much less than 70 mph for safety reasons) to recharge the battery enough to get to the next town is something that could presumably be done by just a…

> However, towing for awhile ... is something that could presumably be done by just about any passing car A Tesla model 3 weighs over 4000 pounds. A base model Toyota Tacoma isn't even rated to tow that weight, there's no way any passing sedan could. Now remember that for regenerative _braking_ to work, the tesla's brakes have been on! Definitely not something any passing car can handle.

Towing ratings are given in lbs but you have to remember that is a "full system" number which includes the load on the hitch, the tongue weight, and (the usual limiting factor) the ability for the vehicle to reasonably accelerate and stop the entire mass in a reasonable time and distance.

I estimated the required power and the equivalent "trailer weight" of towing a tesla model s under full regen elsewhere in this thread; it's roughly equivalent to towing a trailer of approximately 2000-2500lbs, depending on speed. A vehicle with a similar tow rating and a minimum 250hp engine should do the job.

Re: Towing a Tesla at 70 MPH replenishes battery at fast charger rates

#378

Post by throwaway0a5e »

Gustomaximus wrote: post #305
dahfizz wrote: post #273

Earlier quoted context omitted.

> However, towing for awhile ... is something that could presumably be done by just about any passing car A Tesla model 3 weighs over 4000 pounds. A base model Toyota Tacoma isn't even rated to tow that weight, there's no way any passing sedan could. Now remember that for regenerative _braking_ to work, the tesla's brakes have been on! Definitely not something any passing car can handle.

> A base model Toyota Tacoma isn't even rated to tow that weight Isnt this rating more about supporting and breaking force for the combined vehicle and load weight. My understanding in this case tow means 'pull the vehicle' vs carry the front weight. So I think even a smaller car would be fine to pull a Tesla.

>Isnt this rating more about supporting and breaking force for the combined vehicle and load weight.

There are braking requirements in the test but the bottleneck is almost always (i.e. I want to say always but I don't put it past Dodge to show up with a 700hp vehicle) the acceleration and minimum speed up a grade portions of the test. Given the weight that can be accelerated and pulled up a grade per the test requirements basically every vehicle winds up passing the braking portion with room to spare.

But if you get all your vehicle performance advice on Reddit and HN I can see why you'd think brakes are always the bottleneck.

Re: Towing a Tesla at 70 MPH replenishes battery at fast charger rates

#379

Post by iancmceachern »

iancmceachern wrote: post #313
rootusrootus wrote: post #253

Earlier quoted context omitted.

I'm surprised it was that good. I get 8 MPG from my F250 when towing a travel trailer that doesn't have the brakes engaged.

I too exist where the vent diagrams of HN reader and F250/travel trailer owners intersect.

Why the down votes?
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