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Fixing Cold Weather Loss of Regenerative Braking

Fixing Cold Weather Loss of Regenerative Braking

If you live where the temperature drops below 10C/50F, you know regenerative braking becomes unreliable as the weather gets colder. You must drive your Tesla like an ICE vehicle using the brake pedal to stop. The reason is the cold battery cannot accept all the power produced by the motor(s) when they act as generator(s). The vehicle’s kinetic energy is converted to heat by the brakes and thrown away. A possible solution to this problem is placing a suitably sized resistive heating element in the liquid heating/cooling loop. Regenerated electricity that cannot be stored in the battery could be dumped into the heating element to warm the fluid which is used to heat the battery and the cabin. Electricity consumption to provide heat is a major power user in winter so recovering kinetic energy as heat makes sense.

Two benefits come from this proposal. Regenerative braking would function consistently regardless of the temperature and energy currently wasted through the brakes would be captured to provide heat.

Any thoughts?

tes-s | 25 novembre 2019

A 60kW heating element?? That is a lot of heat.

For example, tankless hot water heaters use 9kW heating elements - the largest I know of uses 4 of those elements.

The added cost, space, and complexity are probably not worth the limited benefit.

SnowFlake | 25 novembre 2019

Great idea

Aerodyne | 25 novembre 2019

Keep the car plugged in, set charge time to just before you leave, or use smart preconditioning. In any case, battery will be heated by shore power.

Problem solved.

Bighorn | 25 novembre 2019

Order of magnitude mismatch was my first thought.

barrykmd | 25 novembre 2019

Much ado about nothing was mine

Roger1 | 26 novembre 2019

AERODYNE | November 25, 2019
Keep the car plugged in,

Your solution doesn't work for people who have to leave their cars outside in sub-zero weather. Perhaps they are at a hotel or work nights. The car battery weighs 500 - 1,000 pounds. How much power does it take to raise the temperature from 20F to 60F? How long does it take? Dumping power into a resistive load maintains normal car driving characteristics with regenerative braking and works immediately.

Roger1 | 26 novembre 2019

tes-s | November 25, 2019
A 60kW heating element?? That is a lot of heat.

You have to factor in the duty cycle. A tankless heater has to run indefinitely providing hot water. A resistive load for regenerative braking needs to dissipate 75 kW for, perhaps, 5 seconds. The heating element should have at least 30 seconds to cool before the next brake application in normal driving. Driving on a long downhill grade does not generate the high peak power of a stop from high speed so much of the energy will go into the battery with some spillover into the resistive load. The real question is what kind of materials and design are best suited to handling the sudden, high heat load without going into some kind of thermal shock problem.

SpaceGhost | 26 novembre 2019

Roger1, Agreed... a resistive load would make regen braking consistent feeling. i.e. heating the battery would be nice, but not necessary to keeping the regen braking working all of the time. i.e. the resistive load would variably come on depending on how much current the battery can absorb. The heat could be dumped into the coolant leading into the battery, or optionally vented to the air. The coolant idea would likely be smaller and easier to implement, however the existing battery heater is only 6kW (I think), so like tes-s says, you would need something to dump up to 6 times as much heat.

Bighorn | 26 novembre 2019

@SG
10 times since regen routinely is providing 60+ kW, aka an order of magnitude.

Roger1 | 26 novembre 2019

Bighorn | November 25, 2019
Order of magnitude mismatch was my first thought

barrykmd | November 25, 2019
Much ado about nothing was mine

As drivers, we all have our own views about vehicle performance. I think consistent operation of the braking system is important and is my primary reason for suggesting the idea.

Energy management is a critical process in electric car operation. Tesla and other electric car makers put a great amount of effort into reducing energy use in the propulsion system and recovering energy by regenerative braking. The value of regeneration depends on where you drive. If you get on a clear highway and drive for hours across flat land then regeneration will not be very important. On the other hand, if you drive in traffic where the speed goes up and down repeatedly and the terrain is hilly then regeneration becomes important. So, why should recovering energy be important in summer but "much ado about nothing"in winter?

If a car brakes for 5 seconds at 75 kW then 375 kW/seconds are generated. If you convert to Watts per hour, 375 kW/sec is 104 Watt/hours. In my S100D, 104 W/h is good for about 1/2 kilometer of range in typical driving conditions. If the car can scavenge 100 W/h of energy as heat and leave the equivalent amount of electricity in the battery for propulsion then it makes sense to me.

Aerodyne | 26 novembre 2019

I went to Reno NV last December. Called ahead and asked if I could plug in to an outside 120V outlet. They said sure as many hotels have such outlets. Brought drop cord and was able to heat the car, but not charge much. 20 deg f overnight.

Smart preconditioning still works even if you are not plugged in as long as SoC above %

Bighorn | 26 novembre 2019

It’s not that anyone disagrees it’s a good idea, and lack of search puts you at a disadvantage, but you can imagine this was thoroughly discussed the first winter back in 2012-13. Ate you suggesting there’s a way without a capacitor to feed 60 kW into a 6 kW heater without wreaking havoc?

Bighorn | 26 novembre 2019

So math works, the units are Watt-hours, a product not a quotient.

Roger1 | 26 novembre 2019

Yes Bighorn, a product not a quotient. Thanks for the correction.

The resistive heater probably has to handle a peak of 300 amps at 400 volts (120 kW) but only for a short duty cycle measured in seconds. The thermal mass has to be large enough to act as a buffer and accept the power (100 Watts in the 75 kW for 5 seconds example) then feed heat into the coolant loop at an acceptable temperature. I imagine something like a ceramic heater element with a coolant jacket. The 'regen' heater might replace the existing 6 kW heater thereby saving a component and installation cost. The regen heater would require a controller capable of modulating the current delivered during braking and software additions to allocate power between the battery and heater. The controller would allow operation of the heater at lower power levels for conventional heating. If the vehicle is connected to a supercharger then power levels greater than 6 kW might be appropriate to speed battery heating while keeping the cabin comfortable in cold weather.

So, my scheme doesn't require a capacitor because the heater is specified to handle high current and voltage delivered in short period. However, if the energy recovery from the 75 kW for 5 seconds example is roughly correct then you have to wonder about the cost for 100 Watt hour of storage using ultra capacitors. The cold weather performance problem would be solved and the capacitors could be used in all seasons to enhance performance and buffer the battery from the need handle high currents during hard acceleration and regenerative braking.

Bighorn | 26 novembre 2019

Interesting and novel ideas. I can’t really vet without the assist of an EE.

tes-s | 26 novembre 2019

"I think consistent operation of the braking system is important"

I guess it all depends on what "consistent" means. :)

I find the current braking consistent in that it always does what I expect.

I know what you are getting at - I just think adding the complexity and cost of a whole new system to handle this rare case is not warranted.

Battery buffer to reduce stress on the battery?? That could have possibilities.

SnowFlake | 26 novembre 2019

In my expectation just 20-30kw breaking is enough. Compare to 0 regenerative of first few miles.
I can forsee that the cybertruck will have all these included

Roger1 | 27 novembre 2019

tes-s, consistent operation means the same behavior of the system regardless of temperature. I expect the same braking force from foot-off-the-accelerator whether the temperature is freezing or 20C/70F. If the regen braking force is excessive at 0C/32F because the road is slippery then I expect the anti-lock system to operate. I proposed that dumping electricity into a resistive load as the means to allow normal regen operation and capture the energy as heat. The alternative means to achieve consistent braking force would be software controlled application of the mechanical brake to augment the regen braking. Less energy recovery but consistent braking behavior can be achieved through software changes alone.

tes-s, you describe the inconsistent behavior as a "rare case". Perhaps you live in a warmer climate than I do. In my case, the temperature in the garage drops below 10C/50F in November and stays there until April. The battery is normally cold and regen largely absent. Outside temperatures are typically below freezing and commonly drop below -18C/0F for periods of days. Li batteries are happy at 20C/70F not at the temperatures encountered in the northern US states and most of Canada during winter. Everything is worse for an electric vehicle in the cold because battery capacity drops at the time when energy use increases for propulsion and cabin heating.

tes-s | 27 novembre 2019

"tes-s, consistent operation means the same behavior of the system regardless of temperature."
Why did you choose to qualify it with temperature? How about SOC on a 100F day? How about when roads are slick?

"If the regen braking force is excessive at 0C/32F because the road is slippery then I expect the anti-lock system to operate."
That answers my last question - you prefer inconsistent regen in that case.

"tes-s, you describe the inconsistent behavior as a "rare case". Perhaps you live in a warmer climate than I do. In my case, the temperature in the garage drops below 10C/50F in November and stays there until April."
I live in CT. I would guess that less than 10% of 174k miles driven are with limited regen - probably less than 5%.

I understand what you are asking for. I would rather save the cost (purchase and maintenance) and weight and deal with it. Has not been a problem for me.

Roger1 | 28 novembre 2019

tes, "Why did you choose to qualify it with temperature? How about SOC on a 100F day? How about when roads are slick?"

Cold temperatures alter the braking performance of the car. Tens of thousands of Tesla owners live in wintery climates where regen braking degradation occurs. Braking can be made more consistent by using the mechanical brake to augment regen braking, installing ultra capacitors or dumping power into a resistive load and scavenging the heat. The mechanical brake augmentation approach is likely the least expensive because it doesn't require any additional hardware and the updated software can be downloaded. If Tesla wants to offer one pedal operation to the existing fleet of pre Raven S and X cars then this functionality will be required anyway. Pre Raven S and X models don't have a permanent magnet motor that makes one pedal driving possible in Model 3.

High state of charge certainly reduces regen braking and can be a surprise if you don't habitually charge near 100%. The brake augmentation and ultra capacitor solution can fix the high SOC problem as well as the cold problem. The resistive load approach will not help with high SOC loss of regen at high ambient temperatures but could be helpful if the ambient temperature is below 20C/70F. Brake augmentation could still be used to manage loss of regen at high temperatures even with the resistive heater installed.

The driver has the responsibility to control the vehicle with due regard to road surface conditions among other things. ABS prevents wheel lockup that makes braking inconsistent when the car goes into a skid. Braking force from a turning wheel is higher than a stopped wheel sliding on a surface. More important, a turning wheel can be used to steer the vehicle while a stopped one cannot. So, if the car can optimize braking force by modulating brake application then that's what I want.

Regenerative braking is just one aspect of the larger discussion of the user interface between the driver and car. In the ICE world, acceleration and braking are completely separate and controlled with dedicated pedals. In the Tesla, one pedal controls all the acceleration and part of the braking. The brake pedal controls the traditional friction brake to provide the balance of the braking. In the one pedal mode, all normal driving can be accomplished with the accelerator.

I wonder how 'non car enthusiast drivers' transitioning to Teslas adapt to the braking arrangements? Are significant numbers of drivers continuing to use the accelerator and brake like an ICE car and wondering why they don't get the advertised range from the vehicle? If you received your new Tesla during the winter when regen is dodgy might you continue using ICE driving habits and not change behavior when the weather warms up? Could the one pedal mode be a means of getting drivers to keep off the brake pedal and let regen do its job?

I was riding in a model 3 yesterday and the driver was very happy using the one pedal mode. I want to find out how Tesla has dealt with limited regen in cold weather and if the one pedal mode uses the mechanical braking system at all.

nukequazar | 28 novembre 2019

What about having a secondary battery that would only be charged by regenerative braking, then the captured energy could be used to apply energy to the motors to slow the car from <10 MPH to a stop?

Mathew98 | 28 novembre 2019

Makes as much sense as adding a REX engine and call it a true EV.

If you can't even charge your Tesla at 120V plug overnight in extreme weather, perhaps EV's may not make much sense to you at all.

Up here in the NE, we've been through 6 winters without issues. All we needed to do was to schedule charging to complete about half an hour before we left for work. Why is this a blocking issue?

nukequazar | 28 novembre 2019

So @Mathew98, the only driving you do is from home to work in the morning? You don’t even drive back home? And, no, not everybody has a way to charge where they live.

nukequazar | 28 novembre 2019

I left out that the braking battery may need to be heated by power from the main battery.

This system would be advantageous in all weather because it could, potentially, allow one-pedal driving regardless of motor type. This is a great option for driving and also saves the brakes.

NKYTA | 28 novembre 2019

Everyone realizes that there is a company, named Tesla, with a $59 Billion market cap, that has over 46 thousand employees...and just purchased the Maxwell battery company?

And you know more than they?

Happy Thanksgiving!

nukequazar | 28 novembre 2019

@NKYTA, you’re right. There’s no reason for any discussion here. Let’s all stop posting, starting with you. Thanks.

tes-s | 28 novembre 2019

"I wonder how 'non car enthusiast drivers' transitioning to Teslas adapt to the braking arrangements?"

This "non car enthusiast driver" found no issue transitioning. Regen was second nature after the first day. Low regen (first experienced with 100% SOC) was an "oh", and then natural after that.

Based on the forum posts, people are more concerned about cup holders and coat hooks than low regen in the cold.

bryan.hopkins | 28 novembre 2019

Also, what’s wrong with having to occasionally use your brakes(I.e. when the battery is cold and regen is low)? If you live in a northern state that uses salt or other products, you need to use the brakes occasionally to keep them from seizing up. Living in mid-MI, it takes me about 18 miles of driving for the battery to warm up sufficiently to get full regen. Not an issue as I just leave a longer distance to stop (necessary anyway if the roads are snowy and icy) or I use the brakes to stop in a shorter distance.

PrescottRichard | 28 novembre 2019

I tried that with Darth one time and he told me ‘you first.’

sbeggs | 29 novembre 2019

Flagged for Thanksgiving. Hatchets should be laid down for one day!

sbeggs | 29 novembre 2019

Fixed! I've suffered cold weather loss of regen, had to really keep on top of it, especially going downhill on the Pines to Palms highway to Palm Springs in 2014. Relieved!

buckyuk | 13 gennaio 2020

I usually stick mine on ludicrous + mode for 10 mins which heats the battery enough to obtain the usual level of regen braking.
However i just realised this isnt an option for everyone without performance model, and would be really annoyed if this wasnt an option.
Living in scotland, you would be pretty much driving for 6 months of the year with no regen braking.
Its not a good way of driving, its like going into corners in neutral in an ICE car, its not a very controlled way to take corners. Always remember my driving instructor shitting himself when i done it in my lessons.

Its a common sales point that the brake pads would last x amount longer than ICE car, but in a cold climate, it would end up being the opposite, causing more break wear than an ICE car as they would normally be using gears/engine to help slow down.

Scheduled departure seems to be the only option, but if you dont have a regular schedule like me.... its kind of annoying.
Would love a better solution. even having the option on the app to warm battery at the same time as turning climate on 10 mins before using the car, would be suffice.

GTM | 13 gennaio 2020

Buckyuk, my app shows battery heating when the climate function is activated. 2020 MS performance on software version 2019.40.50.7. My regen has changed noticeably since delivery. Both the amount of regen available and the amount of decel provided. If I depart with ambient temps below 55F I have no, or barely any, regen and full regen can take as much as 40 miles if ambient temps are around freezing. My "work around" is to start charging and preheating the interior, via the phone app, 20 minutes prior to departure. That get me 25 to 30Kw or regen on starting out. Full regen in 10 miles (Full regen still doesn't provide the same level of decel). The battery heating icon seems to take half a minute to show up and does not require that the car be charging, so I think you can do what you want (multiple software versions not withstanding). I can only confirm that this works plugged in, I have not tried it away from home.

Bighorn | 13 gennaio 2020

Battery heating comes on with with climate request on the app for a couple years now.

GTM | 13 gennaio 2020

Buckyuk doesn't seem to be getting this function. I need to amend my comment that it takes a half minute for the icon to show on the app. I timed it. It takes a full minute to show up so maybe buckyuk just needs to keep his phone screen alive longer to see that it is working.