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Retry: Limited Regen

Retry: Limited Regen

This is a new post as my prior post was incoherently written in sleep. Let me ask differently now that I am awake and realize what confusion i caused.

- We know that regenerative breaking is limited in 2 states. State of Charge over ~93% and separately due to low temperatures
- Traditional arguments for limited regen at SOC vary, but cover grounds from efficiency to whether voltages can exceed safe levels in high state of charge + high voltage from regen process
- What i am looking to understand is the limited regen in low temperature which seems less understood. What do we think is the key factor in limited regen? All we know for certain is that the limitation is a software limitation - but why?
- Cold Battery has been stated as a common reason: I personally find this hard to believe as limiting factor. If i can plug a cold car to supercharger and battery accepts that, the regen power cant be that much higher. I think estimates on regen peaks put the max around 60kw but normally lower
- So what i am looking for is a non-battery reason for the limitation on regen braking in cold and seeking input if other have observations/prior learning/thoughts about whether the low temperate regen is related to limitations of motor, inverter or other mechanical parts, etc.
- Separately, I have reversed back to the roll mode from hold mode. I suspect that in cold weather, there is too much traditional braking at lower speeds as regen cant be that effective at very lower speeds (have to overcome internal fixed losses before regen is a net positive). Hold mode seems to be more stylistic than related to real efficiencies. However, the other reason I am reverting back is to test regen (without brakes) and see if i can reduce the time it takes for regen braking to step up by utilizing it more

Bighorn | 14 november 2019

Battery temp can slow supercharging or even make it impossible to accept a charge it temp is -4C or below. Current of charging will warm the battery pretty effectively in modestly cold situations.
Regen can be used aggressively with acceleration-deceleration cycles to warm the battery, though it the regen is limited, it obviously hampers that technique. Regen is optimized at about 30 C or 90F

Fuzzball | 14 november 2019

thats surprising because (4C)/24F is not a very low temp for winter in northeast. If the battery refuses to charge around that temp, then is this a miss on the internal temperate regulation of the pack?
I do recall slower than expected super charge rates a few times last year when i traveled around 20F but assuming whatever minimal heating on way to super charger means its not cold soaked 24F when getting to supercharger..

hokiegir1 | 14 november 2019

@Fuzzball - If plugged in, the car will use "shore power" to warm the battery so that it can charge at lower temps. It's not that it will never charge that low...it just takes longer to start/ramp up than in warmer temps.

Bighorn | 14 november 2019

It would take several hours or more to cold soak to -4C. Ambient temps don’t matter much after the battery is warm. I sat at a supercharger for about an hour before the battery started to charge after sitting at about -15C overnight.

82bert | 14 november 2019

I can attest that the car will not readily accept a supercharge at low battery temps. It taught me an important lesson about cold weather travel and preparing the battery.

Fuzzball | 15 november 2019

Wow - I cant believe i was not aware of this limitation around -4C/24F or lower Cold Soaked Battery. I had always assumed the battery pack management took care to burn energy to ensure battery is not harmed, but this is a totally separate and distinct risk to manage.

I feel like this is one of the top precautions that the community needs to be aware of - I traveled up and down the coast last winter and there were some very cold days. I guess I got lucky that i always super-charged before parking overnight because i never wanted to wake early.

Also - if super charging is limited at very temps, is regular charging also limited? I cant recall anymore...

Bighorn | 15 november 2019

@Fuzz
It’s not dangerous for the battery until you get about 50 degreesF lower than that for a sustained period, ~-25F x 24 hours. So no, it’s not important to alert people to a non issue, aside from the impracticality of charging while cold. Everyone who has learned it from experience doesn’t repeat the miscalculation.
All charging is limited at low temperature threshold.

derotam | 15 november 2019

@Fuzzball, allow me to point you to my thread here https://forums.tesla.com/forum/forums/temps-precondition-regen-data-graphs Specifically in the OP look at the "Full vs Limited Regen 70-0mph" graph.

derotam | 15 november 2019

@Fuzzball, I do have a planned test concerning battery self protection but I can't even think about that until get temps at -7C/19F. There is a parameter that I want to look at in the CANbus data from the car at that temp. Unfortunately I won't get those temps till probably January around here, though we were close the other day.

Bill Korea | 15 november 2019

Since Lion batteries would be damaged if charged at freezing temperatures, the battery management system doesn't allow it. What's needed is a way of offering consistent regen in all conditions. It the batteries are fully charged, dump regen energy as heat. If the batteries are cold, use the regen energy to heat the batteries, or the cabin. It's not that difficult, but it has a cost - gladly paid by drivers wanting consistent performance, or in cold climates. I won't say this is low-priority in California, but I'm thinking it.

Mike UpNorth_ | 15 november 2019

@bill

100% agreed. Would love full regen all the time. Dump that energy anywhere if possible!

andy.connor.e | 15 november 2019

I think supercapacitors are going to play a roll in optimizing regen.

gballant4570 | 15 november 2019

andy.connor.e, hence the acquisition of Maxwell Technologies. I really like my Model 3, but I am quite aware that coming improvements in battery tech will date it at some point. But the pickup truck should have some of it.

Zeus140 | 15 november 2019

Bill Korea +1
There should be a way of automatically assigning excess power generated by regen to whatever else could use it. Heating the battery, cabin or seats in cold weather (I believe these are resistive loads) would seem to be easily done.

andy.connor.e | 15 november 2019

Ya if battery temp is going to be that volatile for regen, then i imagine we are going to see another approach to optimizing regen.

Bighorn | 15 november 2019

So what some consider to be not enough regen creates 60 kW of power. No auxiliary consumer comes close to being able to absorb that load. Battery heater could take 10%.

Mike UpNorth_ | 15 november 2019

@big

You think Max Regen can create that much power?
Is just dumping the power/energy an option?

andy.connor.e | 15 november 2019

Could just have the cars motor polarity switch and could output a small amount of energy to "brake". Would be the same as if you put the car in reverse and gave it a small amount of "gas".

Thats at least a solution until the battery warms up. And it would at least save your brakes.

Bighorn | 15 november 2019

@Mike
Yes. The S is about 65 kW. I can watch it in my power meter surpass 60 kW routinely. I think the computer code for the 3 was originally closer to 50 kW. That might have been for RWD and increased for AWD plus FW updates have made it more aggressive since the first deliveries.

Bighorn | 15 november 2019

I don’t know how you’d dump it without a capacitor. Maybe a Batmobile afterburn would look cool.

CharleyBC | 15 november 2019

@Bighorn: " I sat at a supercharger for about an hour before the battery started to charge after sitting at about -15C overnight."

How did this appear to you as the user? If I plugged in at a Supercharger and got zero result, I probably would have thought I was at a bad stall--unless the UI clued me in somehow to tell me what was going on. Hence my question. How did you know to wait that hour? What was the screen telling you? Thanks!

Mike UpNorth_ | 15 november 2019

@big

Thanks. I'm surprised it's that much.

Bighorn | 15 november 2019

As I recall, it showed a 1kW draw. I’d probably heard if it, but not experienced because I don’t usually stop overnight. I really had no options as I was at a low SOC in Butte, MT.

pjwheeler83 | 16 november 2019

Im also very curious. I've seen videos of LR/P variants still having half a bar of regen after sitting in the cold for days. On my SR+ even in my garage when it sits for 8+ hours in the cold (<40°) if i don't pre heat the car for 45 minutes to an hour i have 0 regen. MAYBE 1/20 of regen is available. In temperatures in the teens/ 20s I'm driving for 20+ miles at freeway speeds and still not getting full regen capabilities.

I understand that my battery is ~ 25% smaller than a LR battery and I fully expected lesser range due to heater usage but the hit on regen is pretty bad.

kevin_rf | 16 november 2019

This is why the new scheduled departure time is a miss. It stops charging (warming) your battery at 6am. In many cases, letting it cold soak for a few hours.

Last winter with a 45 mile mostly highway commute, I would often arrive at work with regen still limited. It takes a fair amount to heat the battery at these temps.

gballant4570 | 16 november 2019

kevin_rf +1. The TOU stop charging setting should be user adjustable, like enter your own TOU time or turn it off completely. As it is, a working scheduled departure feature is still something I am waiting for.

Fuzzball | 29 november 2019

So current temp is around 32F/0C (cold soaked battery) and wall charger already limited to 16/48 amps. There is some fluctuation on the charge rate that I suspect is more due to Volts (old house on shared loop with 2 other houses, very old NYC infrastructure)..