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Regen Braking - Charge Dependent?

abianucci

New Member
Jan 11, 2021
3
1
Illinois
Recent M3 purchase.

I understand varieties, regen, etc are effected by weather.

Our weather has been 30-35 recently. I have noticed that even after the car sits for 8h, then warmed up for 10m that if the battery is above 55% I get limited regen braking messages. However if the battery is less then 55% I constantly get full regen braking.

Any ideas? Is that by intention or? Trying to figure this out as even if I turn the climate on for 30m when the battery is above 55% it doesn’t matter which leads me to think it’s my understanding of regen braking or software.
 
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elptxjc

Member
Dec 15, 2019
833
168
El Paso, TX
Regen decreases when battery is cold, when battery is full, or both. Depending how cold the battery (and ambient temp) is, how much you need to heat the battery for proper regen. If no regen, you might need to heat it more... but reportedly it's not worth to waste energy doing that, unless you're going on a long trip. But I'm still learning all the details of EV ownership :).
 
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DaveRZ

Member
Nov 19, 2019
178
253
Murrieta, CA
Yes, the amount of regen available is directly related to how full your battery is, and how cold it is (and potentially how cold the motors and inverters are - though I don't know that for certain). I'm sure someone at Tesla has an exact mapping of battery temp, SoC (State of Charge), etc... but for the rest of us, the colder it gets, or the fuller the battery is (or both), the less regen you have. Also, if you bought the single motor M3, you're starting off with about half the regen capacity of the dual motor.

I don't like giving advice on anything related to cold weather seeing as how we are having a 85-90 degree weekend here in Murrieta, CA. But my experiences (and it DOES drop into the low 20's here at night) tell me that preheating the car for the purposes of gaining more regen is not worth it. You'll use far more power than you save via regen. I only pre-heat or pre-cool for the occupants of the car. I'm going to mirror the advice I've heard others say on this forum, let the car worry about the battery temp.

Plus, it is important that you use your brakes occasionally anyhow. Keeping them clean and moving freely is vital given how little use they'll get compared to an ICE car.
 

abianucci

New Member
Jan 11, 2021
3
1
Illinois
Gotcha.

just threw me for a loop that at 75% after charging you have limited regen but at lower percentage you do. Given it was storing energy I just assumed it would be there (weather aside).

Thanks for the clarity.
 

Resist

Member
Mar 24, 2019
513
240
San Luis Obispo
It seems to me that regen doesn't have to be limited in any situation. Regen doesn't provide that much juice anyway but I'd think Tesla could have had the regen power not charge the battery at all, if the battery was at a high state of charge.
 
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GZDongles

Member
Feb 23, 2020
225
283
Michigan
It seems to me that regen doesn't have to be limited in any situation. Regen doesn't provide that much juice anyway but I'd think Tesla could have had the regen power not charge the battery at all, if the battery was at a high state of charge.
What would the power be used for then?
 

run-the-joules

Turgid Member
Aug 13, 2017
3,785
6,868
SF Bay
The fact that they don't blend in the friction brakes when regen is limited, thus providing a consistent experience, is mind blowing to me. Such an obvious win.
 

SigNC

Active Member
Aug 23, 2017
1,553
1,425
NC
Why does it have to used for anything? The power output could just be switched off.
Then there would be no resistance in the motor to slow you down. For instance, when they don't want to store the power on large electric/diesel trains they simply dump the power into a huge resistance heater and blow the heat out of the top of the train. :)
 

woof

Fluffy Member
Supporting Member
Apr 30, 2009
1,587
1,837
Why does it have to used for anything? The power output could just be switched off.
Why? Physics. The whole point of regenerative braking is to slow down. If a generator is running with no load, it's very easy to spin. Like a bike going downhill is easy to pedal. In order to make it harder to spin, the load on the generator has to be increased, like trying to pedal uphill. So no load, no slow. High load, high slow. Thus, in order to have a braking effect, there must be a load on the generator. In EVs, that load is the battery. In diesel electric trains the load is a giant bank of resistors that get very very hot. In either case, the energy generated must be absorbed by something, else there is little braking effect. If the battery cannot take a charge, or the resistors are too hot, then friction braking must be used, as there is no slowing effect otherwise. See Dynamic braking - Wikipedia
 

Gasaraki

Active Member
Oct 21, 2019
2,010
1,255
Syracuse, NY
It seems to me that regen doesn't have to be limited in any situation. Regen doesn't provide that much juice anyway but I'd think Tesla could have had the regen power not charge the battery at all, if the battery was at a high state of charge.

This comment is totally not true. The reason why regen IS limited in low temperatures and/or state of charge above 80% is regenerative braking from the motors is a lot of KWs.

If you had the Scan My Tesla tool, you can see the regen power limit at anytime. When regen braking the power can be in the 100KW+ range. So if the battery is cold or full, the regen has to be limited just like how they limit charging speed.
 

SlimJim

Member
Jul 25, 2019
907
689
USA
I am not sure if it still uses the motor to slow the car down but I like that it slows the car down when I lift my foot from the pedal even when regen disabled sign is on.
 

MentalNomad

Member
Dec 6, 2018
386
425
USA
I wish the car had a bank of resistors that could be used for resistive braking when the battery is cold. The resistors could heat the battery coolant to warm up the battery quicker while applying braking until it's ready for regen.
 

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