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Regenerative braking in rear... how does it work? System temperature restrictions.
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Thread: Regenerative braking in rear... how does it work? System temperature restrictions.

  1. #1
    Senior Member PLP's Avatar
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    Regenerative braking in rear... how does it work? System temperature restrictions.

    Two questions on regenerative braking.
    1. Rear system
    2. Temperature influence

    So the specs says that the car has regenerative braking.
    Yes, it does. I know.

    While front regenerative braking is quite simple - drive shafts to the motor, motor acts as a generator.
    However, I do not get how rear regenerative braking works.

    Or there is no such thing in rear at all? Simply front axle does the regen braking until you are in ABS or panic mode then friction braking kicks in.
    Just wondering...


    The second part.
    Is regenerative braking limited by ambient temperature or the battery temperature?
    I believe, and that would make sense, the cold battery will not accept much of charge. A few harder starts would warm it up and regen starts working - first slowly, up to 20 kW, and then more...

    The highest I saw ever so far was around 50 kW.
    What is the max?
    2016 KIA Forte5 SX 1.6 T-GDI, A/T
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    I wasn't aware the rear brakes were part of the regenerative system because it's a FWD and using the drive wheels is much easier. I believe Tesla only utilizes the drive wheels for regeneration thus AWD models do not have a significant net energy consumption compared to ICE AWD counterparts. Similar to Tesla, regenerative braking is limited when the battery is >90% and/or the battery pack is cold. It appears that Li batteries perform best, in terms of output and charging, at temperatures between 68F-110F. However, cooler temperatures appears to be best for battery longevity.

    According to FCA, the regen brake system (RBS) uses a unique "blended braking" system that under most cases utilizes 100% regeneration at speeds above 8mph. It was designed to make "gear coasting" and braking feel more like a traditional ICE car instead of the "dragging" sensation that other manufacturers (Tesla) gives. I can tell you that our 2013 Tesla Model S 85 took some getting use to; if feels like you have left the parking brake engaged when you lift off the throttle.

    I think -48kW was the most I've seen; I know the Tesla will do -60kW.

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    Quote Originally Posted by PLP View Post
    Two questions on regenerative braking.
    1. Rear system
    2. Temperature influence

    So the specs says that the car has regenerative braking.
    Yes, it does. I know.

    While front regenerative braking is quite simple - drive shafts to the motor, motor acts as a generator.
    However, I do not get how rear regenerative braking works.

    Or there is no such thing in rear at all? Simply front axle does the regen braking until you are in ABS or panic mode then friction braking kicks in.
    Just wondering...


    The second part.
    Is regenerative braking limited by ambient temperature or the battery temperature?
    I believe, and that would make sense, the cold battery will not accept much of charge. A few harder starts would warm it up and regen starts working - first slowly, up to 20 kW, and then more...

    The highest I saw ever so far was around 50 kW.
    What is the max?
    There is no rear regenerative braking on the 500e... front wheels only. When rapidly decelerating from highway speeds, I've seen as high as -65 kW for a brief period. It tapers off quickly as speed is reduced.
    2013 Fiat 500e

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    Senior Member PLP's Avatar
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    Quote Originally Posted by VeeDubTDI View Post
    There is no rear regenerative braking on the 500e... front wheels only. When rapidly decelerating from highway speeds, I've seen as high as -65 kW for a brief period. It tapers off quickly as speed is reduced.
    I kind of know that as it does not make any sense (you need a motor to produce electricity, right? ), yet specs says otherwise.

    See here (and it was repeated here on the forum)

    BRAKES
    Availability: Standard on 500e
    Front
    Rotor size and type: 11.0 x 0.87 (284 x 22) vented with regenerative braking
    Caliper size and type: 2.13 (54) single-piston with aluminum housing
    Swept area: 218 sq. in (1410 sq. cm)
    Rear
    Rotor size and type: 9.4 x 0.4 (251 x 11) solid with regenerative braking
    Caliper size and type: 1.50 (38) single-piston with aluminum housing
    Swept area: 162 sq. in (1048 sq. cm)

    http://www.fiat500usa.com/search/label/Fiat%20500EV
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    Senior Member zonker's Avatar
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    Marketing boo boo is what that looks like. The regenerative braking handles most all the braking chores above 6 mph.
    2017 124 Spider Classica - modded tastefully
    2013 500e - 189 lb lighter
    1974 X1/9 - modestly restified
    1971 124 Spider(s) - latest resto project

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    Senior Member PLP's Avatar
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    So, I ran some tests.

    Michigan winter is cold-ish, that is sometimes -15 C (garage then is at about +5 C). Even now, with kind of spring time when I get to the car ambient is +5 C, garage would be +11 C.

    Having said that - the first few miles, despite what the battery stare of charge is, the regen is NOT working.
    HOWEVER, if I use Cruise Control (CC) when slowing down, it will pump electrons back.

    Some numbers and an example.

    I leave home, 100% charge, ambient is +10 C. Speed 35 mph. Slowing down with the pedal - no regen. Maybe at 2 kW.
    Then, set cruise at 25 mph. Accelerate to 35 mph again, let off the pedal - CC will "regen" at 7-9 kW despite battery being at 98%. The moment I touch the brake - it will drop back to 2-3 kW.

    Same setup, but the battery is 50 %.
    CC will use about 15 kW to slow down. Pedal still nothing (that is 2 kW).

    And the pedal remains inactive till about 3 miles driven with a few stops. Then regen slowly "wakes up" allowing for 15 kW to then down the road, same speed and application, up to 30 kW.
    On higher speeds, that is above 50 mph, when going downhill, I observed as much as 70 kW.


    In summary.
    There is some weird correlation between temperature (of the battery) and regen. Capacity wise - hard to say. For sure the regen is a tad limited at near full, but still happening.

    I will need to wait till really warm days and nights to be able to tell more.



    And yes. I know rear regen is a "typo". Unless they meant that with the brake application system does not activate any hydraulic brakes (that is in non of the 4 wheels) and uses regen only.
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    Senior Member zonker's Avatar
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    In my experience in the warmer climate of where I live anytime I let off the throttle or brake above 6mph, my gauge will show negative values, so regen is working for me any time I'm off the pedal. As for how much, it seems speed and brake pedal pressure dictate that.
    2017 124 Spider Classica - modded tastefully
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    Quote Originally Posted by PLP View Post
    There is some weird correlation between temperature (of the battery) and regen.
    I'm just speculating, but a possible explanation is that while the battery is relatively cool, the regen power might be applied to heating the battery.

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    Senior Member PLP's Avatar
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    Quote Originally Posted by zonker View Post
    In my experience in the warmer climate of where I live anytime I let off the throttle or brake above 6mph, my gauge will show negative values, so regen is working for me any time I'm off the pedal. As for how much, it seems speed and brake pedal pressure dictate that.
    That's what I am expecting to see once it gets warmer. I believe battery temp barely gets above +50 F on the initial part of the trip. We had maybe two days with +65 F... but it was still too short to get any good results right away.

    Quote Originally Posted by rkw View Post
    I'm just speculating, but a possible explanation is that while the battery is relatively cool, the regen power might be applied to heating the battery.
    That's an idea.
    Might be.
    Yet, when I am thinking of it, and not exactly knowing the design, I would expect to still see "back current" from the motor as I believe that's how it is set up - the display measures current flow to and from the motor.
    But I might be wrong. Happened a few times.

    I recall seeing video of Bjorn when he could not charge half empty Tesla as it was cold. He took it for a ride and no regen either. He did a few WOT (well - how we would call it? FC - full current?) what resulted in slowly bringing up the regen and then when he plugged it in - it was charging at full speed.
    So having this example I would say - regen is limited.
    Yet, I cannot explain why cruise control would allow regen after all? Why I can get -15 kW on CC, but not on the brake?
    2016 KIA Forte5 SX 1.6 T-GDI, A/T
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    Quote Originally Posted by PLP View Post
    I recall seeing video of Bjorn when he could not charge half empty Tesla as it was cold.
    Lithium ion batteries can be permanently damaged if charged while cold (below about 0 C). A properly designed system would warm the battery before beginning to charge.

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