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Tesla Should Update Firmware to Allow Regen Braking When Battery is "Full"

By that I mean most of us charge to a user-specified software limit of say 70%, 80%, 90%, etc. When you first set out on your next drive after charging to that capacity there's a warning that regenerative braking won't function 100% until some juice is used up to make room. Every time I have to use my friction brakes to slow down my car I cringe knowing that the first several stops will be wasted energy and unnecessary wear and tear on the pads and rotors.

Well, we all know there's enough room there to put a few more miles (or feet?) of regen until you use more up through normal driving. There should be a code change behind the scenes that will still stop charging at the 80% user-defined limit you have set but then, when you set out on your drive, actually allows for a certain amount of overshoot. Say... 2% or something? Even at 1% we're talking several miles worth on most size batteries which would be more than enough for most standard users. Not many people will be putting a lot back on unless they live in the mountains and commute downhill each day but that's going to be a smaller portion of owners. Even so you can bump it to 2% or whatever. I don't have access to the data but whoever is writing code for these things does and could easily put something like this into action I would think.

This would 1) keep driving characteristics consistent and 2) not lose that energy that could be recouped. It doesn't seem like much but over hundreds of thousands of cars driving millions of miles this small amount of otherwise wasted energy could add up.

Can anyone think of a good reason why this wouldn't work? As long as they can separate the way the car sees plug charging versus regenerative charging (I assume it already does) then I don't see why this could be a bad thing. Seems like a win win to me.

Thoughts?
 

liuping

Active Member
Jul 23, 2013
2,243
927
San Diego
That warning is only when the battery is nearly full (over 95%?). It does not matter what you set the charging limit to.

If you set to charge to only 90% you should never see that warning...

When the battery is nearly full, it cannot accept the high current from regen without damaging the cell (the same reason normal charging slows near 100%)
 
Regenerative braking is limited because the battery can only recharge so fast when it's at a certain percentage. It's the same principal behind reduced charging speeds when plugged in / super charging.

Increasing the amount of battery headroom by only 2% wouldn't do much.

The real solution to this is for them to start using capacitor technology in the regenerative braking system, which is one of the potential applications of the solid state technology. Then it can distribute the power over a longer period of time for recharging the battery, or recycling it for the drive train.
 
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IIRC the volt did not regenerate to the battery when fully charged - even though there was a ~20% headroom. I believe they had a resistor sink, but several times going down a hill right after charging and pulling the paddle to slow down, it did not give me any appreciable braking.

Possibly the battery might not like the sudden surge of current above say (guessing) ~ 80% charge.
Otherwise, I certainly like your thought.
 
Maybe you are confusing the fact that regen is limited when the battery is cold? I’m not sure if the mornings are cold enough where you are to limit regen, but when the battery is cold, regen is limited until the battery warms up. As others have said, the limit you set has nothing to do with regen being enabled. The car aggressively uses regen to the extent that physics allows it to.
 
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ewoodrick

Well-Known Member
Apr 13, 2018
5,285
4,292
Buford, GA
Regen is the process by which a motor is making power instead of using it to turn the wheels. If the motor isn't connected to a load, then the power is not able to be sent anywhere and there is no power created and hence no regen force.

When the battery is full, it can't take anymore charge, aka won't accept anymore power. Hence regen is not available.

You can see this is small hobby type cheap science experiment motors. Connect to a battery, the motor spins, connect to a light bulb and spin the motor fast, the light lights. You can feel a little resistance when spinning the motor. Compare that to hooking the wires to nothing and then shorting the wires together.

Regen just doesn't work when the battery is full.

There are applications in which a very large resistor pack is used to feed the power to, but these take room and waste heat. Just not feasible, or even needed.

When regen doesn't exist, user the brake!!!!
 
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Just to clarify, I'm not talking about cold weather here. I realize this limitation and I'm not talking about that in any way.

I'm also not talking when the battery us full @ 100% either. I'm talking about a software capped "full" at 80% or 90% and NOT a full 100% filled battery.

I'm talking about when I charge my car to 80% and then upon first driving it my first few stops are reduced regen amount and I have to use the brakes more. I assume this has to do with the battery thinking it's "full" at the preset 80% and then once it gets down to 79% or so through normal driving regen returns to normal force.
 
There is no current ‘software’ capped battery. You charging it to 80% has nothing to do with the other 20% at all. It’s still empty.

I have no idea what we’re discussing.

I'll try to explain it one more time:

When you set your batteries SoC limit to 80% as the end-user it uses software to tell the battery when to stop charging. Then, when you go to drive, this 80% software limit you set is still in effect so when you come to your first stop the regen braking isn't as aggressive as it will be in another mile or two after you've gone under your 80% limit that you set. It's almost as if the car sees the 80% as a hard cap the same as the 100% cap. It's not. It's a software cap that we as end-users can adjust in order to not let the battery set at 100% for extended periods to prolong the life. I'm saying that, for regen purposes only, when it's at 80% it should allow regen at full strength even though it will exceed the 80% that the end-user set the car to for charging purposes. This won't pose any additional wear on the batter since we will literally be using up whatever regen we just captured in the next few minutes.

In other words, I can set the battery to 70% and charge to "full" of 70% where it stops. Upon driving if I increase this setting to 80% or something above regen braking will be normal. If I don't and leave it at 70% regen braking will be reduced until some of that space is used for forward motion. I shouldn't have to manually adjust the setting to activate the full benefits of regen braking when a simple software program would allow the behavior to just automatically do this. It's not "lying" to you because it would display the actual 81% or whatever it was at if the first portion of your drive had enough regen braking to actually add a full percent. It's just a software exception to allow for a slight buffer above whatever the current setting is for regen only.

I really don't know how I can explain that any clearer. This really has nothing to do with cold weather, permanent battery software caps (ie 70/75 or 85/90) from the factory or supercharging.
 

liuping

Active Member
Jul 23, 2013
2,243
927
San Diego
When you set your batteries SoC limit to 80% as the end-user it uses software to tell the battery when to stop charging. Then, when you go to drive, this 80% software limit you set is still in effect so when you come to your first stop the regen braking isn't as aggressive as it will be in another mile or two after you've gone under your 80% limit that you set.
I've been driving Tesla's since 2013 and never seen this behavior.
 
I'll try to explain it one more time:

When you set your batteries SoC limit to 80% as the end-user it uses software to tell the battery when to stop charging. Then, when you go to drive, this 80% software limit you set is still in effect so when you come to your first stop the regen braking isn't as aggressive as it will be in another mile or two after you've gone under your 80% limit that you set. It's almost as if the car sees the 80% as a hard cap the same as the 100% cap. It's not. It's a software cap that we as end-users can adjust in order to not let the battery set at 100% for extended periods to prolong the life. I'm saying that, for regen purposes only, when it's at 80% it should allow regen at full strength even though it will exceed the 80% that the end-user set the car to for charging purposes. This won't pose any additional wear on the batter since we will literally be using up whatever regen we just captured in the next few minutes.

In other words, I can set the battery to 70% and charge to "full" of 70% where it stops. Upon driving if I increase this setting to 80% or something above regen braking will be normal. If I don't and leave it at 70% regen braking will be reduced until some of that space is used for forward motion. I shouldn't have to manually adjust the setting to activate the full benefits of regen braking when a simple software program would allow the behavior to just automatically do this. It's not "lying" to you because it would display the actual 81% or whatever it was at if the first portion of your drive had enough regen braking to actually add a full percent. It's just a software exception to allow for a slight buffer above whatever the current setting is for regen only.

I really don't know how I can explain that any clearer. This really has nothing to do with cold weather, permanent battery software caps (ie 70/75 or 85/90) from the factory or supercharging.
My car (2015 MS) doesn't do this. I charge to 90% every night and have full regen the first mile.
 
Ostrichsak (OP), you have opened a very interesting discussion on the interaction of battery temperature, state of charge and temperature. I have owned my S100D for almost a year and experienced how the battery responds to the full range of temperatures where I live. Battery temperature seems to be the key factor in controlling regeneration. As the weather turns cold, limited regeneration becomes a daily experience. On short trips the battery doesn't warm up enough to achieve normal regeneration. I have experienced limited regeneration in warm weather when the battery is at 100% charge but this only lasts for a short while as energy consumption from driving reduces the SOC.

Regeneration comes in at least two profiles (my names) that I can envision. The 'stopping' profile is typical foot off the accelerator to slow the car which generates high power over a few seconds. The 'hill' profile is continuous regeneration that happens when driving downhill over a long distance. The downhill regeneration peak will be determined by the grade of the hill. The common 'stopping' profile could benefit from using ultra capacitors to absorb the energy peak and allow normal vehicle driving characteristics regardless of temperature or SOC. The 'hill' profile might not benefit at all except in cases of very steep grades when the battery is cold and not able to accept the amount of energy produced. I think ultra capacitors would be a great addition to electric cars as they would normalize driving characteristics regardless of battery temperature and SOC. Ultra capacitors would also reduce peak power draw on the battery during acceleration which would probably contribute to longer battery life.

I will pay more attention this winter to the relationship between temperature and regeneration at different levels of battery SOC. I wonder if lower battery SOC will result in a shorter periods of limited regeneration during cooler temperatures particularly when the ambient temperature is above freezing?
 

Rocky_H

Well-Known Member
Feb 19, 2015
8,351
10,746
Boise, ID
When you set your batteries SoC limit to 80% as the end-user it uses software to tell the battery when to stop charging. Then, when you go to drive, this 80% software limit you set is still in effect so when you come to your first stop the regen braking isn't as aggressive as it will be in another mile or two after you've gone under your 80% limit that you set. It's almost as if the car sees the 80% as a hard cap the same as the 100% cap. It's not.
OK, I hear your full explanation on this, and I understand what you are describing.

You are asking for a change to the software to make it not behave like that, but here is the response:

It is most definitely not supposed to be doing that!! I've been on two Tesla forums for about 6 years now, and yours is the first case I have ever heard of doing that. If you car consistently is doing that, you have a really crazy software glitch going on. The cars do not care what your charging limit is set for, when determining the behavior of regeneration--only how full the battery actually is.

So no, Tesla does not need to take your suggestion for improvement to implement a software change. You just need to find out why your car is acting buggy. I would suggest a full power off and then power back on to try to reset it out of whatever crazy mode it thinks it is in, where it is mistaking the charging limit as being a completely full battery. It is not supposed to be acting that way. Good luck to you getting that corrected.
 
That warning is only when the battery is nearly full (over 95%?). It does not matter what you set the charging limit to.

If you set to charge to only 90% you should never see that warning...

When the battery is nearly full, it cannot accept the high current from regen without damaging the cell (the same reason normal charging slows near 100%)
we never see it when charged to 90% or less. In fact this last trip when charging to 100% we never saw it or experienced it.
 

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