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What happens to regen if the battery is at 100%?

e-FTW

New electron smell
Aug 23, 2015
3,347
3,239
San Francisco, CA
I have driven Piuses/Prïï a few times, and had regen braking simply go off going down a long hill as the battery was already full. Had to use mechanical brakes to keep the speed in check. Those cars also have a "B" mode on the transmission selector to engage ICE compression braking, which I could have used.
What happens if your battery is packed to the brim with electrons? Will regen braking go bye-bye when lifting off the go-fast pedal? Does the car smoothly transition to let you know what is happening?
 

TEG

Teslafanatic
Aug 20, 2006
21,967
9,194
All the EVs I have driven go back to plain friction brakes when the battery is 100% full. Some will do it when the battery is very cold too.
That is one reason why I like to charge my LEAF to 80% instead of 100%, so I get regen right from the start.
 

sorka

Well-Known Member
Feb 28, 2015
8,298
6,250
Merced, CA
I nearly rear ended a car the first time I charged to 100% and headed out. I wasn't prepared for the car not slowing down with regen. Had to slam on the brakes.
 

alwaysru

Member
Jul 11, 2015
181
94
Austin, Texas
I reported a bug to tesla after the first time I went to 100 and then went down the steep hill by my house. No regen freaked me out. Whoops!

Probably something a delivery specialist should mention or there should be more than a new dotted line showing "reduced" regen.
 

tstafford

Active Member
Jul 4, 2015
1,039
257
Nashville, TN
Yep. Regen is gone or severely diminished at high SOC. I'd never experienced it before - I have always charged to 90% including when the car was given to me at delivery. However I had the car in for service any they moved the charge limit to 100%. I didn't even notice when I pulled out of my garage. Surprised the crap out of me.
 

LLWS

Member
May 21, 2015
37
1
Woodbridge, VA
Look closely at the speedometer for the dashed lines noting your current limitations. If you charge to 100%, you'll get a message about Regen availability and you'll note a dashed green line on the Regen scale.
 

jimtelsa

Member
Mar 31, 2013
427
127
Simi Valley
Our Smart Ed is just like that
With a full charge paddle shift regen levels dont work till it drops a bit

My Volt is opposite
It can go from 41 full charge to 60 on meter with regen on a long hill.......
so I assume they dont max out the Volts battery pack
Regen still works to slow down car but wont show any higher gains than 60
Regen also works on Hold mode...just gas generator only....

Wild

this is good to know as I still wait for my Tesla to arrive
 

mknox

Well-Known Member
Aug 7, 2012
10,103
1,894
Toronto, ON
Same thing happens in the winter when it's cold. Re-gen will be limited (as shown by a dashed orange line on the power meter) all the way down to zero depending on how cold it is.
 

mikeash

Active Member
Oct 26, 2014
1,105
709
Fairfax, VA, USA
I think this behaviour is counterintuitive. If regen is limited, why not automatically apply some braking so that the deceleration remains the same?

Most hybrids and EVs do it like this, where you push the brake pedal to get the braking effect you want, and the car figures out the right application of regen and friction brakes to make it happen. The advantage is that everything just works the way you expect it to. The disadvantage is that brake feel is worse and you're never completely sure what the car is doing.

Tesla seems to have made the conscious choice to have the brake pedal simply be the actual friction brakes, and the accelerator simply be the motor. This gives us more control and visibility into what's going on, but does make us work a bit more when the regen ability is lower than usual.

Having experienced both (I had a Prius before the Model S), I think Tesla's approach is superior. But it's not without its disadvantages.
 

Skotty

2014 S P85 | 2020 3 P19"
Jun 27, 2013
2,528
1,963
Kansas City, MO
Would it be possible to allow regen in the motor but then throw away the electricity somehow? (I don't know enough about it off the top of my head to know if that is reasonable) If so, I would think this would be the best approach to maintain brake feel without having to add the extra complication of some magic combined braking algorithm like some other hybrids use.
 

mknox

Well-Known Member
Aug 7, 2012
10,103
1,894
Toronto, ON
Would it be possible to allow regen in the motor but then throw away the electricity somehow? (I don't know enough about it off the top of my head to know if that is reasonable) If so, I would think this would be the best approach to maintain brake feel without having to add the extra complication of some magic combined braking algorithm like some other hybrids use.

This has been discussed here in the past, and if you think about it, 60 kW of energy is a lot to have to control through sophisticated electronics and then bleed off in some sort of monster resistor, all of which would just add weight and complexity to the car.

After you get used to it (I've driven through 3 winters where reduced to no re-gen is the "norm" before the car warms up) it really isn't a big deal. You just press the brake pedal like you would on any other car.
 

jgs

Active Member
Oct 28, 2014
1,582
956
Ann Arbor, Michigan
This has been discussed here in the past, and if you think about it, 60 kW of energy is a lot to have to control through sophisticated electronics and then bleed off in some sort of monster resistor, all of which would just add weight and complexity to the car.

Worst of all, it would add a radiator. And a radiator grill. No thank you.
 

Jaff

Active Member
Aug 15, 2010
3,135
326
Grimsby, Canada
Yep, agree Mike

I think it is far easier for the driver to just acknowledge that a full battery = no regen then add a new, somewhat useless system to the vehicle...

This has been discussed here in the past, and if you think about it, 60 kW of energy is a lot to have to control through sophisticated electronics and then bleed off in some sort of monster resistor, all of which would just add weight and complexity to the car.

After you get used to it (I've driven through 3 winters where reduced to no re-gen is the "norm" before the car warms up) it really isn't a big deal. You just press the brake pedal like you would on any other car.
 

scaesare

Well-Known Member
Mar 14, 2013
8,577
15,165
NoVA
I wish there were a set of auxiliary set of high-wattage restive heaters for the battery pack (either heating the coolant, or surface-style heating pads heating the pack directly) so that when regen is disabled in cold weather, the regen power could be dumped in to them, so the pack could be brought up to temp more quickly....
 

jgs

Active Member
Oct 28, 2014
1,582
956
Ann Arbor, Michigan
I wish there were a set of auxiliary set of high-wattage restive heaters for the battery pack (either heating the coolant, or surface-style heating pads heating the pack directly) so that when regen is disabled in cold weather, the regen power could be dumped in to them, so the pack could be brought up to temp more quickly....

Now that (unlike the radiator idea I mentioned) seems like an idea with legs. It would also potentially be useful for preconditioning with shore power. One misgiving is whether the pack can safely absorb X0,000 kW of heat, though. My electric space heater is 1.5 kW and it gets pretty toasty. We're talking up to 40x that.

Of course, a pack heater wouldn't help when charged to 100%, the other case in which regen craps out.
 

scaesare

Well-Known Member
Mar 14, 2013
8,577
15,165
NoVA
Now that (unlike the radiator idea I mentioned) seems like an idea with legs. It would also potentially be useful for preconditioning with shore power. One misgiving is whether the pack can safely absorb X0,000 kW of heat, though. My electric space heater is 1.5 kW and it gets pretty toasty. We're talking up to 40x that.

Of course, a pack heater wouldn't help when charged to 100%, the other case in which regen craps out.

Just quick back-of-envelope calcs, assuming you wanted to use flat surface-style heaters:

- I estimate the pack is ~7' x 5'. That's ~5,000 in^2

- Dumping full regen in to a surface heater that size is 60KW / 5000 in^2 = 12W per in^2

- That rate is less than the ~15.5W per in^2 for a standard battery heater like THIS, and also not constant

- That's only 8W per cell of intermittent heat applied. There's also additional pack mass that would buffer this somewhat.


I suspect you couldn't regen nearly enough to be concerned about overheating that 1200 lb mass in any reasonable time period... unless you were coasting down from the top of Everest...

And to be quite honest, I'm not sure how much faster you'd really be able to heat a cold pack up with typical regen during normal driving... the main advantage might simply be not having to lose the braking feel when you let off the accelerator...
 

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