How Much Charge Does Regen Braking Put Back in the Battery?

[jerry33]

If we start with a speed where half of total losses is road friction and half from aero, and


1)mgV1 + rho*CdA*V1*V1

.. increase speed 10% ..

2)mg*1.1*V1 + rho*CdA*1.1*V1*1.1*V1

Then losses will increase ~ 15%

.....
Of course if you just bleed off some speed before the crest this arithmetic can be ignored.

A better starting point would be where 25% is drivetrain and road friction losses.

 

[SageBrush]

A better starting point would be where 25% is drivetrain and road friction losses.

At what speed ? By the way, I wrote the drag equation wrong. Should be

m*g*CCR + 0.5 * Rho*CdA*V*V

Terms cancel leaving the same result

 

[jerry33]

At what speed ? By the way, I wrote the drag equation wrong. Should be

m*g*CCR + 0.5 * Rho*CdA*V*V

Terms cancel leaving the same result

An assumption would be that you crest the hill at 50-55 mph (80-90 km/h)

 

[Hugh Mannity]

One immediate thing I noticed when I test drove a Model S is that the regen braking is much more aggressive than my Volt...even in L which, in the Volt is just a way to get more regen.

Yeah that ^^ The Leaf has two regen settings as well and even the more aggressive of the two is nowhere near as much as a Tesla

 

[Villa-Lobos]

From my experience driving up the Mountain road in Whitefish to the ski area it was a 5 mile drive gaining 2200 feet of vertical. In the winter at 25f it took 20 miles of range to get there. On the way down I gained 4 miles at the same temp. Have yet to do it in the summer at say 72f but I am guessing maybe it takes 15 miles to get up the same elevation with a similar gain of 4 miles on the way down. So figure for every 4-5 miles of rated miles on the way up you use you gain one on the way down.

 

[ThosEM]

Wow. Much ado about something simple. The question the OP wants to ask is: how efficient is regenerative braking? Forget about hills and up and down and all that. The answer is in general, normal driving conditions, including hills (not the most extreme ones) and regular braking (not race track level) regen is in my experience somewhere around 50% efficient.

How did you measure or infer that? Do you have data?

I would claim it should be about 90% or more, based on the likely generation efficiency of the generator-inverter. Why would it be less efficient in regeneration mode than in drive mode.

The OPs data as interpreted above, suggest that there is little difference (6 out of 270 WH/mi) between driving on the level and driving through hilly country with no net elevation change. That agrees with other user experience including my own, and requires very high efficiency of the regenerator mode.

 

[abasile]

Right. My reasoning for posing this question is that I will likely be the first in my (remote, mountainous) community to own a pure electric vehicle. I am posting a couple of essays in the local paper sort of evangelizing the coming of the long(er) range electric car. Our round trip commute to town is about 150 miles, so the Bolt and the M3 are really the first "affordable" solutions for making that run without having to worry about charging.

Something for mountain residents (or those with weekend cabins, etc.) to keep in mind is that you may need a somewhat larger battery to comfortably make a round trip from high elevation, down to low elevation, and back up to high elevation. That is in comparison to the more typical, easier scenario where a "flatlander" gets a full charge, visits the mountains, and then descends back home.

The reasons for this are as follows:

1. If you start down a mountain with a high state of charge, the battery won't have "room" for regenerative braking. Not only is this bad for efficiency, it means more brake wear, and on a large mountain your brakes could become quite hot since there's no such thing as engine braking in an EV. Further, when you do leave room in the battery, you can't necessarily count on getting a full charge from regen because battery charge rates have to be tapered as the battery gets close to full. So you're generally forced to work with less overall energy for the roundtrip.

2. Your battery will generally be colder if it's spending most of its time at a higher elevation. While Tesla battery packs do benefit from some heating while charging, and can be pre-heated, you're still generally working with a colder battery. Colder batteries hold less energy and do not allow as much regenerative braking. (This is a much bigger issue in a Nissan LEAF due to its lack of thermal management on the pack. When I take my LEAF "down the hill" with a cold battery, I'm forced to rely almost exclusively on the brakes and have to stop on the way down to let them cool.)

Therefore, with a 150 mile round trip from high-->low-->high elevation, I'd recommend paying extra for a larger battery. The base model with the 215 mile battery may not be enough for comfort, especially after several years and a bit of degradation. More like 250+ miles would be ideal, unless of course you have easy access to charging while down the mountain.

 

[CoastalCruiser]

Good points abasile! Also serves as a reminder that regen works because the motor is under load. The motor is under load because it's doing work. It's doing work because it is generating electricity to charge the battery. If the battery is full (near full) than the is no work to do. Thus no load. Thus no axillary braking.

Yet another nuance of electric cars.