Regenerative Breaking ... A Hypothetical Question

[Kingske]

THEORETICALLY (perfect efficiency) all you need is a downhill gradient. ...ANY gradient at all ...with any car (not just an EV) and you should be able to roll to the bottom without using ANY energy. And THEORETICALLY all you need is a final elevation that is below the starting elevation. IE: Going down a 50 ft hill will THEORETICALLY overcome any 49 ft uphill which follows. THEORETICALLY (100% efficiency), it doesn't matter what the changes in elevation are between the start and finish (as long as nothing in the middle exceeds the starting elevation). Just release the brakes on our theoretical vehicle and you will roll to the bottom... You don't need ANY energy for a theoretically perfect vehicle.

Taycan PM (Perpetuum Mobile)

Sponsored

 

[daveo4EV]

Taycan PM (Perpetuum Mobile)

it’s a $737,950 option on the Porsche Exclusive configurator - but can only be configured with the “white” PCBS calipers - painting the calipers any other color causes “losses” through the paint thermals - so most people don’t see this option “enabled” when they are configuring their vehicle…

 

[feye]

Its a fairly simple analysis (except for the details) ;-)

Lovely, but meaningless, if you do not consider the downhill driving style!

 

[evanevery]

405 wh/mile

well if there is 50 ft “downhill” and you start 49 ft “uphill” - there was some point in time that you were at true “zero” ft - and you have to account for that energy originally that got the object to “1 ft” mark…

there is no free lunch, and conservation of energy requires that you can not recieve more energy than you put in - if there is 50ft of “energy” available downhill - then there was 50 ft of energy available to raise the object to that height to begin with.

no free lunch.

and no 100% efficiency

so you can not run a Taycan/EV on uphill/downhill grades forever - at a minimum regen is NOT 100% efficient - far from it - and there are losses, and there is energy draw in the vehicle running systems you don’t control and can not be shut down (power steering, ECU, BMS, brake boost, PDCC, etc…) there are many many electrical mouths to feed in a modern vehicle - so even _IF_ the drive train is using zero power - the rest of the systems are running…

the best you can achieve is than Regen can overcome the non-drive train based power consumption - you can start at the bottom of a hill with say 75% battery - be at 50% at the top - and reach the bottom on the other side with maybe 60% battery (more than you had at the top, but not more than you had when you started). But you can never recovery all the power spent - you lose at least 2-3% the moment power is consumed “from the battery” due to heat losses and conversion inefficiency (chemical to electrical)…

Yeah... Its all about the efficiency factor here... And we really don't know what that is. However, before we introduce any consideration for inefficiency, its best to understand the theoretical physics behind the event.

A controlled experiment would probably be best to try and determine the efficiency of the vehicle. However, so many things can effect this and we simply don't have a controlled environment large enough to do this accurately. The factor will also vary significantly with the speed of the vehicle I think. So its likely not a true "constant" for any given set of environmental factors. The "precise" equation starts to expand exponentially with variables... But I do think we might be able to get somewhere in the ballpark (within 5%) for a resonably well defined scenario.

I once worked on a classified project (since declassified) to calculate combined fuel consumption for fleets of "unknown" ships based on satellite photos. I'm betting 80% efficiency would be a reasonable guess... We used that number a lot for thermo-mechanical systems... ;-)

 

[daveo4EV]

well for one - if you want to come close - we’ll have to stay below 40/50 mph on the downhill - otherwise the aero drag losses have a v^2 component - and you’re never recovering the drag losses…below 40 mph - the dominate factors are the Newtonian object physics (cost to move based on weight and friction and inertia) - leaving aero-drag out of it will make the system “simpler”…

so I’ll throw my contribution into the ring…

you can’t be rolling downhill at more than 40 mph

I agree we can’t recover all the energy - but we can minimize the losses

• stay below aero drag speeds that cause loss (40 mph)
• turn off HVAC
• wash the car, wax the car - make it as slippery as possible

 

[Mouse House]

I'll test this out and post charge data in October. My chalet is at 1700m and the Rhone valley is at about 400m, distance is 27km... so pretty much a 5% gradient all the way, though it's not that even of course.

ABRP shows the car using up 14% charge on the way up, and gaining 1% on the way down. We will see.

 

[daveo4EV]

ABRP shows the car using up 14% charge on the way up, and gaining 1% on the way down. We will see.

data will be great I simply can't wait - ABRP doesn't account for HVAC losses and such (only drive train) so I would be unsurprised at a slight level of consumption at the bottom on the hill - but when driving my Model X and Model S between SF and LA in California I often see more percentage at the end of the hill than the top of the hill - so brake-regen can effectively make going downhill "free" for the miles covered (but in reality it's just reducing the cost of going up hill)…

my very first trip with an EV from SF to LA in a 2013 Model S P85 - start at 100% battery at the bottom of the grapevine on I-5 south (it was one of Tesla's original 3 west coast locations) - got to the top of the hill and had used sooooo much battery it was clear we couldn't make it to Long Beach (my parents home) - and at that time there were not a lot of superchargers - it was a huge act of faith that we could make the distance from the top of the hill to Long Beach - we had used sooooo much battery on the up hill segment I was really really concerned…

but with regen and some careful driving it turned out the entire distance from the top of the grape vine (Gorman, CA) to Santa Clarita is basically "free" in an EV w/Regen - that returned a bit of battery - and ultimately we got to Long Beach with battery to spare…

but the battery % at the top of the hill was very very scary…it was that day that I realized this whole EV thing can actually really work - but it requires a different approach and trade offs…

 

[louv]

Real world case. No hypermiling, just normal driving.
Start: elevation: 1584 feet, SoC: 70%
Top of Mt Washington: 6154 ft, 55%
Bottom/End: 1584 ft, 61%

 

[daveo4EV]

@louv - bit more specific - I"d love to see ABRP's estimate - I know you have real data

too many mt. Washington's in the US for maps to help me without some specifics

thanks.

 

[Gogs]

Its a good question, the way I look at it is imagine a windmill on the front of your car, spinning like crazy passing charge to your battery. The windmill creates resistance to the car though so again, no free power, you use more pushing the resistance than you do generating power.

 

[daveo4EV]

ABRP - says a LR Model 3 with 18" Aero wheels will pick up 2% on this route…

 

[louv]

@louv - bit more specific - I"d love to see ABRP's estimate - I know you have real data

too many mt. Washington's in the US for maps to help me without some specifics

thanks.

THE Mount Washington.
The one with the highest recorded windspeed. (231 mph, if I recall)
The one near ME.
The one in New Hampshire, USA.
(The one that costs $35 to drive up the mountain, or you can take a cog railway car)

 

[BayAreaKen]

Real world case. No hypermiling, just normal driving.
Start: elevation: 1584 feet, SoC: 70%
Top of Mt Washington: 6154 ft, 55%
Bottom/End: 1584 ft, 61%

How far (in miles) was this round trip that only consumed 9% of your battery?

 

[louv]

How far (in miles) was this round trip that only consumed 9% of your battery?

15.6 miles round trip.

 

[wrhencke]

I'm not an engineer nor did I do any math, but I did drive a Tesla Model S for 7 years before I got my Taycan. I frequently drive over the hill from Santa Cruz County to San Jose.

I discovered that I could pick up a few miles of charge going downhill, but only if I drove gently. Hard cornering uses energy.

Sponsored