Temperature vs. Energy Use

[T@oo]

I think that the real limit on regen isn't the tire traction, it's the rate at which the battery can accept charge. This doesn't seem to be anywhere near the limit of the tires. Recall that as far as the tires are concerned, speeding up and slowing down at the same rate and initial speed requires applying the same amount of power (ignoring air drag), and the regen power is way lower than the accel power.

Of course it is. You have to account two times the internal losses of motor and PEM.

Also, I disagree with the statement that "The amount of kinetic energy that is regen-able is a lot higher for FWD or AWD than RWD." I think you're confusing power (rate of energy recapture) and energy (total watt hours put back into the battery) here.

No I'm not. I was talking about kinetic energy. The word power is nowhere to be found. The harder you "brake" the more weight gets distributed to the front wheels, the more energy you could possibly regen on the front wheels and the less on the rear wheels. Quite clear to me.

If the limit really was the tires rather than the battery, then you'd be able to put it in faster with RWD, but it for a shorter amount of time (because you'd be stopped).

Someone mentioned a 0.2g (deceleration) rate for regen for the roadster. That isn't much and nowhere near the maxium of around 0.9g on the brakes possible for a street car. During full accel they suck out north of 200 kw to feed the 185 kW motor accounting the losses. That is a about 4C discharge. I have a hard time to believe that at least a 2C charge (~ 100 kw) is not be possible if tire traction and the regen circuits wouldn't limit it. The former is a conceptual thing, the latter a cost thing.

TM puts a 45 minutes charge from zero to full into the Model S specs. Assuming this is for the small 42kWh battery, that would make it a 56 kW charge. Certainly more than the 40 kW that's on the kW gauge for the roadster for regen.

 

[bolosky]

No I'm not. I was talking about kinetic energy. The word power is nowhere to be found. The harder you "brake" the more weight gets distributed to the front wheels, the more energy you could possibly regen on the front wheels and the less on the rear wheels. Quite clear to me.

OK, fine. Then you're just wrong. The front wheels are free running during regen braking, so only the rolling friction is converting kinetic energy into heat on the front. The rolling friction is tiny compared to the 36kW regen on the rear wheels, regardless of the weight distribution (if it wasn't, the car would be inefficient when not being braked, and it's not).

Think about it this way: there's a certain amount of kinetic energy in the system, 0.5 mv^2. Energy is conserved. When the car comes to a stop, all of the 0.5 mv^2 has gone somewhere. Assuming you didn't use the friction brakes, very little of that was from the front wheels, since they're free running. The rest of it went into aerodynamic drag and to the rear wheels. Very little of it went into drag (evidence: you slow down much more quickly on regen braking than by putting the car in neutral). Consequently, most of the kinetic energy was transferred through the rear wheels. Some (most) of that went into the battery, and the rest was lost in electrical or mechanical inefficiencies in the transmission/motor/PEM/wiring, etc (and turns into heat).

This analysis doesn't depend on the weight distribution and only relies on time to the extent that it requires that regen braking slows you down a lot faster than drifting. Consequently, the fraction of the kinetic energy that you can recover isn't affected by the weight distribution to the extent that it slows you down faster than coasting to a stop.

QED

PS. I think that the regen POWER could depend on weight distribution, because the frictional force depends on the weight applied through the tires, but by the analysis in my previous post I also don't think that it's the power limit, either.

 

[TEG]

Here is something else to consider:
In a performance car with the acceleration potential of the Roadster (and how easy it is to get up to high speed quickly), one can be prone to needing prodigious use of the brakes. That can lead to brake fade due to overheat. The brake system tends to be a bit beefier in front to handle the extra heat buildup due to front biased portioning (due to forward weight transfer during deceleration) but I don't think it totally makes up the difference during performance driving. What I am saying is that you could be more likely to overheat your front pads & rotors during spirited driving. So, in that case, having regen on the front wheels for the sake of transferring energy in the form of electricity rather than heat can be useful to avoid brake fade.

Also, max regen could be limited for reasons other than max recharge power to the battery pack. Even if, for instance, you had .9 or even .6g of deceleration traction available you don't want to unsettle the handling during off pedal coast down due to temporary traction anomalies. In other words you don't want the vehicle to just start slipping and sliding on its' own (with no brake pedal effort) when you hit a small patch of sand or ice. So they may be a little conservative with the regen for that reason. Another thing is that the car doesn't offer regen when the pack is already full, so the difference between available regen and disabled regen is less dramatic if you don't allow full regen to be so strong.

So, some of the regen behavior may be based on drivability perceptions and not purely based on efficiency optimization.

 

[T@oo]

OK, fine. Then you're just wrong. The front wheels are free running during regen braking, so only the rolling friction is converting kinetic energy into heat on the front. The rolling friction is tiny compared to the 36kW regen on the rear wheels, regardless of the weight distribution (if it wasn't, the car would be inefficient when not being braked, and it's not).

I thought it's quite clear that I was not talking about the Roadster in peticular but a hypotetic FWD EV, which (of course) would regen on the front wheels too. And I fail to see why this hypotetic EV couldn't regen much more on the front wheel as the roadster could on his rear wheels (ingnoring other limits like bat and regen circuits). That's pretty much basic physics. It's the same reason why you can brake harder with front brakes than with rear brakes.

 

[bolosky]

I thought it's quite clear that I was not talking about the Roadster in peticular but a hypotetic FWD EV, which (of course) would regen on the front wheels too. And I fail to see why this hypotetic EV couldn't regen much more on the front wheel as the roadster could on his rear wheels (ingnoring other limits like bat and regen circuits). That's pretty much basic physics. It's the same reason why you can brake harder with front brakes than with rear brakes.

Right. Like I said in the first place, you're confusing power and energy. Remember, power is energy (work) divided by time, so it's rate of energy transfer. Here I'm using "power" and "energy" in the physics sense; I mean joules and watts (1 watt = 1 joule/s).

Aside from the small effects of aerodynamic drag and rolling friction, the amount of energy you get from braking doesn't depend on how hard you brake, though the power does. There's a certain amount of kinetic energy in the car that depends on the mass and the speed (0.5 mv^2). When you go from that speed (v) to stopped, all of that energy comes out, regardless of how quickly you stop. Some of it is lost in rolling friction, some to aerodynamic drag, some to losses in the regen/battery system, and the rest lands in the batteries. If you do 0-60 in 2s or in 10s, it's still the same amount of energy once you're stopped.

If it turns out that the tire traction is the limit in regen braking (which I'm sure it isn't on the Roadster), then using all four wheels would result in higher braking power. However, you'd then stop sooner, and so cease getting energy sooner. Brake less hard, and you get energy more slowly, but for longer. Ignoring the small losses, they work out to the same energy, just different power.

 

[bolosky]

It's the heater

I did the math, and I think that the largest source of different efficiency based on outside temp is the cabin heater.

I turned the heater on full blast with the car stopped. The current draw went from 1A to 9A (after briefly going to 11A). At 375V, this 8 amp difference is 3 kW. So, if I was running it at half power (which is roughly what I usually did) then the 1.5 kW times the 40 minute drive is 1 kWh. Dividing that by the 17 mile drive is about 60 Wh/mile, or the difference between 238 Wh/mile and 298; about 25%. I haven't measured the AC, but I suspect that it's lower than the heat.

I guess one advantage of an ICE is that due to its very low efficiency, there's a ton of waste heat around that can be used to heat the cabin for free.

 

[mpt]

Cabin heater: yes; I've noticed the big difference in power consumption, I was 275-300 last couple of months and now I'm 225-250 and I'm seeing for the first time, ideal < estimated range. I accidentaly put the heater on and saw 300.

ChargeIT: when I see parity between ideal and estimate I think I'm around 245Whr/Mile, so I'm thinking that's the hard coded number in my firmware. Maybe it varies depending upon temperature?

Regen: MINI regen > Tesla regen. MINI's front wheel drive. However, I think it's more taste and TC programming. When I asked TM about adjustable regen I hear that TC mapping is the biggest problem. Makes sense I supose as MINI regen comes in slowly regardless of how quickly you release the pedel; Tesla; instant. Could be dodgy on a fast bend.

 

[bolosky]

I did the math, and I think that the largest source of different efficiency based on outside temp is the cabin heater.

This morning it was 50F or a little colder. I sucked it up and drove in without the heat or seat warmers. I got 234 Wh/mile, which seems to support my hypothesis.

 

[Alpine Driver]

I am not the right guy to participate in this discussion, but on my long wait for the roadster, I have read somewhere that regen amount is limited to 80Amps per design, which was a "number" like 1C or 2C, dont remeber, I am not familiar with that numbers. In that context, it was said that the 80A number was chosen due to battery longevity.
I can confirm that 80A number, very selldom get a very little larger number (83A, 85A, even if I fully regen at 150km/h.

 

[edo]

I am not the right guy to participate in this discussion, but on my long wait for the roadster, I have read somewhere that regen amount is limited to 80Amps per design, which was a "number" like 1C or 2C, dont remeber, I am not familiar with that numbers. In that context, it was said that the 80A number was chosen due to battery longevity.
I can confirm that 80A number, very selldom get a very little larger number (83A, 85A, even if I fully regen at 150km/h.

53000Wh/375V =140Ah... 80A seems to be about 1/2 C

In general, a 1C charging rate fully charges in an hour. (= Ah of the battery for 100% efficient systems)

 

[edo]

I did the math, and I think that the largest source of different efficiency based on outside temp is the cabin heater.

I turned the heater on full blast with the car stopped. The current draw went from 1A to 9A (after briefly going to 11A). At 375V, this 8 amp difference is 3 kW.

Good data points! I plugged them into my spiffed-up version of Straubel's spreadsheet and I get a maximum theoretical range for the roadster of 220 miles at a constant 34 mph (as opposed to a max range of 410 miles@17mph or 340 miles at the 34 mph operating point) Looks like the heater "on full blast" is about a 35% range hit in normal operating conditions...