[SoCalGuy]

OK, so keeping in mind Brian's comments about the performance and my own firsthand experience, it got me thinking.

When you switch in to Sport mode, the Karma uses battery and the ICE to give you all 403hp... Individually, they produce 240hp and 260hp respectively, for a total of 500hp. What I don't quite get is how that extra 100hp is supposed to be able to recharge the battery which is discharging at 240hp? Is it because you're not always using the full 403hp when in Sport mode?

Also, another related question - at what point does the Sport mode really become 'limp home' mode? If I have 3 miles battery range left, is the Karma gonna let me use that power to augment the ICE and give me the full 403hp? My guess is no, but I've read no guidance from Fisker on what the magic battery charge state is (range or %). Curious if anyone here knows.

 

[karma1]

My understanding is that sport mode does not replenish the battery, but actually drains it to provide increased performance. Then at some point, I don't know when, it changes to maintenance and neither depletes nor replenishes the battery.

 

[ct-fiskerbuzz]

OK, so keeping in mind Brian's comments about the performance and my own firsthand experience, it got me thinking.

When you switch in to Sport mode, the Karma uses battery and the ICE to give you all 403hp... Individually, they produce 240hp and 260hp respectively, for a total of 500hp. What I don't quite get is how that extra 100hp is supposed to be able to recharge the battery which is discharging at 240hp? Is it because you're not always using the full 403hp when in Sport mode?

Also, another related question - at what point does the Sport mode really become 'limp home' mode? If I have 3 miles battery range left, is the Karma gonna let me use that power to augment the ICE and give me the full 403hp? My guess is no, but I've read no guidance from Fisker on what the magic battery charge state is (range or %). Curious if anyone here knows.

It might help if you think in terms of watts (or kW) instead of horsepower (hp). They are the same unit (power, or derivative-of-energy-over-time) but "aimed at different people" as it were: kW is for electricity, hp is for motors. As it happens, there's also a scaling ratio (of about 4/3 or 3/4) depending on which way you go, so that 400 hp is about 300 kW and vice versa.

Now, the electric motors can draw 300 kW (of electric power, to produce 400 hp of "motor makes car go"). (It's never been clear to me how much heat loss there is in this process. 300.000 kW is 402.307 hp, to be more precise, but any energy lost as heat means that to get 402.307 hp "out" you have to put more than 300 kW "in"; or, equivalently, if you put 300 kW "in" you might get, say, 400 hp "out" and roughly 2.3 hp worth of heat. But let's ignore losses for now, since we hope they are small enough to ignore.)

Anyway, the 300 kW is, as you note above, a maximum (sustained anyway) power draw. Most of the time, one uses far less energy than 300 kWh per second (remember, energy is the integral of power so we have to measure the power draw for some time, like one second at a time, and talk about "energy used per unit time") to sustain speed. It's only when climbing a big hill or, as in the case of the Veyron someone mentioned elsethread, going 250 mph through air that at that speed seems more like treacle / molasses, that you need huge amounts of power for long sustained periods. (Incidentally "air drag" is a terribly complicated calculation because there are laminar and turbulent flows and you need to know the "Reynolds number" and look at turbulence boundaries and so on, but there's an approximation using "coefficient of drag" and the area of the car that's usually good enough, that uses square of velocity, so that 250 mph is four times more drag than 125 mph. Using this rule, and ignoring coefficients of drag and car surface areas, if the Veyron requires ~1000 hp to overcome 4x the drag force, the Karma should be able to sustain 125 mph with a mere ~250 hp, or about 185 kW.)

Now, the motor-generator in the Karma produces roughly 175 kW (from memory—you said "260 hp" which is closer to 190 kW). At a sustained 75 mph on an overall-flat-road, the Karma should need roughly 140 kW of electric drive (using the numbers from the earlier parenthetical aside). If the generator cranks out the full 175 (190?) kW, then all the "extra" electrical power can go into recharging the batteries. That should be at least 35 kW (175 - 140 = 35). If you were to sustain that for one hour, that would produce 35 kWh, which is more than the battery holds (roughly 20 kWh).

Using all the above numbers (which are all theoretical rather than measured), it should be possible to drive the Karma 75 mph for about 40 minutes and still go from "empty battery" to "full battery". However, this would mean running the gasoline engine pretty close to flat-out for that entire time. Turn the engine output down a bit (and/or account for various system losses) and you need more time to recharge, or you have to go slower than that.

(I had a bunch of interruptions writing this, and have another thing to get to now, so I hope there are no major glitches above. )

 

[kabalah70]

CT,
I made some calculations very similar to this in another thread, without aerodynamics, based on the fact that 95 mph max in Stealth is a battery output limitation and computed the output required to maintain 70 mph, which came up with the same or near 35kw of excess power that the generator would provide to maintain 70. I derated it and even with 15kw excess an hour twenty minute drive should full charge the battery, and guessing burn about 5 gallons of gas at the 20 mpg EPA. Considering the extra 40 miles on the battery that could be driven without burning more gas, one could make the claim that the the Karma gets 26.67 miles to the gallon. Which is my short test drive of about 10 miles in Sport cruising at 70 mph and an increase of 3 miles on the battery range would support.

 

[siliconkiwi]

Don't forget that standard lithium polymer batteries can only be charged "slowly" (technically around 0.7C) or battery life degrades. This typically results in around a 3 hour charge time to full for a lithium polymer battery. If Fisker is smart they will be moderating the charge rate in Sport mode to match this maximum charge rate.

 

[SoCalGuy]

Don't forget that standard lithium polymer batteries can only be charged "slowly" (technically around 0.7C) or battery life degrades. This typically results in around a 3 hour charge time to full for a lithium polymer battery. If Fisker is smart they will be moderating the charge rate in Sport mode to match this maximum charge rate.

hmm... that's interesting to note. I'd be happy if Fisker put that option back into the software. Thanks CT/kab for your calculus.

By the way, I was reminded of why the hybrid is the way to go. I was charging my Karma Sunday morning and wanted to meet some family for breakfast. I only had 9 miles of range left - it would've taken a few more hours to store enough juice to do the 18 mile roundtrip to/fro the restaurant. So it was great I had the gas option. This happened to me again on the freeway coming back from LA. Got stuck in bumper to bumper traffic with 6 miles left in battery - I can only imagine if I were in an EV and got stuck in traffic as my battery gauge ticked down and down...

 

[karma1]

CT,
I made some calculations very similar to this in another thread, without aerodynamics, based on the fact that 95 mph max in Stealth is a battery output limitation and computed the output required to maintain 70 mph, which came up with the same or near 35kw of excess power that the generator would provide to maintain 70. I derated it and even with 15kw excess an hour twenty minute drive should full charge the battery, and guessing burn about 5 gallons of gas at the 20 mpg EPA. Considering the extra 40 miles on the battery that could be driven without burning more gas, one could make the claim that the the Karma gets 26.67 miles to the gallon. Which is my short test drive of about 10 miles in Sport cruising at 70 mph and an increase of 3 miles on the battery range would support.

I think there is an error here. You can't get 20 kWh out of 15 kW over 1/3 of an hour. That only gives you 5 kWh.

You guys are assuming that the programming is such that the car "tries" to recharge the battery. My understanding is that it does not use the gas engine to "recharge."

Also, is the range based merely on charge remaining, or does it also consider driving style, load, etc... like the range calculator in my Mercedes. I think that the 3 mile increases seen during test drives could be the result of driving style and not actual battery charging.

 

[ct-fiskerbuzz]

Don't forget that standard lithium polymer batteries can only be charged "slowly" (technically around 0.7C) or battery life degrades. This typically results in around a 3 hour charge time to full for a lithium polymer battery. If Fisker is smart they will be moderating the charge rate in Sport mode to match this maximum charge rate.

Good point. Although at the moment the software seems to moderate it to "zero average charge rate".

hmm... that's interesting to note. I'd be happy if Fisker put that option back into the software. Thanks CT/kab for your calculus.

By the way, I was reminded of why the hybrid is the way to go. I was charging my Karma Sunday morning and wanted to meet some family for breakfast. I only had 9 miles of range left - it would've taken a few more hours to store enough juice to do the 18 mile roundtrip to/fro the restaurant. So it was great I had the gas option. This happened to me again on the freeway coming back from LA. Got stuck in bumper to bumper traffic with 6 miles left in battery - I can only imagine if I were in an EV and got stuck in traffic as my battery gauge ticked down and down...

Generally, "stuck in stop and go traffic" is not that big a problem for a battery-only vehicle (because they are lighter and lose less range to this sort of problem). And I ran out of gas (well, virtually, had to take the very next exit) in a gasoline car back in the 1980s when traffic followed by a snowstorm paralyzed the DC beltway. I hit the traffic at the merge from 270 into the Beltway (where 270, 355, and 495 all meet on the inner loop, technically I guess that's Bethesda) around 4 PM I think, and by about 6:30 PM I had gotten the one mile or so down to the Connecticut Ave exit. Short of a Mr Fusion nuclear power plant , there is no perfect solution.

Still, batteries alone are not going to do the trick for me, given recharge times and drive distances and the lack of charging infrastructure around here. The LA and SF areas in Calif, and Portland and Seattle further north, are far better about that, and still not really good enough! :s Maybe in a decade or so, with lithium-air or fluid electrolyte swaps and/or "swappable rented batteries" (a la Better Place)...

 

[mattjs33]

It might help if you think in terms of watts (or kW) instead of horsepower (hp). They are the same unit (power, or derivative-of-energy-over-time) but "aimed at different people" as it were: kW is for electricity, hp is for motors. As it happens, there's also a scaling ratio (of about 4/3 or 3/4) depending on which way you go, so that 400 hp is about 300 kW and vice versa.

Now, the electric motors can draw 300 kW (of electric power, to produce 400 hp of "motor makes car go"). (It's never been clear to me how much heat loss there is in this process. 300.000 kW is 402.307 hp, to be more precise, but any energy lost as heat means that to get 402.307 hp "out" you have to put more than 300 kW "in"; or, equivalently, if you put 300 kW "in" you might get, say, 400 hp "out" and roughly 2.3 hp worth of heat. But let's ignore losses for now, since we hope they are small enough to ignore.)

Anyway, the 300 kW is, as you note above, a maximum (sustained anyway) power draw. Most of the time, one uses far less energy than 300 kWh per second (remember, energy is the integral of power so we have to measure the power draw for some time, like one second at a time, and talk about "energy used per unit time") to sustain speed. It's only when climbing a big hill or, as in the case of the Veyron someone mentioned elsethread, going 250 mph through air that at that speed seems more like treacle / molasses, that you need huge amounts of power for long sustained periods. (Incidentally "air drag" is a terribly complicated calculation because there are laminar and turbulent flows and you need to know the "Reynolds number" and look at turbulence boundaries and so on, but there's an approximation using "coefficient of drag" and the area of the car that's usually good enough, that uses square of velocity, so that 250 mph is four times more drag than 125 mph. Using this rule, and ignoring coefficients of drag and car surface areas, if the Veyron requires ~1000 hp to overcome 4x the drag force, the Karma should be able to sustain 125 mph with a mere ~250 hp, or about 185 kW.)

Now, the motor-generator in the Karma produces roughly 175 kW (from memory—you said "260 hp" which is closer to 190 kW). At a sustained 75 mph on an overall-flat-road, the Karma should need roughly 140 kW of electric drive (using the numbers from the earlier parenthetical aside). If the generator cranks out the full 175 (190?) kW, then all the "extra" electrical power can go into recharging the batteries. That should be at least 35 kW (175 - 140 = 35). If you were to sustain that for one hour, that would produce 35 kWh, which is more than the battery holds (roughly 20 kWh).

Using all the above numbers (which are all theoretical rather than measured), it should be possible to drive the Karma 75 mph for about 40 minutes and still go from "empty battery" to "full battery". However, this would mean running the gasoline engine pretty close to flat-out for that entire time. Turn the engine output down a bit (and/or account for various system losses) and you need more time to recharge, or you have to go slower than that.

(I had a bunch of interruptions writing this, and have another thing to get to now, so I hope there are no major glitches above. )

Arrghh!! My brain hurts!

 

[ct-fiskerbuzz]

I think there is an error here. You can't get 20 kWh out of 15 kW over 1/3 of an hour. That only gives you 5 kWh.

He said "an hour twenty", i.e., 80 minutes. 15 kW times 80 minutes = 1200 kW-minutes = 20 kWh.

I think, though, that yes, it's been changed in the software to "sustain" rather than "recharge". I'd like to have an option to do the "recharge", so that I can go "full stealth mode" at end-points and "recharge" on freeways for certain trips. (An option other than riding the brakes ... not that I know whether that even works, although it might!)

I don't know if the mileage meter uses recent driving history to calculate miles-remaining, but given that almost every other car's does, it seems likely.

 

[karma1]

He said "an hour twenty", i.e., 80 minutes.

I stand corrected. Thank you.

 

[kabalah70]

Thanks CT, saved me from having to type. ;-)
What would really be nice, would be to see the instantaneous and trip average demand in KW of the motors and be able to set how much power you want from the battery and how much power you want from the ICE/Gen. Settings of 0-10 from each would be nice, with 10 meaning 100%. Or a simplier design might be a simple blend dial where it would start at one end with all ICE/Gen and all Batt on the other with a 50/50 blend in the middle. This could be set in the Command Center software, and activated with the Stealth/Sport paddle, shifting into three modes, Stealth, Sport, and User Defined (which would then use the current Command Center setting). The dash display could read the mix: 0/10, 1/9, 2/8, 3/7, 4/6, 5/5, 6/4, 7/3, 8/2, 9/1, 10/0 (batt/gen) where the Sport or Stealth word would be. Tons of possibilities for third-party development.

 

[karma1]

Thanks CT, saved me from having to type. ;-)
What would really be nice, would be to see the instantaneous and trip average demand in KW of the motors and be able to set how much power you want from the battery and how much power you want from the ICE/Gen. Settings of 0-10 from each would be nice, with 10 meaning 100%. Or a simplier design might be a simple blend dial where it would start at one end with all ICE/Gen and all Batt on the other with a 50/50 blend in the middle. This could be set in the Command Center software, and activated with the Stealth/Sport paddle, shifting into three modes, Stealth, Sport, and User Defined (which would then use the current Command Center setting). The dash display could read the mix: 0/10, 1/9, 2/8, 3/7, 4/6, 5/5, 6/4, 7/3, 8/2, 9/1, 10/0 (batt/gen) where the Sport or Stealth word would be. Tons of possibilities for third-party development.

Really? With the way my car has functioned thus far, I think this would be a bit of a stretch.

It does seem like the electric analog of the variable displacement ICE.