EV power in BHP
Discussion
Is this a straight calculation of max discharge kW into BHP or is it more involved?
If its a straight calc then the M3 perf is more like 575bhp as Bjorn's latest vid shows max discharge kW at over 430
https://youtu.be/5r-BeOc83Uo?t=357
If its a straight calc then the M3 perf is more like 575bhp as Bjorn's latest vid shows max discharge kW at over 430
https://youtu.be/5r-BeOc83Uo?t=357
As mentioned one horse power is 745.7 watts, or 0.7457 kW (a kW is rather obviously 1000 watts)
In the case posted, we don't actually know where "max discharge power" is measured. I would assume given it's name, that this is power measured at the battery, and in that case it would be expected to be significantly higher than motor mechanical power, because the transmission losses, the inverter efficiency, and the motor efficiency all need to be taken into account. If we take some very typical efficiencies (97%, 95% and 96% respectively) that's a total efficiency from battery power to motor mechanical power of 88%, so for a battery power of 430 kW (576 bhp) you would expect a motor power of around 380 kW (510 bhp).
Simarly, if you measured the fuel flow out of the tank of a ICE vehicle, you would find that because of the efficiencies of that system (way lower than for an EV) for an engine with 100 kW at the flywheel, the "fuel power" would be in the order of 300 kW.
It's worth noting for an EV, absolute peak battery power is a short term effect, because voltage sag due to battery internal resistance quickly causes a reasonably large drop from that peak number to a lower but sustainable number (sustainable across a wider SoC)
In the case posted, we don't actually know where "max discharge power" is measured. I would assume given it's name, that this is power measured at the battery, and in that case it would be expected to be significantly higher than motor mechanical power, because the transmission losses, the inverter efficiency, and the motor efficiency all need to be taken into account. If we take some very typical efficiencies (97%, 95% and 96% respectively) that's a total efficiency from battery power to motor mechanical power of 88%, so for a battery power of 430 kW (576 bhp) you would expect a motor power of around 380 kW (510 bhp).
Simarly, if you measured the fuel flow out of the tank of a ICE vehicle, you would find that because of the efficiencies of that system (way lower than for an EV) for an engine with 100 kW at the flywheel, the "fuel power" would be in the order of 300 kW.
It's worth noting for an EV, absolute peak battery power is a short term effect, because voltage sag due to battery internal resistance quickly causes a reasonably large drop from that peak number to a lower but sustainable number (sustainable across a wider SoC)
Max_Torque said:
As mentioned one horse power is 745.7 watts, or 0.7457 kW (a kW is rather obviously 1000 watts)
In the case posted, we don't actually know where "max discharge power" is measured. I would assume given it's name, that this is power measured at the battery, and in that case it would be expected to be significantly higher than motor mechanical power, because the transmission losses, the inverter efficiency, and the motor efficiency all need to be taken into account. If we take some very typical efficiencies (97%, 95% and 96% respectively) that's a total efficiency from battery power to motor mechanical power of 88%, so for a battery power of 430 kW (576 bhp) you would expect a motor power of around 380 kW (510 bhp).
Simarly, if you measured the fuel flow out of the tank of a ICE vehicle, you would find that because of the efficiencies of that system (way lower than for an EV) for an engine with 100 kW at the flywheel, the "fuel power" would be in the order of 300 kW.
It's worth noting for an EV, absolute peak battery power is a short term effect, because voltage sag due to battery internal resistance quickly causes a reasonably large drop from that peak number to a lower but sustainable number (sustainable across a wider SoC)
thanks that's very helpfulIn the case posted, we don't actually know where "max discharge power" is measured. I would assume given it's name, that this is power measured at the battery, and in that case it would be expected to be significantly higher than motor mechanical power, because the transmission losses, the inverter efficiency, and the motor efficiency all need to be taken into account. If we take some very typical efficiencies (97%, 95% and 96% respectively) that's a total efficiency from battery power to motor mechanical power of 88%, so for a battery power of 430 kW (576 bhp) you would expect a motor power of around 380 kW (510 bhp).
Simarly, if you measured the fuel flow out of the tank of a ICE vehicle, you would find that because of the efficiencies of that system (way lower than for an EV) for an engine with 100 kW at the flywheel, the "fuel power" would be in the order of 300 kW.
It's worth noting for an EV, absolute peak battery power is a short term effect, because voltage sag due to battery internal resistance quickly causes a reasonably large drop from that peak number to a lower but sustainable number (sustainable across a wider SoC)
Its a good question and one without a simple answer. Back in about 2014/15 after much criticism over installed motor power v battery power (and in related topics why the DVLA often rate our cars as having a much lower power output than you'd imagine, something to do with continual power delivery) they wrote an article to talk about it.
Can't say I understood everything back then and not read it since (although found the article for you) but the comparison to ICE is largely meaningless. I think what we can say is there's plenty at typical driving speeds - not so good over time or at higher speeds but how often do we drive with 3 figure speeds? I think tis is because the torque curve is almsot the complete opposite of Petrol engines which love to be rev'd out.
https://www.tesla.com/en_GB/blog/tesla-all-wheel-d...
Can't say I understood everything back then and not read it since (although found the article for you) but the comparison to ICE is largely meaningless. I think what we can say is there's plenty at typical driving speeds - not so good over time or at higher speeds but how often do we drive with 3 figure speeds? I think tis is because the torque curve is almsot the complete opposite of Petrol engines which love to be rev'd out.
https://www.tesla.com/en_GB/blog/tesla-all-wheel-d...
Pica-Pica said:
Remember an EV car has to carry all the fuel it uses. An ICE car supplements it liquid fuel with air.
An EV also carries the weight even after the fuel (energy) has been used - an ICE gets lighter although I suspect the benefit is only really noticable in race cars.- Thinking back to my physics at school, electrons weigh 9.109×10−31 kg each (I did look it up) so I suspect every one except Rob would accept thats a neglible amount even if the cars technically get a little lighter.
Heres Johnny said:
An EV also carries the weight even after the fuel (energy) has been used - an ICE gets lighter although I suspect the benefit is only really noticable in race cars.
The electrons don't actually get used up, they just move about!- Thinking back to my physics at school, electrons weigh 9.109×10?31 kg each (I did look it up) so I suspect every one except Rob would accept thats a neglible amount even if the cars technically get a little lighter.
RichardM5 said:
The electrons don't actually get used up, they just move about!
True, but strictly speaking charged batteries are heavier than discharged ones (there's a common misconception that Einstein's E=MC^2 only applies to nuclear reactions but it applies to storage of electrostatic charge and indeed chemical storage as well). However, for a typical car battery you are looking at a weight difference of something like a millionth of a gram. 
kambites said:
RichardM5 said:
The electrons don't actually get used up, they just move about!
True, but strictly speaking charged batteries are heavier than discharged ones (there's a common misconception that Einstein's E=MC^2 only applies to nuclear reactions but it applies to storage of electrostatic charge and indeed chemical storage as well). However, for a typical car battery you are looking at a weight difference of something like a millionth of a gram. Heres Johnny said:
I think tis is because the torque curve is almsot the complete opposite of Petrol engines which love to be rev'd out.
According to the BMS of our 75D X - the slowest AWD car Tesla ever sold, even at 36% SOC the max battery output is 300KW or 400bhp. 
Personally I find it hard to believe it can sustain that kind of powerout for more than a few seconds if that, but thinking back I was able to keep up with a V8 M3 convertible going WOT up a hill till the legal limit for the road (as none of us would obviously go over 60mph)
.The M3 is rate at 414bhp, there is transmission loss but weighs some 500kg+ less. The fact it couldn't pull away from our base/slowest AWD car Tesla sells would suggest that the car can deliver the equivalent of a 400bhp+ combustion car in real life, at least to national speed limit speeds.
Similarly on a open bit of dual carriageway at WOT our X was catching a F355 Spider up to the legal limit - in fact at one-point I had to lift off as the rear bumper of the Ferrari was starting to get a bit close. The F355 is 'only' 388bhp new, so these days probably running sub 350bhp, still it does show the real life performance of EVs is pretty impressive regardless how long the 'peak' power is sustained for. When you than in factor in fuel costs of 3-4p per mile at worst it really is the best of both worlds, great usable every day performance without having to worry at all about mpg etc.
Track work is a different matter, but for public road usage even the slowest Tesla is more than quick enough, and the 'P' cars are just nuts, am surprised there hasn't been more high profile accidents - AutoPilot stuff excluded
https://youtu.be/j-uf9hf0jBk
Edited by gangzoom on Thursday 28th November 19:23
gangzoom said:
Heres Johnny said:
I think tis is because the torque curve is almsot the complete opposite of Petrol engines which love to be rev'd out.
According to the BMS of our 75D X - the slowest AWD car Tesla ever sold, even at 36% SOC the max battery output is 300KW or 400bhp. Personally I find it hard to believe it can sustain that kind of powerout for more than a few seconds if that, but thinking back I was able to keep up with a V8 M3 convertible going WOT up a hill till the legal limit for the road (as none of us would obviously go over 60mph)
.The M3 is rate at 414bhp, there is transmission loss but weighs some 500kg+ less. The fact it couldn't pull away from our base/slowest AWD car Tesla sells would suggest that the car can deliver the equivalent of a 400bhp+ combustion car in real life, at least to national speed limit speeds.
On a open bit of dual carriageway at WOT our X was catching a F355 Spider - to the point where I had to lift off, the F355 is 'only' 388bhp new, so these days probably running sub 350bhp, still it does show the real life performance of EVs is pretty impressive regardless how long the 'peak' power is sustained for. When you than in factor in fuel costs of 3-4p per mile at worst it really is the best of both worlds, great usable every day performance without having to worry at all about mpg etc.
Track work is a different matter, but for public road usage even the slowest Tesla is more than quick enough, and the 'P' cars are just nuts, am surprised there hasn't been more high profile accidents - AutoPilot stuff excluded
https://youtu.be/j-uf9hf0jBk
Edited by gangzoom on Thursday 28th November 19:21
t off a shove off the line but then wilt slightly at speed whereas an ICE is rubbish off the line but as the reva build they can go like stink- unfortunately for the ICE we tend to drive more in the EV sweet spot than the ICEsIt doesn't matter how you deliver power, once rolling you have to achieve decent power to accelerate a 2-3ton machine through the air.
How long does our 75D X sustain 300KW at sub 40% SOC? I guess I can find out by looking at the data log. But what ever the figure for public roads real life usage I would say a EV with a 300KW rated BMS output offers similar performance to a 400bhp rated combustion car.
How long does our 75D X sustain 300KW at sub 40% SOC? I guess I can find out by looking at the data log. But what ever the figure for public roads real life usage I would say a EV with a 300KW rated BMS output offers similar performance to a 400bhp rated combustion car.
Heres Johnny said:
I think you misunderstand - on an ev the peak torque is from zero whereas on an ICE it builds with speed. Power is a more complicated function of torque and rotational speed (too many beers to explain fully) - but that’s why EVs go like st off a shove off the line but then wilt slightly at speed whereas an ICE is rubbish off the line but as the reva build they can go like stink- unfortunately for the ICE we tend to drive more in the EV sweet spot than the ICEs
Depends on the ICE. Most state of the art ICE would be petrol turbo and have perfectly flat regulated torque curves. The BMW B58 is flat 1500 - 5000 rpm or something. Old school V8 ICE would be fairly flat to, my Monaro V8 is. Exotica and certainly motor bikes are more peaky.t off a shove off the line but then wilt slightly at speed whereas an ICE is rubbish off the line but as the reva build they can go like stink- unfortunately for the ICE we tend to drive more in the EV sweet spot than the ICEsAn EV would have a constant power curve above base speed which I guess would be 30 - 40 mph. So flat torque up to this with rising power, then falling torque constant power above.
So ICE would tend to catch up later if the race given enough time and space. Typically not though in road situations, the EV gets out in front and then the legal limit comes up and game over.
granada203028 said:
Depends on the ICE. Most state of the art ICE would be petrol turbo and have perfectly flat regulated torque curves. The BMW B58 is flat 1500 - 5000 rpm or something. Old school V8 ICE would be fairly flat to, my Monaro V8 is. Exotica and certainly motor bikes are more peaky.
An EV would have a constant power curve above base speed which I guess would be 30 - 40 mph. So flat torque up to this with rising power, then falling torque constant power above.
So ICE would tend to catch up later if the race given enough time and space. Typically not though in road situations, the EV gets out in front and then the legal limit comes up and game over.
Doesn’t really depend at all - no ICE delivers max torque at zero revs An EV would have a constant power curve above base speed which I guess would be 30 - 40 mph. So flat torque up to this with rising power, then falling torque constant power above.
So ICE would tend to catch up later if the race given enough time and space. Typically not though in road situations, the EV gets out in front and then the legal limit comes up and game over.
And you've gone on to explain exactly my point about the ICE catching up later which is the post about 3 above yours
Edited by Heres Johnny on Friday 29th November 04:56
https://www.semanticscholar.org/paper/An-Investiga...
Some graphs there for model S torque and power as a function of speed. These are induction motors do not sure if the permanent magnet motors are different. But you reach peak power just as that flat headline torque begins to fade. You then get a flat power band as torque drops with speed. However, there is then a second torque inflection where it begins to die off quicker than speed builds so you begin to lose power.
I guess this is due to heat building from back EMF effects?
Some graphs there for model S torque and power as a function of speed. These are induction motors do not sure if the permanent magnet motors are different. But you reach peak power just as that flat headline torque begins to fade. You then get a flat power band as torque drops with speed. However, there is then a second torque inflection where it begins to die off quicker than speed builds so you begin to lose power.
I guess this is due to heat building from back EMF effects?
Heres Johnny said:
granada203028 said:
Depends on the ICE. Most state of the art ICE would be petrol turbo and have perfectly flat regulated torque curves. The BMW B58 is flat 1500 - 5000 rpm or something. Old school V8 ICE would be fairly flat to, my Monaro V8 is. Exotica and certainly motor bikes are more peaky.
An EV would have a constant power curve above base speed which I guess would be 30 - 40 mph. So flat torque up to this with rising power, then falling torque constant power above.
So ICE would tend to catch up later if the race given enough time and space. Typically not though in road situations, the EV gets out in front and then the legal limit comes up and game over.
Doesn’t really depend at all - no ICE delivers max torque at zero revs An EV would have a constant power curve above base speed which I guess would be 30 - 40 mph. So flat torque up to this with rising power, then falling torque constant power above.
So ICE would tend to catch up later if the race given enough time and space. Typically not though in road situations, the EV gets out in front and then the legal limit comes up and game over.
And you've gone on to explain exactly my point about the ICE catching up later which is the post about 3 above yours
Edited by anonymous-user on Friday 29th November 04:56
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