Tesla Roadster - you can't have your cake and eat it

unkel

Just happened to browse their website and decided to put some of their figures into Excel as they looked a tad optimisitic to me. Here's the score:

The car uses 6831 batteries with a rating of about 2000mAh. That is 13662Ah of storage. The batteries are rated 3.8V and the battery pack in total is 375V, so the battery pack is good for 13662*3.8/375 = 138Ah which multiplied by the voltage of 375V gives it a storage of 51kWh. This is consistent with the figures Tesla provide here. So far so good.

Tesla claims the car has a range of 220 miles with mixed driving. Let's not even use mixed driving. Let's use low (constant) speed motorway cruising at 55mph (88km/h), which is much more efficient than mixed driving. So we're stacking the odds in favour of the Tesla.

At 55mph the car should be able to drive for 4 hours (220/55). So over those 4 hours it has used up the full battery capacity of 51kWh, using a constant power of just under 13kW (51/4). This is the equivalent of 17BHP. That's not a lot of power for a sports car

Using the full 185kW the engine is rated for according to Tesla and taking off the speed limiter and the calculated range is 15 miles (13*220/185). Taking into account air resistance and the range will me no more than a handful of miles :eek:

And my calculations are simplistic. I'm not even taking into account the fact that there are inefficiencies in the engine, drive train etc.

BTW to recharge a 51kWh battery using a standard 13A 220V socket would take 18 hours (51/(220*13/1000))...

Are my calculations wrong? If not, why hasn't the press picked up on this? The figures for electric cars in general, and the Tesla Roadster in particular, just do not add up

colm_mcm

if the car is doing a constant speed, why would bhp come into it, as long as it can maintain the speed?

L-M

I think that Tesla was more of a "look, us electric cars aren't that boring after all". I think practicality is the last of there worries.

Just another mile stone, nothing to see here, move along :P

zilog_jones

I don't see what's so unreasonable about that, maintaining 55 MPH in 5th gear in a normal car shouldn't require much power either.

The only thing I see wrong in your calculations is that our mains is 230V, so it would be closer to 17 hours. Still a lot though - can't be right surely?

Guy:Incognito

Nothing that cant be sorted by a long pole and overhead power lines.:)

leeroybrown

I think they supply a high current charging unit (70A) that allows the unit to charge three or four times faster. Just don't turn on the tumble dryer, oven or electric shower at the same time...

Seth Brundle

unkel wrote: »

And my calculations are simplistic. I'm not even taking into account the fact that there are inefficiencies in the engine, drive train etc.

the engine?

Anan1

kbannon wrote: »

the engine?

Ok, the motor.;)

zilog_jones

leeroybrown wrote: »

I think they supply a high current charging unit (70A) that allows the unit to charge three or four times faster. Just don't turn on the tumble dryer, oven or electric shower at the same time...

IIRC the main fuse in some houses (older installations I guess) is 70 or 80A! :eek:
You'd need a separate mains supply alltogether so - I'm sure that'll be cheap...

air

A well thought out post like the original one based on fact and reason is most welcome.

Having said that you are actually way off the mark and as Zilog says, maintaining 55mph does not use much energy.

In fact 13kW sounds generous to me for a small, lightweight car with a (presumably) low drag coefficient, travelling at 88km/h.

Ballpark example
One litre of diesel has approximately 12.44kWh of energy.
ICE cars are generally accepted to be about 30% efficient - you get 1/3 motive power 2/3 waste heat (split between exhaust & radiator primarily).
So we see that the car outputs approx 4kWh of mechanical energy per litre of diesel consumed.
Take your average small modern efficient diesel car with a fuel consumption of 5L / 100km.
This equates to roughly 20kWh of mechanical energy consumed per 100km.
The 220miles quoted equates to approx 354km which gives a figure of 70.8kWh of energy used for that journey - not a million miles away from that quoted by Tesla.

air

zilog_jones wrote: »

IIRC the main fuse in some houses (older installations I guess) is 70 or 80A! :eek:
You'd need a separate mains supply alltogether so - I'm sure that'll be cheap...

They're probably talking about a US fuse at 110V so it'd likely be a 32A circuit over here - not unheard of in domestic houses.
IIRC the chargers use 220V circuits in US houses though (they have 220 to certain high load circuits normally) so I cant be sure.

air

unkel wrote: »

Using the full 185kW the engine is rated for according to Tesla and taking off the speed limiter and the calculated range is 15 miles (13*220/185). Taking into account air resistance and the range will me no more than a handful of miles :eek:

IIRC the Bugatti Veyron runs out of juice in 12mins or something at full bore, so not entirely unprecedented.
Granted you'd get a bit more than 15miles in a Veyron at full bore for 12mins

zilog_jones

Yeah from what I saw it was 70A @ 220V, so what's that, 67A @ 230V?
And on closer inspection, 63A is the more common standard for the main fuse in Irish households. I guess American households are designed for a lot more current with their ACs, massive hot water tanks, massive washing machines, etc.

leeroybrown

The electric motor should actually yield an extremely high battery-to-wheel efficiency.

The high-speed range calculations above definitely aren't correct. They don't stand up to a simple dimensional analysis. Also, the efficiency of the motor to burn electricity won't be increased by drag but the ability of the car to accelerate using that power will be decreased.

At maximum power usage the motor will still last for:

(51kW/hr) / (max power draw) * 60 minutes

Assuming only 80% internal (not to wheel) efficiency that would still be 13 minutes which at the limiter speed would make the range around 30 miles. With the limiter off it should be better. I'd suggest that realistically the 185kW (presumably dyno flywheel equivalent) figure would be achieved slightly more efficiently than that possibly pushing up another bit.

Zube

zilog_jones wrote: »

I guess American households are designed for a lot more current with their ACs, massive hot water tanks, massive washing machines, etc.

No, most of their stuff is rated at 110V. Only ovens and A/C are wired to the 220V supply.

zilog_jones

Wikipedia article claims 90% battery-to-wheel efficiency, but I'm not sure where they've sourced that from

leeroybrown

zilog_jones wrote: »

And on closer inspection, 63A is the more common standard for the main fuse in Irish households.

Yeah. Most Irish domestic houses are powered via a single 63A fuse/breaker. Assuming idealised power factor of 1.0 that only gives you 14.5kW to play with.

Mr Benevolent

17hp is easily enough to keep a car moving at 88kph. A petrol engined car will use around 20% of its bhp output at motorway speeds. Yes, the Roadster will only go for 15 miles... if you keep it floored.

A car engine is only inefficient when it's accelerating. That's why urban mileage is so much worse than motorway mileage. Toyota figured this out when they developed the Prius. The engine they used is modified to used a set of fixed revs that are efficient as possible. The problem with this is that fixed revs are not compatible with manual gears - they'd make it nasty to drive, so they used a modified (epicyclic?) auto box that solved the problem.

So yes, your figures are way out. Understandable mistake though.

Zube

Confab wrote: »

A car engine is only inefficient when it's accelerating.

This is so wrong that it's hard to imagine what you were trying to say.

robtri

no the tesla has a crap range, 15 miles sounds around right... dricving it like a lunatic...
anyone catch the top gear show when the tested it.... didn't last too long before the power ran out.....

any don't forget what the handling in the tesla must be like with all that battery weight...

air

Just watched the TG review on AOL Video - http://video.aol.com/video-detail/top-gear-reviews-the-tesla-roadster/4052367588
Jeremy mentioned 55 miles range on their track which doesnt sound bad to me assuming that involved driving like that in the review.
If the batteries were swappable & it was cheaper (which should come in time) it looks great.

leeroybrown

robtri wrote: »

anyone catch the top gear show when the tested it.... didn't last too long before the power ran out.....

The show where they faked it running out of power? I'm no fan of the thing but it does have a longer range than they made it look.

Matt Simis

air wrote: »

In fact 13kW sounds generous to me for a small, lightweight car with a (presumably) low drag coefficient, travelling at 88km/h.

FWIW:

Comparing the Tesla S (the newer 4 Door saloon) to Tesla Roadster
The 300-mile range is possible (vs the Roadster's 244-mile range) because the S has 8,000 battery cells vs. 6,000 in the Roadster, the batteries have been improved in mass and volumetric performance, and there is more advanced cell chemistry in each cell, and the S has a cd of about .27 vs. the Roadster's drag coefficient of .35.

http://www.autobloggreen.com/2009/03/26/tesla-model-s-50-000-ev-sedan-seats-seven-300-mile-range-0-6/
A Cd of .35 isnt very good (but it is a roadster, so what can you do).

For comparison:
0.27 Nissan GT-R 2008
0.27 Toyota Camry Hybrid 2007
0.27 Tucker Torpedo 1948
0.27 Volkswagen Passat B5 (sedan) 1997
0.27 Mercedes-Benz S Class 2000-2005
0.26 Hotchkiss Gregoire[citation needed] 1951
0.26 Lexus LS 430 2001-2006
0.26 Lexus LS 460 2006
0.26 Mercedes-Benz W221 S-Class 2006
0.26 Opel Calibra 1989
0.26 Toyota Prius 2004-2009
0.25 Audi A2 1.2 TDI 2001
0.25 Honda Insight 1999, 2003, 2005
0.25 Toyota Prius 2009
0.24 Mercedes E-class coupe 2009
0.212 Tatra T77 1935
0.195 General Motors EV1 1996

0.35 puts it in line with:
0.35 BMW Z4 M coupe 2006
0.35 Dodge Viper GTS 1996
0.35 Jaguar XKR 2005
0.35 Lexus RX 2003-2009
0.35 MINI Cooper 2008
0.35 Tesla Roadster[9] 2008
0.35 Toyota MR-2 1998
0.35 Toyota Sequoia 2007
0.35 Volvo 940 (sedan) 1990

robtri

leeroybrown wrote: »

The show where they faked it running out of power? I'm no fan of the thing but it does have a longer range than they made it look.

faked it running out of power?? haven't heard that one, any reason why or links?

Cuddlesworth

If you are going to drop money on a electric car, why would you charge it out of a 63amp fuse?

Gregor Samsa

robtri wrote: »

faked it running out of power?? haven't heard that one, any reason why or links?

http://www.guardian.co.uk/media/2008/dec/24/jeremy-clarkson-top-gear-tesla-electric-car

air

Thanks Matt, yes I had no idea of the coefficient - hence the "presumably". I guess the same tech in a longer smoother shape would be a better job alright. I still think the power output inferred by Telsa for cruising speed sounds reasonable though.

Carawaystick

What's the range when the heating is on????
or do you only drive it while the temperature is just right, so no need for aircon either???

This is the biggest issue with battery powered cars.
fuel cell will have greater waste heat to heat the interior as well as much greater range.

Yakult

Honda FCX Hydrogen fuel car ftw

enda1

Battery technology is developing quickly. To reach the energy density of petrol/diesel is not needed due to the inherent inneficiencies of internal compustion engines, so an energy density of ~ 1/3 is required.

So to achieve this type of energy density we are llking at around the 2030 mark when we will have higher energy density and higher energy/mass batteries which reach the level of current electric engines.

Motors with electrical to mechanical efficiencies of 98% at 100kW exist also so that hurdle is easily overcome.

In terms of fuel cells, the main technological barrier is in the storage of hydrogen. This is very difficult!
No one has yet ot find the optimal technologies and we need breakthrough technologies such as solid oxide storage perhaps to make the storage of hydrogen feasible in a domestic car. Current technologies of liquid/gas storage are too heavy and costly and dangerous and have too low evergy densities.

Elessar

enda1 wrote: »

Battery technology is developing quickly. To reach the energy density of petrol/diesel is not needed due to the inherent inneficiencies of internal compustion engines, so an energy density of ~ 1/3 is required.

So to achieve this type of energy density we are llking at around the 2030 mark when we will have higher energy density and higher energy/mass batteries which reach the level of current electric engines.

Motors with electrical to mechanical efficiencies of 98% at 100kW exist also so that hurdle is easily overcome.

In terms of fuel cells, the main technological barrier is in the storage of hydrogen. This is very difficult!
No one has yet ot find the optimal technologies and we need breakthrough technologies such as solid oxide storage perhaps to make the storage of hydrogen feasible in a domestic car. Current technologies of liquid/gas storage are too heavy and costly and dangerous and have too low evergy densities.

The Tesla is impressive but in order for electric cars to really take off, batteries need to improve drastically. Some rather big R&D will have to be done on higher storage capacitor batteries or something similar. You would really want to be able to drop into a "petrol" station and charge the batteries in no more than 5 minutes like conventional cars.

With current lithium technology I cannot see electric vehicles really taking off.

air

The idea behind current plans for EV infrastructure as I understand them is that you will be able to pull into a refuelling station and have your battery swapped out by a mechanical arm in a similar length of time to that which it takes to fill a tank of petrol. You then continue on your merry way having paid for the swap.

woody33

How about liquid electricity? Refill your battery like your petrol tank.

http://peswiki.com/index.php/Directory:Vanadium_Redox_Batteries

air

Terrible volume to energy ratio unfortunately so not suited to mobile applications, great for standby storage etc though.

unkel

air wrote: »

The idea behind current plans for EV infrastructure as I understand them is that you will be able to pull into a refuelling station and have your battery swapped out by a mechanical arm in a similar length of time to that which it takes to fill a tank of petrol. You then continue on your merry way having paid for the swap.

I suppose that can only work if the batteries come with a lifetime guarantee?

Also, all EVs need to have the exact same battery pack?

And the battery packs weigh half a tonne at present?

Call me a cynic, but I feel by the time these issues are sorted, the world has moved on from electric cars...

And I'm still driving V8 petrols :cool:

air

I think the idea is that you dont own the battery at all, as I said, the idea is that it's swapped at the filling station, not refilled.
It's not true that every EV would need an identical battery pack but certainly some sort of modular standard would have to be established whereby the battery pack would be made up of individual units.
This would also address the issue of handling by splitting them up into more manageable chunks. In any case I dont think it would be beyond modern engineering and robotics to swap a half ton pack if that were required (unlikely) - think of the weight a small forklift can shift.
Now I've not given this any thought but I'd imagine that a certain voltage may become standard for EV packs - say 300V. A certain cell size might be chosen also, say 50Ah. When you pull in you could choose how many packs to swap. Bigger cars / vehicles would simply use more of the same packs.

leeroybrown

Elessar wrote: »

You would really want to be able to drop into a "petrol" station and charge the batteries in no more than 5 minutes like conventional cars.

And that's where things get genuinely scary. If you start trying to provide that much power instantaneously then you're into power station territory.

The only way it would really work is through a combination of night-time (home) charging systems and standardised replaceable battery packs that allow high volume users to replace charge easily during the day. None of this will happen without higher capacity, smaller and lighter batteries.

For the moment (clean/cleaner) combustion provides the only logical source of that much energy for many road users.