Is kWh a good measure for battery capacity?

[djp]

I love this thread... Thought calories is easier to understand for most people without scientific degrees. 60 kWh battery has approx 52000 kcal.

If we're going to simplify it, how about in terms of houses or days of food?

The typical American male consumes about 2,600kcal per day so the 60kWh battery is the equivalent of 20 days of food.

The typical house consumes 2kW of electricity so a 60kWh battery can power a house for 30 hours, or 42.5 hours for an 85kWh battery.

The Model S Performance can draw a peak of 310kW or the equivalent energy draw of 155 homes.

 

[ItsNotAboutTheMoney]

Unless people discuss electricity use in kJ, the capacity is in kWh.

The real answer is "Of course I meant capacity, if I were discussing performance I'd be talking in Newtons."

 

[Johan]

The 85kWh battery holds 73 135 kcal and will from now on be the "135 Big Macs battery" in my book.

 

[hiroshiy]

@Johan: that sounds small! In Japan Big Macs cost $3.4 each, so 135 Big Macs will cost $459... wait, that's way more expensive than electricity!

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[Johan]

@Johan: that sounds small! In Japan Big Macs cost $3.4 each, so 135 Big Macs will cost $459... wait, that's way more expensive than electricity!

Sent from my HTC One using Tapatalk 4 Beta

In Norway it's the equivalent of over $7. The world's most expensive Big Mac actually.

Big Mac Index - Wikipedia, the free encyclopedia

Purchasing power is high here and the Model S relatively "cheap".

 

[doug]

kWh is one of the more silly units of measurements in the world. It's basically k(J/s)h. Or kilojoules-per-second hour.

Or kJ h/s. What this really illustrates is that we didn't go far enough with the metric system. Too bad we never adopted that decimal clock.

Or better yet, we should have converted to base twelve.

 

[WarpedOne]

Sorry for reviving this thread but an idea crossed my mind that has to be mentioned here: There is another rather common designation of battery capacity - Ampere Hours or Ah.
We know that:

* 60 kWh battery tops off at 352 V
* 85 kWh battery tops off at 402 V

Also there is a sticker on the batteries saying 350VDC and 400VDC. Quick calculation gives 170 and 210 Ah of MIBC (Meaningless Indication of Battery Capacity).
Still worlds better then reading about 60kW or 85 kW Model S...

Coincidently those Ah numbers are also rather close to miles of range that you can count on i.e. in very bad circumstances you'll get at least that range.

 

[jerry33]

Or kJ Or better yet, we should have converted to base twelve.

Bingo. Base twelve is a far better base than ten. Math just becomes easier to do because fewer "places" are required, division has multiple factors (1,2,3,4,6,8,10,12 compared to 1,5,10), repeating decimals are less frequent. It's just a more human base, unfortunately some dull person counted on his fingers and the decimal system was created.

 

[Reykjavik]

If we are going to change number systems, I think hexadecimal is an option worth looking at. Computers do the heavy lifting as far as numbers are concerned, so adapting to use a more computer friendly system has merit.


I do think measuring energy in Joules is a good option, and we should do the same for gas tanks. At first ICE cars will look amazing with their massive energy densities, but then when you look at J/mile, the EVs will blow them away completely.

 

[doug]

Bingo. Base twelve is a far better base than ten. Math just becomes easier to do because fewer "places" are required, division has multiple factors (1,2,3,4,6,8,10,12 compared to 1,5,10), repeating decimals are less frequent.

I agree, but lets not get carried away. The factors of 12 are 1, 2, 3, 4, 6, and 12 (not 8 and 10). But yeah, a third could be expressed as 0.4 (instead of 0.333333...).

Another admittedly esoteric argument for base twelve is that the sum of all the natural numbers (i.e., 1+2+3+4+5+6+7+...) is equal to -1/12. (This is a controversial result for some but is required, for example, in string theory.) So in base twelve, the sum of all positive integers then would equal -1/10 or -.1, as if that the base the universe prefers.

I do think measuring energy in Joules is a good option, and we should do the same for gas tanks.

I'd prefer we just used Joules. It is our proper SI (mks) unit for energy. So we'd be talking about 306 MJ instead of 85 kWh.

 

[jerry33]

I agree, but lets not get carried away. The factors of 12 are 1, 2, 3, 4, 6, and 12. (8 and 10 are not.)

Another admittedly esoteric argument for base twelve is that the sum of all the natural numbers (i.e., 1+2+3+4+5+6+7+...) is equal to -1/12. (This is a controversial result for some but is required, for example, in string theory.) So in base twelve, the sum of all positive integers then would equal -1/10 or -.1, as if that the base the universe prefers.



I'd prefer we just used Joules. It is our proper SI (mks) unit for energy. So we'd be talking about 306 MJ instead of 85 kWh.

Right, 8 and 10 are multiples of the factors.

 

[lolachampcar]

nerd content is high

 

[DrComputer]

But if everything changes, how are we going to order a $5 foot long at Subway for lunch at noon while reading a contract printed on legal paper?

 

[Reykjavik]

Right, 8 and 10 are multiples of the factors.

10 is not.

 

[ThosEM]

Maybe this confusion about units is the reason Tesla shows miles of range as the unit of stored energy, despite how indirect that can be? I personally like to claim that my Model S features the power of time travel, 1.21 GW.

 

[jerry33]

10 is not.

2 x 5 = 10. I wasn't implying that you could only use the factors in multiplication.

 

[brianman]

Base twelve is a far better base than ten.

Religion and mathematics might agree on this one...

If we are going to change number systems, I think hexadecimal is an option worth looking at. Computers do the heavy lifting as far as numbers are concerned, so adapting to use a more computer friendly system has merit.

... but computer science might disagree.

Maybe just go minimalist (base 2) and build from there.

 

[VolkerP]

I personally like to claim that my Model S features the power of time travel, 1.21 GW.

Who needs stinkin parking assist. I travel to the past, just halt in the desired spot, then warp back to the future until the city is built around me. Doesn't work in some places in Netherlands, though. Too wet.

 

[Peter_M]

To the original question, why not use the root units? 85 kWh = 3.06 x 10[SUP]8[/SUP] kg m[SUP]2[/SUP]/s[SUP]2[/SUP]. Yes, I'm being sarcastic. You could use Joules, Watt-seconds, Coulomb-Volts, Newton-metres or Kilowatt-hours. All are valid, but kWh is what people relate to because it's what's on their electricity bill, and because they can relate it to a kilowatt (say ten 100-watt lightbulbs) running for an hour, or for that matter, a Model S running at 20kW on the highway for 1/20 of an hour (3 minutes). As for people saying kW when they mean kWh or vice versa, well they should just get it right :wink:.

 

[WarpedOne]

why not use the root units?

Because it is not the unit that is wrong, it is it's similarity with other unit that people have heard before and use it instead. And that happens to be totally wrong and nonsensical.
My humble suggestion is to use a unit that is not similar to anything.

a Model S running at 20kW on the highway for 1/20 of an hour (3 minutes).

Umm, say what?
Even such simple math is just wrong because you use at least 100kWh when charging a 85kWh battery from empty to full, maybe even more.
And 85kWh battery won't last for an hour if 'you' draw' constant 85kW from it. Because you just cannot draw constant 85kWh and because at 1C discharge its capacity is not 85kWh anymore.
Things are complicated thus such simplified math gives no usable answer. Its results are just wrong, simple but wrong.