Is kWh a good measure for battery capacity?

[ItsNotAboutTheMoney]

nerd content is high

I'm sure you meant "The Nerd Content is strong with this thread." (Or maybe that was a reference and you just went to 11 on the nerd control).

kWh is fine for capacity, just capacity isn't perfect for expressing range. But with the simplified relationship (capacity ~ range ~ performance ~ charge rate ~ 1/discharge rate) it does give some very strong hints about the vehicle.

 

[deonb]

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

I like base 2 - doubling and halving are one of the most natural mathematical constructs. On the downside, base 2 is cumbersome to use due to its verbosity.

HOWEVER, base 8 and base 16 are trivially convertible into base 2, and still is human-relatable, making either of those even better than base 12 .


I personally far prefer cm over inches, due to conversions to other things, but there is one thing that inches has for it - which is when you start halving things:
1inch - 1/2 inch - 1/4 inch - 1/8 inch - 1/16 inch - 1/32 inch - 1/64 inch

When you work in cm's you get:
1cm - 5mm - 2.5mm - 1.25mm - 0.625mm - uhhh... 0.3125mm -- uhhhh 0.15625 - I give up.


This isn't cm's fault. It's decimals fault. If we had base 16 (and yes 0x10 mm in a cm, and 0x100 cm in a meter), it would provide the best of both worlds.

 

[ThosEM]

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.

There are other energy losses than the intended or monitored ones. That's what leads to inefficiencies, and I think it is not that hard to accommodate that complexity in ones thinking. But if you head west up I70 from Denver I suspect you can indeed draw something like 85 kW for at least a substantial fraction of an hour and correspondingly flatten your battery. What is this about a coulomb of charge? I'm curious if there are other limits of which we aren't aware...

 

[Johan]

There are other energy losses than the intended or monitored ones. That's what leads to inefficiencies, and I think it is not that hard to accommodate that complexity in ones thinking. But if you head west up I70 from Denver I suspect you can indeed draw something like 85 kW for at least a substantial fraction of an hour and correspondingly flatten your battery. What is this about a coulomb of charge? I'm curious if there are other limits of which we aren't aware...

The C is not for Coulomb in this setting. It is for a constanct factor C which means discharge rate. 1C = draining the battery in one hour = for an 85kWh battery it means 85kW of power, 2C would be 170kW and so on.

 

[Peter_M]

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.

Looks like you missed my sarcasm. Anyway, saying not to use kWh because it's similar to kW is like saying not to use km/h because it looks similar to km, or not to use degrees C because it looks similar to degrees F. Different units are different...

Umm, say what?

It's an approximation - something most people can relate to.

- - - Updated - - -

... Because you just cannot draw constant 85kWh ...

I think you meant kW :wink:

 

[stopcrazypp]

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.

It's a decent approximation, although to get a exact answer in usable energy you need to factor in the resistive losses (the battery does contain enough energy for 1 hour of 85kW discharge, it's just part of that 85kW goes to heat and not electricity). But all units will be subject to the same issue.

 

[keithz]

Two points:

1) Here in North America we pay electric bills according to kWh. So it's an easy way to calculate bills for most people by multiplying their utility rates by "tank size" to get a rough cost of each "fill". For example, if I know that my car takes $60 in gas and I get 400 miles. I can compare that to an S85 and say that at peak, the S85 would cost me $17 to fill and at worst mileage would get me at least 200 miles. So that means 400 miles would cost me $34 or just over half of what I pay for gas now. And that's at peak.

2) Just like the iPhone ended debating of phone specs. I believe you'll get there with electric cars. The user experience will be so positive that the only thing that will matter is EPA rated range. Acceleration won't matter because it will be good anyway. Top speed won't matter, to anyone except autobahn drivers, and even there Tesla will have it. And fuel economy won't matter because the public is already understanding that EVs are cheap to fuel. So the only datapoint to cause discussion is range. And a shopper will simply be able to say x battery capacity yields y range. They'll consider their needs and buy the battery capacity accordingly. Not all that different than shopping for capacity on an iPhone.

 

[ThosEM]

I like to invoke the EPA figure of 33.7 kWh as the energy content of 1 gal of gas. So the 85 kWh Model S battery holds the electric equivalent of about 2.5 gal of gasoline and costs a similar amount to charge, depending on local prices. On that energy, it will go 250 miles instead of 62.5 miles (@25 mpg). This approach has the benefit of being easy to relate to experience with an ICE powered car, and it becomes immediately apparent how great are the savings of high efficiency electric motors and automobiles.

 

[WarpedOne]

So it's an easy way to calculate bills for most people by multiplying their utility rates by "tank size" to get a rough cost of each "fill".

And such result is plain wrong. The real cost is much even multiple times higher and depends on many uncontrollable factors like temperature, line amperage, etc.

I believe you'll get there with electric cars. The user experience will be so positive that the only thing that will matter is EPA rated range.

Nope. Forums are full of 60kW batteries and 320kWh motors ...

 

[Al Sherman]

Nope. Forums are full of 60kW batteries and 320kWh motors ...

Yes. This is clearly true. However, the "nerd content IS high" in this thread. Most people want to know how far the car will go, and how much that will cost compared to their ICE car. When I try to sell a Model S I tell folks that the EPA says 265mi, but I routinely get over 300. I also tell them that if they drive VERY aggressively they'd be hard pressed to get less than 180 miles.

It's just too confusing for the average person. Even "smart" people confuse energy and power. I have a buddy who is relatively smart and pretty knowledgeable on a wide range of subjects. He constantly tells me that my car is capable of 114 HP because he confuses energy with power.

We (and Tesla) have to sell this car (and future EV's) to the masses. They simply don't want to, or are incapable of minutiae. If they want to eventually try to understand, they can. For now they want to know; how far will it go, and how much does that cost.

 

[brianman]

We (and Tesla) have to sell this car (and future EV's) to the masses. They simply don't want to, or are incapable of minutiae. If they want to eventually try to understand, they can. For now they want to know; how far will it go, and how much does that cost.

How about:

0-60 in 4.4
200+ miles for sane climates
Your grandmother can do both in this car.
Name another car that can say that.

 

[WarpedOne]

@sherman:
Agreed.

 

[yme]

Yeah, I know it is a standard way of doing it, but is it good?

What would be much better is representing the capacity in the right unit - Joules.

60 kWh battery should be advertised as 216 MJ battery and 85 kWh battery should be advertised as 306 MJ battery.

I like this idea.

Also, energy divided by distance is force. So, instead of measuring "gas mileage" in Wh/mile (or kJ/mile), you could measure it in pounds or newtons. The interpretation is that it's the force with which the wheels drive the car forward against air resistance, rolling resistance, and, when accelerating, inertia. (Energy used for other purposes, like heating, cooling, power steering, power brakes, and running the computers, can't really be interpreted in this way, although the units still work out.) Possibly, this is slightly silly, but I still like it.

 

[ElSupreme]

I like this idea.

Also, energy divided by distance is force. So, instead of measuring "gas mileage" in Wh/mile (or kJ/mile), you could measure it in pounds or newtons. The interpretation is that it's the force with which the wheels drive the car forward against air resistance, rolling resistance, and, when accelerating, inertia. (Energy used for other purposes, like heating, cooling, power steering, power brakes, and running the computers, can't really be interpreted in this way, although the units still work out.) Possibly, this is slightly silly, but I still like it.

MPG is really just area. Lots of times you use units that make sense, rather than reducing them down to simplest terms.

Droppings