kwh total capacity versus usable kwh. How to compare cars

[cove3]

The i3 specs list the battery at 500 lbs and 21.6 kwh 18.8 usable. The usable number is what BMW apparently figures is what's available to drive the car before shutting the battery down. The concept seems to be that in order to extend battery life, don't let it run down past a certain level

The problem I'm having is e-Golf and the others only list one kwh which for e-Golf is 24.2 Supposedly this it total kwh, but since the battery at 700 lbs is 200 lbs (40%) heavier than the i3, it seems not much bigger than the i3 21.6 considering the added weight. I couldn't find anything about useable kwh The new MB class B lists kwh at 28, but it was revealed on a B forum that the total capacity is really 36, thus their published kwh listing seems to be usable, not total.

The other concern is how to compare companies as to whether they are equally cautious in how they conserve battery life via this "usable" measure. Couldn't one company claim more range by pushing the battery harder before shutting it down, and sweep battery life implications under the rug? That is, the i3 with it's spread of 21.6 vs 18.8 seems to be conservative on battery abuse, vs a company who might have 24.2 total but 22 useable, indicating they let they let the battery run down more in order to get more range.

Ron

 

[mindmachine]

The i3 specs list the battery at 500 lbs and 22.5 kwh 18.8 usable. The usable number is what BMW apparently figures is what's available to drive the car before shutting the battery down. The concept seems to be that in order to extend battery life, don't let it run down past a certain level

The problem I'm having is e-Golf and the others only list one kwh which for e-Golf is 24.2 Supposedly this it total kwh,but since the battery at 700 lbs is 200 lbs (40%) heavier than the i3, it seems too close to the i3 22.5 considering the added weight. I couldn't find anything about useable kwh The new MB class B lists kwh at 28, but it was revealed on a B forum that the total capacity is really 36, thus their published kwh listing seems to be usable, not total.

The other concern is how to compare companies as to whether they are equally cautious in how they conserve battery life via this "usable" measure. Couldn't one company claim more range by pushing the battery harder before shutting it down, and sweep battery life implications under the rug? That is, the i3 with it's wide spread of 22.5 vs 18.8 seems to be very conservative on battery abuse, vs a company who might have 24.2 total but 22 useable, indicating they let they let the battery run down more in order to get more range.

Ron

Then on top of your questions you can add differences in battery chemistry, as not all lithium batteries use the same chemistry and thus will perform different and have differing life expectancies.
.

 

[ultraturtle]

The naked emperor in this spot-on observation is Tesla. One of the reasons the Model S can post such incredible EPA battery ranges is that Tesla allows its owners access to the full capacity, the tradeoff being that they specifically deny any capacity warranty. From the warranty page (34) of the current Model S owners manual http://www.teslamotors.com/sites/de...ts/model_s_quick_guide_-_na_rev_d_for_web.pdf:

The Battery, like all lithium-ion batteries, will experience gradual energy or power loss with time and use. Loss of Battery energy or power over time or due to or resulting from Battery usage, is NOT covered under this Battery Limited Warranty.

That said, and to their credit, they do advise their owners to not charge to full capacity, nor run the battery flat on a regular basis. The massive battery capacity itself also limits the ability of owners to eat into a significant percentage of state of charge on a daily basis. I personally feel a great deal of sympathy for folks that need to spend more than 150 miles per day in traffic.

I much prefer the approach of BMW, Toyota, GM, and Nissan in limiting our access to the battery pack's full capacity in order to protect its longevity. I also appreciate the capacity warranties (BMW's is 70% "state of health" over 8 years/100,000 miles). You might be surprised to find that BMW's approach is about the same as Nissan's, allowing the i3 BEV to tap approximately 87% of the i3's total capacity, as does Nissan with the Leaf (a BEV), and more aggressive than Toyota and Chevy at ~81% for the REx - the Prius Plug-in and Volt (both PHEVs, like the REx) only tap ~65% of total battery capacity. To be fair, at more than double the EV range of either vehicle, the REx is significantly less likely to deplete its useable capacity on a regular basis.

That said, the NMC chemistry of the i3 battery pack is inherently longer lasting than the LMO chemistry of the Volt and Leaf and on par with the longevity of the NCA chemistry of the Prius Plug-in, which (at 11 miles EPA range) gets depleted frequently: http://batteryuniversity.com/learn/article/types_of_lithium_ion

 

[Zzzoom3]

The i3 specs list the battery at 500 lbs and 22.5 kwh 18.8 usable. The usable number is what BMW apparently figures is what's available to drive the car before shutting the battery down. The concept seems to be that in order to extend battery life, don't let it run down past a certain level

The problem I'm having is e-Golf and the others only list one kwh which for e-Golf is 24.2 Supposedly this it total kwh,but since the battery at 700 lbs is 200 lbs (40%) heavier than the i3, it seems too close to the i3 22.5 considering the added weight. I couldn't find anything about useable kwh The new MB class B lists kwh at 28, but it was revealed on a B forum that the total capacity is really 36, thus their published kwh listing seems to be usable, not total.

The other concern is how to compare companies as to whether they are equally cautious in how they conserve battery life via this "usable" measure. Couldn't one company claim more range by pushing the battery harder before shutting it down, and sweep battery life implications under the rug? That is, the i3 with it's wide spread of 22.5 vs 18.8 seems to be very conservative on battery abuse, vs a company who might have 24.2 total but 22 useable, indicating they let they let the battery run down more in order to get more range.

Ron

Yep, it can be misleading which is why I go by the EPA sticker.
https://drive.google.com/file/d/0B5LVemnP3B4sYUY4a3VkR0Z2VFk/edit?usp=sharing

I assume they actually measured the kw-hrs used over a 100 mi and if they did it would pretty representative of any i3 under the same driving conditions. Based on these numbers, using the published usable battery capacity of 18.8 kWh, I come up with these range numbers:

• Highway: 63mi

• City: 75mi

• Combined: 70mi

Notice that's not the 81mi range the BEV is advertised. Guess what, if you use the max battery capacity of 22 kWh, you get:

• Highway: 73mi

• City: 88mi

• Combined: 81mi

Looks to me like the EPA uses total capacity in their range estimates not usable!

 

[cove3]

Thanks. Very helpful. Am I correct that the 87% for the i3 you quote is simply approximately the 18.8/21.6? Which as you point out is surprisingly, for me at least, more aggressive than Toyota or Chevrolet.

If so, it seems these 3/4 car comparison web sites which all the car manufacturers have should add a line for total/usable kwh, the %, and some qualitative/quantitative ranking to be able to compare the cars. Also, something about battery technology used as the earlier poster suggested, although I would think the battery technology due to evolution would gravitate to all cars using pretty much the same technology. Is the i3, for example, markedly better or worse than e-Golf. Leaf or MB B class?

Ron

 

[ultraturtle]

Looks to me like the EPA uses total capacity in their range estimates not usable!

No. The EPA uses wall to wheel measurements. The i3 internal charger operates at about 85% efficiency, so the %15 loss looks very close to the 13% SOC BMW keeps as unusable.

18.8 kWh useable divided by 21.6 kWh total equals 87%.

33,705 divided by 124 MPGe equals 272 Wh per mile (see http://en.wikipedia.org/wiki/Miles_per_gallon_gasoline_equivalent)

272 Wh per mile times 81 equals 22 kWh, which is what is drawn from the wall.

At 85% efficiency (typical of all EVSEs) 22 kWh from the wall puts 18.8 kWh into the battery, so the math works.

 

[Zzzoom3]

Looks to me like the EPA uses total capacity in their range estimates not usable!

No. The EPA uses wall to wheel measurements. The i3 internal charger operates at about 85% efficiency, so the %15 loss looks very close to the 13% SOC BMW keeps as unusable.

18.8 kWh useable divided by 21.6 kWh total equals 87%.

33,705 divided by 124 MPGe equals 272 Wh per mile (see http://en.wikipedia.org/wiki/Miles_per_gallon_gasoline_equivalent)

272 Wh per mile times 81 equals 22 kWh, which is what is drawn from the wall

At 85% efficiency (typical of all EVSEs) 22 kWh from the wall puts 18.8 kWh into the battery, so the math works.

You've given me a headache! What does the charger have to do with range?

All I am saying is what the EPA is telling us. That is that the i3 BEV uses the following:

• 27 kw-hrs per 100 miles in their combined driving test

• 25 kw-hrs per 100 miles in their city driving test

• 30 kw-hrs per 100 miles in their highway driving test

However they measured it, those are the numbers EPA is advertising. If 18.8 is all BMW allows us to use, then the range will be what I stated above for the driving conditions used by the tests.

 

[ultraturtle]

Am I correct that the 87% for the i3 you quote is simply approximately the 18.8/22? Which as you point out is surprisingly, for me at least more aggressive, than Toyota or Chevrolet.

Yes, the 87% number is 18.8 divided by the actual BMW i3 battery capacity of 21.6 kWh. It is no more aggressive than the only BEV in the comparison - the Nissan LEAF.

Unlike a BEV which (for obvious reasons) will never reach a zero useable State of Charge (SOC), a PHEV will frequently run the useable battery SOC down to zero, so smart manufacturers will add an extra margin - in this case 35% for Chevy and Toyota. I would not claim that BMW is significantly more aggressive in allowing only a 19% margin for the REx, because the battery capacity for a 72 mile range EV is likely to be depleted far less frequently than a 37 mile or 11 mile range EV.

I would think the battery technology due to evolution would gravitate to all cars using pretty much the same technology. Is the i3, for example, markedly better or worse than e-Golf. Leaf or MB B class

Can't speak for either car's battery chemistry, as neither is readily available by internet search. I can point out that the Mercedes B class's efficiency is abysmal at 84 MPGe. The i3 is 48% more efficient.

 

[Zzzoom3]

I also appreciate the capacity warranties (BMW's is 70% "state of health" over 8 years/100,000 miles).

According to Consumer Reports, "General Motors and Nissan, the first two companies to introduce electric cars in the United States, are providing battery warranties of eight years or 100,000 miles. In California and 15 other states that follow California emissions laws, GM will have to warranty the batteries to 10 years or 150,000 miles."

As far as I have been able to gather, the determination for whether a battery meets the warranty is whether it holds 70% of it maximum capacity. Less than 70% before 8 or 10 years (depending on where you live) has elapsed gets you a repair/replacement at the manufacturers expense. This does not necessarily mean you get a brand new battery, only that they are obligated to return it to you with at least 70% usable.

Going on a little bit of a tangent, the BMW battery pack is designed in a modular fashion whereby each module provides a certain amount of capacity. Therefore, the entire battery pack doesn't have to be replaced if there is a problem in 1 or more modules only those bad modules need be replaced. That's good for BMW and the consumer depending on who has to pay!

 

[ultraturtle]

You've given me a headache! What does the charger have to do with range?

All I am saying is what the EPA is telling us. That is that the i3 BEV uses the following:

• 27 kw-hrs per 100 miles in their combined driving test

• 25 kw-hrs per 100 miles in their city driving test

• 30 kw-hrs per 100 miles in their highway driving test

However they measured it, those are the numbers EPA is advertising. If 18.8 is all BMW allows us to use, then the range will be what I stated above for the driving conditions used by the tests.

I do not disagree at all. All I am saying is that the EPA used 22 kWh of electrical energy from the wall to put 18.8 kWh of electrical energy into the i3's battery system. That is simply how they conduct their tests.

Check the math, and it works. 27.2 kWh per 100 miles is the same as 22 kWh per 81 miles. 22 kWh from the wall at 85% efficiency equals 18.8 kWh, which is the useable capacity of the i3 battery system.

 

[Zzzoom3]

You've given me a headache! What does the charger have to do with range?

All I am saying is what the EPA is telling us. That is that the i3 BEV uses the following:

• 27 kw-hrs per 100 miles in their combined driving test

• 25 kw-hrs per 100 miles in their city driving test

• 30 kw-hrs per 100 miles in their highway driving test

However they measured it, those are the numbers EPA is advertising. If 18.8 is all BMW allows us to use, then the range will be what I stated above for the driving conditions used by the tests.

I do not disagree at all. All I am saying is that the EPA used 22 kWh of electrical energy from the wall to put 18.8 kWh of electrical energy into the i3's battery system. That is simply how they conduct their tests.

Check the math, and it works. 27.2 kWh per 100 miles is the same as 22 kWh per 81 miles. 22 kWh from the wall at 85% efficiency equals 18.8 kWh, which is the useable capacity of the i3 battery system.

Yeah Okay I don't want to argue over it as I'm really not sure what they do.

I recalculated using 22 kWh max and that 81 miles is what is advertised for the BEV range. That's using max capacity not usable right? We can't actually achieve that since we're not allowed to use the full capacity so it's somewhat misleading. Is this a big deal? It can be if I'm really going to get 70 miles of range when I think I'm capable of 81.

I'm not trying to quibble about the little details really. Whether it's 70, 81 or 90 that's probably enough for most of us on a trip to work and back. At least if you believe those studies that say something like 80% of us average less than 40 miles a day. In my case, I can say that is absolutely true. After living with 40 miles of range for the last 2 years, I'm very comfortable with the i3 BEV range for my daily use.

So all this was essentially for me to try and say that the answer to the "How to compare cars" question is look at the EPA numbers. You should be able to draw direct comparisons between cars from them.

 

[I33t]

You've given me a headache! What does the charger have to do with range?

The charger efficiency doesn't effect range at all. But it does effect the number of watts consumed from the grid to fill the battery.

Effectively, ~15% of the power consumed to charge the battery is lost. That is a valid consideration when comparing vehicles as a more efficient battery and charger may suffer lower charging losses.

 

[Zzzoom3]

You've given me a headache! What does the charger have to do with range?

The charger efficiency doesn't effect range at all. But it does effect the number of watts consumed from the grid to fill the battery.

Effectively, ~15% of the power consumed to charge the battery is lost. That is a valid consideration when comparing vehicles as a more efficient battery and charger may suffer lower charging losses.

Wonderful .. and where do you find the numbers that compare charging losses and charger efficiency?? Good luck

 

[I33t]

I recalculated using 22 kWh max and that 81 miles is what is advertised for the BEV range. That's using max capacity not usable right? We can't actually achieve that since we're not allowed to use the full capacity so it's somewhat misleading. Is this a big deal? It can be if I'm really going to get 70 miles of range when I think I'm capable of 81.

Nope. You have 18.8 usable BUT it takes 22 (or whatever) to charge it. The EPA is only going to measure range with usable battery capacity. The difference is that they measure the kWh from the mains power plug, not from the battery.

 

[I33t]

You've given me a headache! What does the charger have to do with range?

The charger efficiency doesn't effect range at all. But it does effect the number of watts consumed from the grid to fill the battery.

Effectively, ~15% of the power consumed to charge the battery is lost. That is a valid consideration when comparing vehicles as a more efficient battery and charger may suffer lower charging losses.

Wonderful .. and where do you find the numbers that compare charging losses and charger efficiency?? Good luck

I don't need to find them. The EPA measures them and publishes the results so we can compare EV efficiencies without having to do the maths ourselves. That's what you pay them for...

 

[ultraturtle]

According to Consumer Reports, "General Motors and Nissan, the first two companies to introduce electric cars in the United States, are providing battery warranties of eight years or 100,000 miles. In California and 15 other states that follow California emissions laws, GM will have to warranty the batteries to 10 years or 150,000 miles."

As far as I have been able to gather, the determination for whether a battery meets the warranty is whether it holds 70% of it maximum capacity. Less than 70% before 8 or 10 years (depending on where you live) has elapsed gets you a repair/replacement at the manufacturers expense. This does not necessarily mean you get a brand new battery, only that they are obligated to return it to you with at least 70% usable.

I have spent a lot of time trying to decipher the meaning of various battery warranty claims, and I greatly appreciate your help shedding new light on the subject. From what I have been able to gather, there is a difference between a battery warranty and a battery capacity warranty. I have been unable to uncover any relationship between the two. My understanding of the battery warranty is that if, after eight years or 100,000 miles (10 years/150,000 miles in California emission standard states) the battery is capable of moving the car a few feet, it is still operational and not legally required to be replaced under warranty. A capacity warranty would require replacement should the battery capacity drop below the warranted level, and is provided as a value-added service by BMW and others. That Tesla specifically declines to offer a capacity guarantee is puzzling. Would you be so kind as to post a reference to to the 70% capacity requirement? It would certainly make a Tesla more appealing to me.

Going on a little bit of a tangent, the BMW battery pack is designed in a modular fashion whereby each module provides a certain amount of capacity. Therefore, the entire battery pack doesn't have to be replaced if there is a problem in 1 or more modules only those bad modules need be replaced. That's good for BMW and the consumer depending on who has to pay!

Agreed. Smart design!

 

[ultraturtle]

So all this was essentially for me to try and say that the answer to the "How to compare cars" question is look at the EPA numbers. You should be able to draw direct comparisons between cars from them.

Good point. Absolutely agree.

 

[Zzzoom3]

Would you be so kind as to post a reference to to the 70% capacity requirement?

There is a distinction between battery "defects" and loss of capacity which is not something most people probably consider. The 8 years is against defects not loss of capacity. Nissan does define both in their warranty language. Here is the wording in the case of Nissan Leaf:
“In addition to the existing lithium-ion battery coverage provided under the Nissan Electric Vehicle Limited Warranty for defects in materials or workmanship, the lithium-ion battery for your Nissan LEAF is also warranted against capacity loss below nine (9) bars (or approximately below 70 percent) as shown on the vehicle’s battery capacity level gauge for a period of 60 months or 60,000 miles, whichever comes first. “

I'm still looking for what BMW publishes in this regard.

 

[cove3]

This link relates to our battery discussion
http://tinyurl.com/lm5u3yo

State of the Art are the cells that are used in the BMW i3. A nickel-cobalt-manganese cell with solid core. That is, with an aluminum enclosure, by laser welding. Which is constructed so that it has 20 years of life.

Coming back to the mentioned Tesla, whose ranges are impressive. Why can not the other car manufacturers do not obtain such coverage? Is this due to the cell or the whole package?

The ranges are so good because Tesla built a very large battery packs in the vehicle. But they get with a 18650-cell back yet 20 years lifetime.

How long you trust the Tesla batteries to specifically?
When the battery is spared, maybe eight years. If you frequently go full power or live in a hot area like in California where to come across the road 60, 70 degrees, then perhaps only five or four years.

So the impression is wrong that Tesla has the better battery?
First off, Tesla has a battery that works. But the battery in the BMW i3 is a quantum leap in comparison. BMW has an industrially manufactured, engineered part with it tuned cells, not a laptop, cell that has been modified for an automotive application. That's a huge difference. Tesla will have a problem because the customer will be ready after five or six years, hardly spend time just 20,000 euros for a new battery.

 

[ultraturtle]

I'm still looking for what BMW publishes in this regard.

Warranty
• 8 years / 100,000 miles
• Battery is replaced when State of Health (SoH) is 70% (State of Health to be determined by the Diagnosis System)

See page 19 of this document: http://darrenortiz.com/website_pdfs/BMWi3PG.pdf