GM Comes Up With Model To Balance Energy Density & Fast Charging

APR 2 2018 BY MARK KANE 50

High-energy density batteries usually struggle to accept high-power charging, while low-energy density batteries work just fine under high-power charging

GM thinks it may have a model to show the perfect compromise here.

Related – High Energy Density Battery Cells Brings Performance Trade-Offs Of Durability And Chargeability

Chevrolet Bolt EV w/Optional CCS Combo

General Motors researchers Mark W. Verbrugge and Charles W. Wampler picked up the topic of balancing two major parameters of electric cars – range and fast charging capability.

Manufacturers need to consider the trade-off between those two, as high range requires high-energy density cells that typically are unable to charge fast without significant degradation in cell life.

The model described in the article “On the optimal sizing of batteries for electric vehicles and the influence of fast charge” takes into consideration the cost of batteries, weight, charging, as well as cost of adaption – corresponding to the days a customer would need an alternative form of transportation, as the EV would not have sufficient range on those days.

The model could help to answer the question of what battery (type and size) would be optimal for each car.

“We provide a brief summary of advanced battery technologies and a framework (i.e., a simple model) for assessing electric-vehicle (EV) architectures and associated costs to the customer. The end result is a qualitative model that can be used to calculate the optimal EV range (which maps back to the battery size and performance), including the influence of fast charge. We are seeing two technological pathways emerging: fast-charge-capable batteries versus batteries with much higher energy densities (and specific energies) but without the capability to fast charge. How do we compare and contrast the two alternatives? This work seeks to shed light on the question. We consider costs associated with the cells, added mass due to the use of larger batteries, and charging, three factors common in such analyses. In addition, we consider a new cost input, namely, the cost of adaption, corresponding to the days a customer would need an alternative form of transportation, as the EV would not have sufficient range on those days.”

According to the Green Car Congress’ extended quotation, with fast charging capability (and a network of fast chargers), the older lower energy dense batteries would be more profitable – as the car would be cheaper and the owner would likely use alternative means for transport from time to time.

Besides there being no decent fast charging network, the major problem to us is that the car is related to the life style and range satisfies a peace of mind need. Consumers will often be willing to pay more for range higher than optimum from an economic standpoint.

Chevrolet Bolt EV battery pack supplied by LG Chem

“Of particular note is the cost of adaptation. When we exercise the model with inputs one can associate with a battery like that of the Chevrolet Bolt EV, we find that the net cell volume and the vehicle range are consistent with an adaptation cost of $165/day, three times the average cost per day to rent a car in the United States. This adaptation cost would be appropriate for customers having a strong desire to avoid relying on some alternative method of transportation for the days in which the EV could not supply the needed miles. … the Bolt EV’s 238 miles range would mean that the 75th percentile drivers would need an alternative form of transportation—that is, they would need to adapt—about 5 days per year.)

The results … allow one to assess whether fast-charge of a conventional lithium ion battery is superior to the implementation of a high-energy density cell that cannot be fast charged. For the parameters chosen, fast-charge of a conventional lithium ion battery offers superior value to the customer relative to the high-energy density cell.”

source: On the optimal sizing of batteries for electric vehicles and the influence of fast charge via Green Car Congress

Categories: Battery Tech, Charging, Chevrolet

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50 Comments on "GM Comes Up With Model To Balance Energy Density & Fast Charging"

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Need vs want will always be present as far as these are humans shopping we are talking about … manufacturers will mostly always offer what consumers want, not what they necessarily need.

It will be interesting to watch this paradigm to develop.

“adaptation cost”
What kind of phony baloney phrase is that?

OK Homer.

Well, this is why PHEVs exist.

A properly designed PHEV with at least 40 miles of range like the Gen 1 and 2 Volt or the I3 Rex can easily make the average US drive all electric on 80-90% of their miles drive in a year.

As a current gen2 volt owner, Ive found 50 miles only covers about 60% of my mileage. I make a 300 mile round trip drive to my parents or my wife’s parents about once a month. In addition, there a a few trips here and there over 50 miles we take. Sure, the PHEV is great, but it is a stepping stone into the EV world, not a complete solution.

The Volt (and most other PHEVs) would be far more useful if they had more than 3.3kW L2 charging.

Yup, the i3 irex comes standard with 7.2kW, and quick CCS. It can pick up 25 Mi of range in 10 minutes vs 2 hours for the Volt. And of course the AER is double the Volt.

There are always people who will fall on either side of a bell curve. That’s why it is a curve.

However, the median Gen II Volt driver uses electricity for 80% of their travel. If the nation could cut its fossil fuel usage for transportation that would be revolutionary. Cutting the remaining 20% would have negligible benefit.

GM reported stats for the Volt fleet a while back and electric miles comprised about 60% of total miles.

That was based on Gen I data. That was around 2/3 or 67%.

Gen II data is higher now. GM expects it to be close to 90%

Yup I’m at 90% with my gen 2 Volt.

Using fleet total miles isn’t a good metric. You could have 100 people that drive daily, plugging in each night, that never use gas. Then you could have 1 person that consistently drives 200 miles/day and never plugs in. It would totally misrepresent the real driving/owning experience for most Volt owners.

I made a chart (bell curve) years ago based on the Gen1 Volt data from, and it lined up with what my experience is as an owner. As you can see, most people were around 86%. With the Gen 2 Volt, that peak will be at an even higher percentage.

Indeed, I drive 500 kms an average per month, I stay where I work and walk for my commute. I drive 20 kms daily to drop and pick up my son from the school. Even an EV with just 20 kms of range would ensure that 80% of my average driving is fully electric.

The benefit of shorter range increases the faster it canaccept a recharge. It makes a smaller, lighter weight, less intrusive, less expensive pack realistic… which is key to competing with traditional vehicles.

battery technology moved , todays is cheaper to have a big pack of low cost low power cell than a smaller pack of fast charging cell

Without actual data, there’s no reason to believe that claim. The devil is the detail. There are tradeoffs, not a single factor to making the decision.

For example, that claim of “with at least 40 miles of range like the Gen 1” was very misleading with so much detail excluded.

2011 = 35
2012 = 35
2013 = 38
2014 = 38
2015 = 38

Notice how none of the model-years ever actually delivered 40-mile ratings. During the warm months, 40 could indeed have been delivered. But in winter, not even close… and heater type makes difference, a factor not mentioned anywhere.

Notice how vague GM’s statement is? They thrive on ambiguity, releasing information that really doesn’t tell you anything specific.

In other words, read with care and look for what’s missing.

Notice how john1701a always says notice? He think’s he’s teaching you something, but instead he just states the obvious, or is trolling.

Notice how he always chimes in on any GM-subject article to try and tell you how GM is doing it all wrong.

Let’s wait a bit, and you will see the pivot to how Toyota is doing everything right. He’s already laid the groundwork here, but hasn’t mentioned Toyota by name yet. (… shh.. don’t scare him away… it will happen)

can easily make the average US drive all electric on 80-90% of their miles drive in a year.

Reality check: The fleet average of Volts — all Volts, not just 1st generation Volts — is currently 66.9%, as reported by While that is certainly very good, and contrary to the claims of some BEV purists, the fleet average is certainly more than 50% all-EV miles for the Volt, it’s not 80-90%.

I’d love to see the Volt, the Clarity, and similar higher-ranged PHEVs get an EV range of something like 60-75 miles. Then maybe we’d get to that 80-90% ratio of EV miles that you and I would both love to see!

a) Previous Tom post not regular Tom. I AM REGULAR Tom. b) The challenge with voltstats is GM changed some of the tech and car counts are going down not up as they’ve had so much technical difficulty. But still. Something about 2/3 sounds about right. I think on the PHEV end eventually the market will work itself out. Probably the Prius Prime / Hyundai Ioniq PHEV model will be one version surviving due to low cost over/above standard hybrid. And I think another model of the range extender with near 100 miles…like the BMW only more mainstream costs. I’m just not sure the Volt (and now Honda) solution of somewhere in between works in the long run. All this commentary on PHEV/extended BEV rather than pure EV. Pure EV is growing yes but numerous restrictions on capacity (such as batteries) is going to throttle things for the foreseeable future. What if one could get a battery pack about the size of BMW’s i3 for half the cost? Would it make sense to put that into a Kona-like vehicle and add a Rex or would it make sense to instead keep the same battery cost and improve the range of… Read more »

Its my understanding that Volt stats is a volunteer system. Not everybody is in it. I had to signup to get my Volt in it.

It is important to note that while the Gen 1 Volt fleet sits around ~66% EV/gas miles, the % of electric TRIPS is much higher….somewhere around 80% I believe. An owner could drive 99% of their trips as all-electric miles, but takes that 1 cross country and back drive a year that kills their EV miles percentage.

The Gen 2 Volt only improves on the above numbers. Take an inferior product such as the Prius Prime and see how far the EV numbers tank.

A “properly designed PHEV” would not include the BMW “i3 Rex” with very low power and range on gasoline (about 20 hp and less than 100 miles range).

The Chevy Volt, Hyundai Ionic PHEV, and Honda Clarity PHEV would be examples of “properly designed” ones.


I found this interesting:” For the parameters chosen, fast-charge of a conventional lithium ion battery offers superior value to the customer relative to the high-energy density cell.” I might be a minority but for the same cost if it was a choice between 200 miles of range with fast charging or 400 miles of range with nothing faster than L2 I would take the latter every time. This also makes me wonder if there might be an opportunity to construct packs consisting of a mix of cell types to capture the best of both worlds for a lower overall cost.

Yeah, I was going to say something really similar. I mean, I think if I had to make a trade off between faster charging and more range, I’d pick the range. But I suspect even with the “range” option, surely that battery would be able to accept at least 25 KW fast charging. And that would be find as I could pick up around 75 miles of range per hour even at that rate. But, ultimately, the more range you have, the less need you have to charge en-route to begin with.

More range makes more sense. If you have more range there’s less need to charge fast. Take it to an extreme, say a car with 1000 miles of range. When would you ever need fast charging? You could drive for a whole day, and while you slept at night, recharge the battery. It’s the short range EVs, like a 70mile BEV, that needs a quick charge.

Or 310 miles of range and fast charging like the Model 3 with a Supercharger network. That pretty much takes care of almost all scenarios.

It just needs to be implemented at a $26K-$30k base model cost which will take maybe 5 more years to achieve.

Yup, that’s what I would say too!! We’ve got to have 300-400 mile range EVs that can fast charge at 10-15 miles per minute if mainstream consumers are going to bite. Seriously.

Interesting. I think I’d take the fast charging option! My thoughts are, for any trip where you need to charge along the way, it’s mostly the charging speed that determines how fast you can travel. Really you have to look at what your frequency distribution of distances is in a given period and decide what’s ideal based on that and on what you’re willing to pay. For me, a car that could do about 300km in the cold going slightly uphill (driving to the alps to go skiing, for instance) without charging would be ideal. If the Ioniq had a bit more range and could take roof racks, I’d be all over that like white on rice. As it is, the Ioniq handily beats cars with much larger batteries on longer trips, because it can reliably charge at a high rate (#rapidgate).

“This also makes me wonder if there might be an opportunity to construct packs consisting of a mix of cell types to capture the best of both worlds for a lower overall cost.”

Li-ion cells are very finicky; all the cells in a pack have to be precisely balanced to match each other. If you want a mix of different li-ion battery chemistries, that would require multiple battery packs in the same car, with each pack having its own BMS (Battery Management System). Of course, physically those could all be inside the same casing, as long as each group/module of different types had segregated wiring and had its own separate BMS.

Not impossible, but certainly more complex and therefore more expensive.

Interesting concept. Could have a smaller fast charging pack, that acted like a buffer to the larger more energy dense pack. Wonder what the percentage split would be. The slower charging pack could charge to whatever percentage it can reach while the quicker pack fills up. Then the slower pack could draw energy from the quick pack to finish charging.

Maybe Like a 25kWh quick charge pack and a 40kWh high density pack?

If you go to the original GCC article, they have charts from the study posted that have a lot of information, with assumptions. I’m still staring at it trying to digest it. A lot of info there with numerous variables.

It’s also worth mentioning the environmental aspect of the whole battery size thing. A smaller high power battery will have a much smaller CO2 footprint than a larger high energy battery. At some point, probably at around 40 kWh, most people will start to get into some pretty massively diminishing returns in terms of travel time saved in a year, while the emissions from the production of the battery continue to rise linearly. If you’re into EVs because of the environmental aspect, is a 100 kWh battery that saves you an hour or two in a year worth the tonnes of extra CO2 and other pollution that you’re causing with that huge battery?

This is why (and I know this is a controversial viewpoint) I don’t think that PHEVs are such a bad thing; if they’re designed well, they have enough range to cover most daily driving, but their batteries are fully utilized and can thus give the car better lifetime emissions than a pure EV with a huge battery. Of course the best of both worlds would be an EV with a moderate battery and REALLY fast charging. Here’s hoping.

We don’t know how long the battery packs in those cars will last. Just the other day I read a claim that some of the older Volts are starting to show their age by reduced power from the battery packs. Still full range showing (because of the high fraction of reserve capacity GM built into the Volt packs), but reduced power, which is almost certainly due to loss of capacity. There are many advantages to larger battery packs. Not only longer range and the ability to safely charge faster, but also smaller loss of capacity over total distance traveled, as a larger pack is cycled fewer times for the same distance. EVs with larger packs should also have higher resale value, since the battery packs are degraded less over a given distance. I think those who claim that smaller battery packs are “better” for one reason or another, should be careful not to overstate their case. In most cases, they are definitely ignoring many of the benefits to EV drivers of larger battery packs. If the original battery pack is so depleted that the pack has to be replaced, either because of reduced range or reduced power, or the owner… Read more »

Is it just me, or does this article seem to be an ‘appetite suppressant’ for EVs intended to keep GM in the game as they roll out PHEVs that (they feel) are more profitable, but keep their oil investors happy?

You do realise that GM is rumoured to be dumping PHEV’s in favour of going all BEV right?

Not from their stated plans

Well, going by just the abstract and the sections quoted in the Green Car Congress article, it certainly does come across as someone restating well-known tradeoffs in different li-ion battery chemistries; restating those facts using fourteen-dollar words, and never using one word where 10 will do. This will no doubt impress the easily impressed.

For example: “…problematic insofar as they cannot at present accept fast charging without significant degradation in cell life.”

Is merely a longer and fancier way of saying “…cannot be fast charged without premature aging”.

In other words: “If you can’t dazzle ’em with brilliance, then baffle ’em with bull****!”

As far as intended to “keep their oil investors happy”… Well, nothing there lead me to that conclusion, but it’s an interesting viewpoint and may be worthy of further discussion.

It is unclear what is unique in their thinking that hasn’t been stated and shown for several years but all *EV manufacturers?!? They came up with this phrase but it has also been talked about forever! “cost of adaption, corresponding to the days a customer would need an alternative form of transportation” — Oddly GM Spark was really fast charging, GM Bolt is slow charging (and really handcuffed at over 50% SOC), and Volt is still very slow on L2 charging (Gen1 3.3-kW & Gen2 huge bump to 3.6-kW /s).

Bolt charging chart example: (Via: ZoomIt Bolt forum)

Very interesting and it highlights the struggle of power density vs. energy density. Though as battery technology evolves hopefully this metric moves to where we can be able to have vehicles with 300 miles of range but with charges from 20% to 80% in 15 mins.

This reads like an excuse for not having built a supercharger network and for building the Bolt with no fast charging capability.

“…an excuse for… building the Bolt with no fast charging capability.”

Indeed. I’m guessing that is precisely why GM would tout this paper, or at least the biggest reason for it.

Or, you know, they could just put in a higher capacity battery pack, which would automatically improve the ability to charge, in terms of miles added per minute, without increasing the “C” rate. As I understand it, Tesla uses cells optimized for high energy density, not high power. But Tesla cars have plenty of power, because they have a large capacity battery pack. For the same reason, they can fast-charge pretty fast (as compared to other production EVs) without significant degradation, altho Tesla has limited the number of times their cars fast charge before the car automatically reduces the very highest power charging available for the first few minutes when the pack is almost fully discharged. Looking forward to the near future: I think at least some EV makers will be transitioning to solid state batteries, perhaps as soon as 2020*, and that’s a whole new ball game as far as fast-charging capability. We’ll have to wait to see what kind of solid-state batteries are actually made, and what their characteristics are, before we can meaningfully discuss this issue in relation to that type of battery. *Altho it’s entirely possible that will prove to be too optimistic. 2022-2024 may be… Read more »

Clarification: I should have specified that Tesla limiting charging of its cars, temporarily and only under certain circumstances, only happens at Supercharger stations. With very, very few exceptions, non-Supercharger fast chargers don’t put out enough power to worry about testing the limits of a Tesla car’s ability to safely fast-charge.

I know of no households with only one car. Most are three or more. In this scenario, a ~200mi nominal range could work for their primary vehicle with ZERO fast charging.

Well, we were a single car family for 25 years! I refused to buy (or even drive a gas vehicle) for years until a suitable EV came along. We bicycled in all weather-20F to 115F. Yes, we had an ICE and used it as needed, but only when absolutely necessary, knowing that every dollar sent to the oil plutocrats would generate destruction in the end. We’ve been driving the Leaf since 2011 and sold the ICE last summer. Sorry, one car families are possible (although we do hav four bicycles as well, plus many more retired).

We have been a one care household since 2005. We do errands by bike and trike. Long distance trips require the car. We want a car with a plug but it has to meet some specific requirements : Not too big, good visibility, 5 passenger capacity, 30+ mile AER ( PHEV ) or 250+ mile range BEV, hatchback optimally or very large trunk, ability to tow a small trailer.

With a PHEV like the Volt, I get 30 to 55 miles per charge, and fast charging will never be needed. As long as it charges fast enough on a 120 outlet overnight to completely recover, no more is needed. On the lower amp setting I just barely fall short in the winter. Close enough. Less than 20 gal a year. If you have the ICE you need to use it a little, move the oil around.

Does the Tesla M3 not have the good balance between range and fast charge ?

I would agree the M3 has a good balance but cars over $40k, $50k, etc aren’t an option for us masses.
– sad face –