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Study finds 20-25% of EV range lost as psychological safety buffer; driver assistance systems could shrink that buffer

Adaptive control of range resources. Drivers compare the current range situation with their range level reference values, which are driven by certain trait and coping skill variables. As a result, coping strategies are adapted (e.g., drive more economically, do not use EV). This leads to a certain efficiency of range utilization. Source: Franke (2014) Click to enlarge.

Between 20 to 25% of the potential range of an electric vehicle is lost as a psychological safety buffer, according to a study by a doctoral candidate at Technische Universität Chemnitz in Germany. The results of the study, which also suggested that assistance systems could reduce the size of that buffer, were based on more than 400,000 km (249,000 miles) of user experience gathered in the research project “MINI E Berlin powered by Vattenfall”. (Earlier post.)

The MINI E project was funded by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety. The Professorship of Cognitive and Engineering Psychology of TU Chemnitz was involved in the project from the start, alongside with BMW and Vattenfall and other university partners. During the field study a total of 79 users drove the MINI E for six months. The psychologists of TU Chemnitz conducted qualitative interviews at several points in time. In addition, questionnaires, diary methods and data loggers were used.

Using the results from the project, Thomas Franke developed and tested a model—the adaptive control of range resources (ACOR)—based on transferrable theories and concepts from related areas of applied psychology. A key element in ACOR is the concept of comfortable range, which represents a psychological foundation for the widely discussed concept of range anxiety.

…in everyday life, users with practical electric vehicle experience rarely experience situations in which range anxiety occurs, given a relatively typical mileage for mobility in Germany. Rather, range interaction is characterized by the avoidance, not the experience, of range anxiety (i.e., range stress). Via the analogy to psychological stress, different variables that influence comfortable range are identified. The comfortable range (i.e., an individual’s preferred range safety buffer) appears to be a variable that shows a high inter-individual variance, which partly seems to be predicated upon differing stress resistance.

In sum, the suboptimal range utilization found in previous studies is explained by the proposition that there are three psychological range levels besides the technical range that characterize the transition from the objective physical to the subjective psychological range situation: (1) The competent (i.e., maximum achievable for the user), (2) the performant (i.e., available on an everyday basis) and (3) the comfortable (i.e., actual usable) range. It shows that 20-25% of the range resources that are available on an everyday basis are lost as a psychological safety buffer.

—Franke (2014)

Franke posits that users adopt a preferred coping style when dealing with limited energy resources: “user–battery interaction style (UBIS)”. This is based on the observation, that although EV energy resources are limited, experience of subjectively critical range situations is still relatively infrequent; therefore, EV users seem to be mostly free to choose how they manage their battery resources in everyday use.

The understanding of user-range interaction enables the development of better informed strategies for attaining higher actual battery usage relative to battery capacity, as well as supporting users in the sustainable use of range resources, Franke suggests. The objective would be to help drivers to fill any gaps between these three range levels (competent, performant and comfortable) and thereby to have more usable range if needed without sacrificing driving pleasure.

[The results imply] that the primary objective of vehicle development should not be to increase battery capacity but to increase the comfortable usable range for the driver. If you consider how much increasing the technical range of electric vehicles by 20 percent costs today, it is very promising if one could achieve such an increase potentially also through optimized information and assistant systems.

—Thomas Franke

The study, which was the basis for Franke’s doctoral dissertation, also covers the preferences for certain range configurations. Here, the pattern of previous research findings indicate that range preferences of car buyers are often far greater than their actual range needs. Franke quantifies the discrepancy based on data from potential electric car customers with practical electric vehicle experience. Among the findings are that:

  1. users with practical EV experience do not necessarily have exaggerated range preferences;

  2. range preferences decrease with increasing EV experience; and

  3. the correlation between actual range needs and range preferences grows as practical experience increases.

This highlights, he sugests, the importance of practical experience for the broad success of sustainable electric mobility systems.

The results of the dissertation are highly relevant for the development of electric mobility systems. Because of the developed theoretical framework the dissertation also contributes to a generally better understanding of the interaction with limited resources. And this is a fundamental issue of our time.

—Prof. Dr. Josef Krems, chair of the Professorship of Cognitive and Engineering Psychology




Finally, someone gets it.


This is one of the reason why future EVs should have a minimum of 500 Km range + 100 Km safety added for a total of at least 600 Km.

Unless future electrified vehicles and batteries are much lighter, future highway EVs should have about 120 kWh battery pack. Future Quick charging facilities should be upgraded to between 200 and 240 KW.

Both are not impossible to do by 2020 or so.


Before anybody has a cow with my post, remember I am an EV lessee (2012 Leaf, which I love).

I think the work suffers a bit in translation, especially in the flow chart. The best summarization of Franke's (et al) work is noted in the highlights of the second linked paper:

(1) Drivers experience range as sufficient after 3 months of electric vehicle use.
(2) Users reserve substantial range safety buffers.
(3) Personality traits and system competence play a role in range utilization.

The word "substantial" in point (2) is driven by the variables in point (3). Fairly obvious, I guess, but not to everyone. When Tesla enthusiasts had a huge fit over the NYT test driver running out of juice a couple of winters ago, as much as anything they were upset that a "typical" driver had trouble interacting with an EV. In particular you would see raging anger addressing the moron driving 65-70mph with the heater on in cold weather. But when people are programmed to understand highway speed as the "most efficient" and heat as a byproduct of engine waste energy, they can screw up. I have spent quite a bit of time reminding my daughters how to drive the car on their occasional visits (and both of them are technically quite competent).

I'm an engineering nerd, so I get distracted by the energy usage screen and the regen lights. I have a very good handle on the dynamics of the battery vs. temperature. The physics of the car are not daunting to me at all. But estimating actual range for a given set of circumstances is not easy -- particularly in cold climates -- and so a-buffering we go when it comes to trip planning. Despite its superb design and interfaces, the Model S is really no better (worse if you take into account the vampirism problem). Of course, the Tesla owner doesn't face these issues except on long jaunts.

There is much time and many opportunities for improvement in sensor, computation, nav and traffic intelligence, and of course battery chemistry. In the meantime EVs will continue to be more for the dedicated consumer.

Good observations, Herman. One if the things overlooked here is that plentiful, fast recharge opportunities would completely change this dynamic. Because those resources are widely shared and used fairly infrequently, the cost per vehicle can be reasonably low.

When a recharge is 2-4 hours and you have to travel an extra 10-20 miles to get there, you have to plan trips that are at the upper bound of your range very carefully.

When a 15 minute recharge will restore sufficient range to complete your trip and get back home to your overnight refueling station, correcting an error in your range planning is a minor inconvenience.

This is why the Tesla performs so well on long trips. A fast recharge, conveniently located, when you need a break anyway, makes the trip easy.


No disagreement, insider. My critique of the Model S interface is meant as an observation of technology limitations in general. Clearly the Tesla charge rate makes things much easier.

Where I live (near I-90) there are very few charging stations. I pretty much operate out of home. I have ventured 61 miles from the Clipper Creek in my garage, where I used the only QC the car has ever had since it left CA. This was a cool event.

Over the past year I have seen roughly a dozen or so Volts in the wild around where I live. Interestingly my count of Teslas slightly outnumber Leafs 3:2, and one of those two Leafs is mine. Range is an issue, especially with our cold winters.

There is a supercharger installation at a mall close to the Interstate with several plugs awaiting Model S. I have never seen it in use, although I'd bet Friday and Sunday evenings it's busy with Northern lake resort traffic.


It is hard to remember how far we come in terms of on the job learning and understanding from many years of background processing related to everyday tasks such as econodriving.

I would like to see a Taxi-meter style adder including a red light that connects to the 'esp friction brake so that learners and instructors become aware of the cost of excessive use of brakes. Many drivers are plainly unaware of even the cost of the go pedal - let alone the woa pedal.

Although EV's are low producers of waste heat, there are likely to always (for the foreseeable future) opportunities for utilising these unavoidable heat sources.

I would look at a salt storage system that flowed past and scavenged - say starting from the input, wall charger via umbilical past on board invertoer, battery pack, engine, transmission (if any) brakes, thaere are also opportunities fo scavenge heat from damper system or possibly use the damper as the dynamic flow pumping - or 'heat pump' engine.

While this may seem to be complicated or small gain, remember that there are many posible uses for utilising and upgrading including cooling or refrigeration as in a cold box for frozen or dairy storage while shopping.
(now that is a popular hobby)

There may not be a need for heating cooling every journey so the large dimension such a system lends itself to could store a more than that imedediately recognisable incuding the possibility of primary grid ( renewable or furnace origin sourced boosting.

Why not integrate the pathways into the bodywork as double triple insulated structural members?

Just athought on what could be achieved with current technology.

I feel that


@Herman that Vampire battery draw you mention for Model S has been fixed. It was some bad code in the firmware for the lead acid battery controller that caused it to recharge repeatedly. Tesla can monitor the Tesla owners cars technical logs when they go online and download software updates. Tesla has used that system to find the Tesla owners that was having this Vampire problem to offer them a firmware update and a new lead acid battery (for free of cause) as that battery was also aged prematurely because of this software bug. But it is solved.

Range anxiety is very real. It has even caused Tesla owners with a 60kwh battery to upgrade to the 85kwh battery because you need those 85kwh to get from supercharger to supercharger in comfort with the heater on also in cold weather. IMO there will never be a mass market for those 24kwh BEVs that can barely do 50 miles in really cold weather in a mountainous terrain. And you need mass market to bring battery prices down. When Tesla get their Model E on the market it will be the best selling EV on the planet. I think it could move at least 300k units per year even at 45k USD/Euro. Nissan or any other 24kwh BEV maker will be lucky if they ever get over 100k units per year per model with a 24kwh battery pack BEV. 24kwh BEVs is a dead end strategy IMO.


Right on Henrik....Current heavy steel vehicles DO NOT have enough range with a 24 to 30 kWh battery pack.

Lighter (0.5X) better insulated composite-aluminium body, lighter (0.5X) wheels, windows, tires, HVAC, e-motors, controller and much lighter ((0.2X to (0.3X) battery pack would be required to get minimum decent range from a 30 kWh pack.

Otherwise, 120+ kWh battery packs are required. That may be where Tesla is heading for 2020 or so.

Lol, Harvey, where do you get these numbers? 120kWh!?

I drive 2 EVs with 24kWh batteries daily. They work perfectly well for a daily 44 mi commute.

When I go on a cross-country trip, like Seattle to San Diego, the 85kWh Tesla model S works beautifully, the best road trip car I've ever driven. Even in cold weather.

I'm sorry Harv, but your estimates have no basis in reality.

Both the Ford Focus Electric and the Fiat 500e are excellent daily drivers, and both are made of steel - no special lightweighting. Both also have nearly identical ICE counterparts, making for easy head to head comparisons. The EVs are superior in every respect, except for long distance road tripping (wouldn't want to use either for that purpose, even with longer range).

Really, what's the basis of a 120kWh battery requirement? A Hummer?


A Tesla Model S-120 would have performances closer to equivalent luxury ICEVs. The current 85 kWh battery pack cannot do it and require too many refills, specially on very cold days.

Of course, much lighter future e-cars could have the equivalent performance with smaller battery packs but 24 kWh will never do it.

I'll keep my hybrids until affordable BEVs, with 500 Km range are available. Will that be the Tesla Model E in 2018+? Maybe.

@harvey > The current 85 kWh battery pack cannot do it.

Cannot do what Harvey? Performance(s). Require too many refills?

This observation is based on what research or what personal experience?

If true, why is the model S outselling all of its ICE performance luxury sedan competition?

Why does it get such glowing praise from stodgy Consumer Reports (2014 "Best Overall" - that includes all ICE competition).

In town, I've never needed to charge the Model S away from home. Not once. Even on 1,200 mile and 1,400 mile drives, the refueling experience was easy and convenient. I never had to "wait" because after 4 hours of driving, I have a meal and a stretch. Probably most people do.
I didn't even need my credit card, the refueling option was included in the price of the car. And no fumes - the most convenient refueling experience ever.

How is the 85kWh battery just not big enough? How far do you typically drive in a day?

Next time you're in San Diego look me up. We can go for a drive - from the ocean to the desert and back - without a single fill-up. Then tell me how much the Model S need a 50% bigger battery.


@ e-c-I-c:

A small EV with a 24 kWh battery back is like a small ICEV with a 1.5 to 2 gallon fuel tank. Not too many of them around!

A large EV with a 60 kWh (usable) battery pack is more like a large ICEV with an 8 to 10 gallon fuel tank. Not too many of them around either!

Small EVs should have 60+ kWh battery packs and large EVs should have 120 to 140 kWh packs, special if they are to be used in our very cold winters.

Near future (2X to 3X) capacity battery packs will not have larger volume nor weight more than current 24/85 kWh packs.

Mid and longer term (4X to 5X) capacity battery packs may even be smaller than current 24/85 kWh packs and use less lithium.

Harvey, you're knowledgeable about a lot of things, but you've missed the point on this one.

> A small EV with a 24 kWh battery back is like a small ICEV with a 1.5 to 2 gallon fuel tank.

The difference is that with an EV, you start every day with a full tank. Most people drive far less than 80 miles per day. So every day, you get back home with fuel to spare. The total size of the fuel tank isn't relevant to most people on a daily basis.

The fuel cost - approximately $1 per gallon equivalent - makes the EVs much more attractive from a financial standpoint, especially with current lease offers and clean air rebate incentives.

No doubt that more energy dense battery packs will improve utility and consumer acceptance. But to say that a steel car with a 24kWh battery pack does not have "enough" range is to ignore the fact that several hundred thousand of these vehicles have been sold and that customer satisfaction with them is very high. The customer satisfaction for the Tesla Model S with a 60 kWh and 85 kWh is among the highest ever measured.

I'm all for higher energy density batteries. But these cars are viable for millions of commuters right now today. For the few times a year they take long trips, a second car, a rental or train or flying would work very well. For many, the annual fuel cost savings would finance a flight from North America to Europe rather than a road trip to Podunk.

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