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Study suggests OEMs should use a modular design for PHEV and EREV vehicle battery packs to offer capacity choices to customers

TCO in € cents/km of EREV as a function of battery size for users with different annual mileages. Redelbach et al. Click to enlarge.

Car manufacturers should develop a modular design for plug-in hybrid and extended range electric vehicles (PHEVs and EREVs), allowing them to offer a choice of storage capacity to meet individual customer requirements rather than forcing a “one size fits all” approach, according to the results of a German-market-specific TCO study by a team from the Institute of Vehicle Concepts, German Aerospace Center (DLR).

The authors of the study, published in the journal Energy Policy, stress that they are not suggesting OEMs offer each customer an individual battery size, but rather than they offer, as an example, three different battery sizes dedicated to drivers with low, average and high mileage. The development of a modular design for battery packs could help OEMs to change the size with less effort and few implications on the rest of the vehicle, they suggested. (This is analogous to the approach taken by Tesla Motors with its two—originally three—pack capacity sizes offered in the Model S.)

In their study, the DLR team analyzed the impact of different driving profiles on the optimal battery setup from the total cost of ownership (TCO) perspective. Results showed that the battery size has a significant effect on the TCO.

For the average German driver who drives 15,000 km/year (9,321 miles), battery capacities of 4 kWh (PHEV) and 6 kWh (EREV) would be cost optimal by 2020. However, the researchers found, the values vary strongly with the driving profile of the user. For example, for an EREV, the optimal size changes to 2 kWh or 13 kWh is the annual mileage is halved or doubled, respectively.

Moreover, the optimal battery size is also affected by external factors, e.g. electricity and fuel prices or battery production cost.

One of the most promising powertrain technologies are [sic] plug-in hybrid electric vehicle (PHEV) and extended range electric vehicles (EREV). They combine emission free driving of battery electric vehicles with the unrestricted driving range of conventional cars powered by gasoline or diesel. However, the battery is still a very critical component due to the high production cost and heavy weight. Therefore, the right sizing of the battery is the key for electric powertrains to meet customer expectations and become cost competitive against conventional technologies.

—Redelbach et al.

While numerous studies have already explored the impact of PHEV battery size on costs and greenhouse gas emissions, existing studies neglect some significant aspects in this context, the DLR researchers said:

  • The studies do not account for the heterogeneity across different driver types;

  • They do not consider that drivers with higher annual mileage typically spend more time on motorways with a higher average velocity than drivers with lower annual mileage—thus affecting energy consumption and the share of electric driving;

  • None consider the technical differences between hybrid architectures such as parallel (PHEV) and serial (EREV) configurations; and

  • Some do not take into account that batteries are subject to degradation and aging processes which require a substantial oversizing of the initial energy capacity.

… this paper aims to close these gaps by introducing a holistic approach for the optimization of the battery size of PHEVs and EREVs under German market conditions by considering the battery degradation and secondary effects of additional mass on energy consumption. The assessment puts special focus on the heterogeneity across drivers, by analyzing the impact of different driving profiles on the optimal battery setup from total cost of ownership perspective for the year 2020 in Germany. Furthermore, specific CO2 emissions (tank to wheel — TTW and well to wheel — WTW) for grid connected cars are analyzed as a function of battery size. The most relevant data for this analysis, e.g., energy consumption or battery costs, is based on own vehicle simulations and cost models.

—Redelbach et al.

Structure of the battery-size optimization problem. CD = charge depleting; CS = charge-sustaining; TTW = tank-to-wheel; WTW = well-to-wheel. Redelbach et al. Click to enlarge.

For their analysis, they used three representative driving behaviors, based on national data: user A, with 7,500 km (4,660 miles) per year; user B, with the average 7,500 km/year; and user C, with 30,000 km (18,641 miles) per year.

Among their findings were:

  • The energy consumption of the PHEV is significantly higher compared to the EREV especially for higher battery sizes.

  • The cost of energy decreased with growing battery size due to the higher share of electric driving.

  • For a given battery size, specific energy costs rise with the mileage of the user.

  • For EREV users A, B and C, last costs are achieved with battery sizes of 2.0, 6.0, and 13.0 kWh, respectively, corresponding to 8, 24 and 51 km of electrical range.

  • For PHEV users A, B and C, least costs are achieved with battery sizes of 1.5, 3.5 an 5.0 kWh.

The results of this paper imply that higher battery capacities would reduce the overall WTW GHG emissions. If the political target is to reduce the GHG emissions even further by encouraging OEMs to design high battery sizes for PHEVs and EREVs, the public authorities may influence the results of TCO by different measure.

This can happen in basically two ways: On customer side financial incentives could be provided in the form of a direct purchase bonus or tax benefits which are linked to the battery size of the electric driving range of the new car. Furthermore, policy makers may increase the petrol price by increasing the taxes on it. On OEM side, the legal CO2regulations could contain norms that reward manufacturer with additional credits depending on the electric driving range of their partial zero emission vehicles (as realized in the ZEV legislation in California). Finally, the electricity to operate plug-in hybrid vehicles in electric mode should be provided from renewable energy sources to reach their full environmental benefit.

It should be noted that this paper assumes a rational customer who has objective to minimize the total cost of ownership. In reality, the behavior of consumers may not be fully rational. Consumers may prefer high electric range due to several reasons. This phenomenon may be analyzed in the future and the factors that may distort a rational choice may be identified.

—Redelbach et al.


  • Martin Redelbach, Enver Doruk Özdemir, Horst E. Friedrich (2014) “Optimizing battery sizes of plug-in hybrid and extended range electric vehicles for different user types,” Energy Policy, Volume 73, Pages 158-168 doi: 10.1016/j.enpol.2014.05.052



I have been saying this for some time (though without the figures).
People should have a choice of battery for PHEVs, much as they have a choice of engine size (or power) with ICE cars.
Manufacturers have been very successful in selling larger engined cars to people (men mostly) for spurious speed and acceleration reasons. The reason people (men) go for this is for pecking order. [ I am guilty too. ]

However, if they sold larger batteries, they would confer real benefits in terms of fuel economy and cost per mile.

These guys have a model which shows how to convert miles to optimal battery size.

You could also give an app which would monitor your current driving for at least one week and suggest an optimal battery size.
This would have a real benefit, not just being able to go from 0-60 in 1 second less than the other guy.

I hope they succeed in their task.

Roger Pham

Three most important things to ensure a mainstream car will sell well:
1. Generous trunk space
2. A full rear bench seat for 3, and foldable rear seat
3. Competitive price tag.

An HEV, PHEV, or EREV are no exception. As such, the engine must be downsized to 2 cylinders make room for some battery in the front, and to cut cost and weight, the fuel tank must be halved to make room for the rear battery pack, in order to ensure a full trunk space. Get rid of the spare tire.

Those wanting more battery capacity may be able to add them to the trunk as an option, knowing that that would degrade handling and acceleration, reduce load capacity, and take away trunk space, while significantly adds up the cost. "Hauling a lot of battery around is not a good idea."


I see one major flaw in the study. For normal driving conditions you need a normal power rating from the battery, lets say 40 kW. With 13 kWh battery you can get away with cheaper cells rated at 3C and basic cooling. With only 2 kWh of those same cells you will never get enough power for normal driving, you will have to use more expensive cells with higher power rating per cell (20 C).

True, there is capacity that is good enough, but going lower than 8 kWh, cost of the pack won't get lower.



IMO VW with their ~10kwh packs have got it pretty much spot on, and the next generation of batteries should allow them in the same space to offer the option of ~20kwh packs and Volt-like AER without having to modify the body.

Toyota OTOH have gone for the 4kwh pack, perhaps intending to up that as better batteries become available to ~8kwh, arguably optimum at any rate for folk in the Far East and many in Europe.

I prefer VW's take though for the reasons you give.


I'd like to see a modular battery pack that could easily slide in/out and be replaced for upgrades and replacements. A lot like a cordless drill.


Standardized Plug-in (under floor) modules would be interesting. Each of the 4 to 6 plug-in modules could have 3 to 5 kWh of stored energy.

Owners could start with 2 modules and add more at a latter date, when the price has dropped etc.

I would love to see your idea in practice Harvey, but unfortunately believe it unlikely.

What I find most interesting is the amount of money manufacturers are leaving on the table by not offering larger battery packs as an option. Tesla buyers at least have shown a great willingness to pay $400 kWh for more range - even though in the Tesla case, it would only be used on long out of town trips.

I am convinced that range options will become standard. It fits the automakers "trim level" pricing and configuration model, and ensures sales will not be lost because of a battery too small, or too large.

Patrick Free

They are missing a big point here, the battery cycles and reselling value of the car linked to battery wearing... A good battery last 3000 x full chache/discharge cycles before loosing more than 20% of its capacity due to wearing. With 1 charge per day that takes 10 years, with 2 charges per day it takes 5Y, and with best practice 1 charge every 2 x days it takes 20 years (although most batteries may have to be replaced a bit before that long due to other wearing issues). So, although I'm in their class C with 50 to 100Km per day average, my target is to do 1 x charge every 2 x days only, and my battery size target is 30KWH.
Other point missing is PHEVs pricing is in the high mid-range and low high end price segments (BMW, Audi and Mercedes hot spots) where you find essentially class C and more people. So in that space the target is 30KWH, not 13KWH ! Also missing is the fact that most 1st gen PHEVs with <10KWH battery have a far too small Electric motor power, so their all electric mode is un-conveniant today and I won't buy them for that reason. Need 100KW to 200KW all electric mode power, nothing less in that high range.

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