If we are 20 years into the EV revolution, why isn't US fuel consumption dropping more?
The short version is on the homepage. This note is the argument, with the public numbers and the limit of what they are. Every SampoVolt figure below is modeled, not test data. The highway figures are public statistics, not SampoVolt measurements.
How can a change to battery pack cooling unlock the highest reduction in fuel per kg? By designing the pack for heat and C-rate, so a small series hybrid can stay on low-cost LFP and graphite and still carry the peak.
The gallons did not leave
US highway fuel climbed for decades, then leveled off. It did not settle into a long decline. From 2007 through 2024, gasoline plus special fuels, mostly diesel, stayed in a band from 159 to 186 billion gallons a year. The latest year, 2024, is 171.8 billion. The low year is 2020. The high year is 2022. The Federal Highway Administration publishes the series as Table MF-27.
The mix changed. Pickups, vans, and sport utility vehicles used 75.9 billion gallons in 2019. Cars used 48.6. Many crossovers that count as car SUVs are still inside the car figure, so the utility-vehicle share is at least that large. The Oak Ridge Transportation Energy Data Book, Edition 40, Tables 4.1 and 4.2, is the source. Through 2008 the split is the FHWA series. FHWA stopped publishing it after that. Later years are the Oak Ridge continuation.
In model year 2021, the EPA Automotive Trends Report, Table 3.2, puts truck SUVs at 44.7 percent of light-vehicle production, pickups at 16.1 percent, and car SUVs at 11.4 percent.
Hydrocarbon fuel is a remarkable store of energy. A tank is the right reserve: the long trip, the cold morning, the day without a plug. A small battery can take the everyday miles, if it can deliver the peak, day after day, without cooking itself.
Why the obvious packs miss this vehicle
For a vehicle that size, a pure battery pack has to be large. You haul that weight every day and rarely use all of it. The weight calls for heavier, more durable parts, and the price moves past most buyers. Building packs that large for so many vehicles also strains the supply chain and often requires more expensive chemistries. Most hybrids do not fix the price either. In a series hybrid the battery alone has to deliver the peak. With conventional cooling, the pack still has to grow large enough that, combined with the engine system, it once again lands at a price few buyers can meet.
Cooling is the bridge. A pack near 29 kWh is light, has the peak power, and stays cool enough for the improved cycle life. Daily driving is electric. The generator covers the road trip. The vehicle stays light enough to be a price a buyer can meet.
"A pack shaped for power and cycle life displaces more fuel per kilogram than a pack shaped for energy density."
The battery has to carry the peak
In a series hybrid, the engine does not turn the wheels. The motor does. The battery feeds that motor at full power. A right-sized pack lives at a high C-rate: a lot of power from not very many kilowatt-hours.
A traditional face-cooled prismatic cell runs out of thermal room near 2.7C. To deliver the same peak, the pack grows until the rate falls back inside the limits. More battery. More mass. Less fuel saved per kilogram.
SampoVolt builds stacked ribbon electrodes. On edge, heat leaves along the metal. Packed appropriately, the stack is a low-resistance heat exchanger. Same LFP/graphite cell. Same coolant. Only the cooling path changes. We design from the pack in, not from a coin cell out.
At a 5C charge, the modeled peak is 34.9 °C on the edge-cooled cell and 47.4 °C on the typical face-cooled cell. The spread inside the cell is 4.7 K against 16.6 K. The limits we design to are a 5 K spread and a 45 °C peak. Inside those limits the edge-cooled ribbon holds 5.15C indefinitely. On a 150 kW SUV, that is the 29 kWh pack. The face-cooled prismatic holds to 2.74C.
A short, hard pull is a different question from a steady charge. Even at a 7C demand, the temperature spread inside the cell stays near 2.5 K after 10 seconds, about half the 5 K limit. It rarely has to hold that alone. The controller starts the engine before the climb or the hard pull, and tow mode keeps the engine on, so the 70 kW generator and the battery share the load and the pack settles at about 4.6C.
The same cooling opens the vehicle. A pack in the 30 kilowatt-hour range can still move the whole vehicle. That is what a series hybrid needs. The motor takes the launch, the grade, and the pass. The generator starts before a hard pull and shares that load. It can be smaller, steadier, and set up for one efficient job. The engine makes electricity. It does not need a transmission to the wheels.
Half the battery, more fuel per kilogram
For an SUV at a 150 kW peak, the edge-cooled pack is 29.2 kWh and 249 kg. The face-cooled pack for the same peak is 55.6 kWh and 436 kg. About half the battery. The same comparison is about 50 kWh against 94 kWh on a pickup, and about 29 kWh against 56 kWh on a van.
That is a practical 60 to 70 mile series hybrid. At that sweet spot, about 33 lifetime gallons of fuel are displaced per kilogram of battery, against about 19 for the larger face-cooled pack. Over a year, about 2.2 gallons per kilogram against about 1.3.
At a cell price of $55 to $95 per kWh, the cells are $1,609 to $2,779, against $3,057 to $5,280. About half the cell cost. Cell cost only. Pack and vehicle are not in those figures.
The pitch is thermal control that lets a small pack take harsh, high-power cycling day after day and still last. Improved cycle life is the design goal from holding the cell evenly. It is not a measured result, and this is not a measured charge time or a measured cycle life. No engine is selected, and no vehicle has been measured.
A smaller pack is also a supply-chain fact. Half the cells per vehicle means half the LFP, graphite, copper, and aluminum to buy for that vehicle. More vehicles can then carry a pack. The aim is straightforward: more packs in the vehicles that burn the fuel, and more fuel displaced. That is a design aim, not a counted fleet.
Low-cost materials are the point
Energy density is the score most battery programs chase. It is the right score for a pure battery car that has to hold a long trip inside the pack. It is the wrong score for this hybrid.
A series-hybrid pack is sized by power and heat. The chemistry can stay LFP and graphite, the low-cost pair the industry already knows how to make. Denser powder, nickel-rich cathodes, and cells built as power specialties are how you chase energy or a rate the cooling could not support. Here the cooling is what supports the rate.
The second note runs the powder grades. In that model, moving from an older mainstream LFP to the densest pilot grade changes the hybrid pack by about 4 kg. Changing the cooling path changes it by 140 to 160 kg. Those figures are modeled, and they are a different cut from the 249 kg SUV pack above. The conclusion lines up: the heat path sets the size, and the pack does not need the densest powder to do this job.
What we are not claiming
Public highway statistics are not SampoVolt test data. Temperatures, pack sizes, cell cost, and gallons displaced per kilogram are model results. The comparison holds the cell and the coolant fixed and changes the cooling path. It does not say a vehicle has been built, a powder lot has been qualified, or a cycle life has been measured.
The figures from the homepage are drawn again at the end of this page: the highway series, the shift from cars to trucks, the 2024 gallons, the fast-charge temperatures, the burst, the pack sizes, and the gallons per kilogram.
SampoVolt builds on the rolled-ribbon electrode invented by Thomas D. Kaun and developed at InvenTek (US Patent 7,195,840, now expired), with research support from Argonne National Laboratory. Not affiliated with the inventor or Rolled-Ribbon Battery Company. The patent is US 7,195,840.
Sources
- U.S. EPA, Automotive Trends Report, EPA-420-R-22-029, December 2022, Table 3.2. Model year 2021 light-vehicle production shares: truck SUVs 44.7 percent, pickups 16.1 percent, car SUVs 11.4 percent. https://www.epa.gov/system/files/documents/2022-12/420r22029.pdf
- Federal Highway Administration, Highway Statistics, Table MF-27. Gasoline, including gasohol, and special fuels, mostly diesel. The 2024 table is the latest year on the chart, 171.8 billion gallons. https://www.fhwa.dot.gov/policyinformation/statistics/2024/mf27.cfm
- Oak Ridge National Laboratory, Transportation Energy Data Book, Edition 40, Tables 4.1 and 4.2, June 2022 update. In 2019 the estimate is 75.9 billion gallons for pickups, vans, and SUVs, against 48.6 for cars. https://tedb.ornl.gov/wp-content/uploads/2022/03/TEDB_Ed_40.pdf
- Federal Highway Administration, Highway Statistics 2024, Table VM-1. Light-duty vehicles are 124.8 billion of the 171.8 billion gallon total. https://www.fhwa.dot.gov/policyinformation/statistics/2024/vm1.cfm
- Thomas D. Kaun, US Patent 7,195,840, rolled-ribbon cell structure. Now expired. https://image-ppubs.uspto.gov/dirsearch-public/print/downloadPdf/7195840