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Solar Impulse 2 used Kokam Ultra High Energy NMC batteries in round-the-world solar flight

The Solar Impulse 2—the solar airplane that recently completed a round-the-world flight—used batteries from Kokam, based on that company’s advanced Ultra High Energy Lithium Nickel Manganese Cobalt (NMC) Oxide (Ultra High Energy NMC) technology.

The Solar Impulse uses four 38.5 kWh Kokam Ultra High Energy NMC battery packs—one in each motor housing—with 150 Ah cells totaling 154 kWh of energy storage. Over the course of 17 flights totaling 26,744 miles (43,041 kilometers), the Solar Impulse 2’s 17,248 mono-crystalline silicon solar cells—mounted atop the wings, fuselage and horizontal stabilizer—produced 11,000 kWh of electricity, much of which was stored in its Kokam Ultra High Energy NMC batteries and then discharged to power the plane at night.


Total mass of the batteries is 633kg (2,077 lb). The four brushless, sensorless motors each generating 17.4 hp and are fitted with a reduction gear that limits the rotation speed of the 4m diameter, two-bladed propeller to 525 rev/min.

The aircraft can fly at an average speed of 70 km/h (43 mph), takeoff at a speed of 44 km/h (27 mph) and attain a maximum cruising attitude of 8,500 m (27,900 ft).

Kokam’s Ultra High Energy NMC batteries feature an energy density of approximately 260 watts hours per kilogram (Wh/kg). This high energy density enables the Solar Impulse 2 to store more energy without increasing the plane’s weight or size. In addition, Kokam’s Ultra High Energy NMC batteries have a 96% efficiency, meaning less energy is wasted when the batteries charge or discharge.

Kokam’s NMC battery technology’s high energy density and efficiency, along with its ability to operate over a wide range of temperature, humidity and pressure conditions, led the Solar Impulse team to select Kokam’s NMC battery technology for both the first prototype, the Solar Impulse 1, which was the first zero-fuel solar airplane to fly between continents and across the continental United States, and the current and second prototype, the Solar Impulse 2, which is the first zero-fuel solar airplane to circumnavigate the globe.

We had to find and use the most advanced solar, material and battery technologies available on the market at the time of the design to build a plane capable of flying around the world using only the power of the sun. What was critical was to get the lightest and most energy efficient solution, and we consequently selected Kokam’s Ultra High Energy NMC batteries, which has been our battery solution since the first flight of Solar Impulse 1 in December 2009 until the final leg landing of Solar Impulse 2 in Abu Dhabi in July 2016.

—André Borschberg, co-founder, CEO and pilot of Solar Impulse

In April, Kokam introduced a variety of new high energy battery solutions based on its advanced Ultra High Energy NMC battery technology for Unmanned Aerial Vehicles (UAVs) and other unmanned systems. In addition, dozens of customers around the world currently use Kokam’s advanced battery solutions for UAV, electric plane and other aviation applications, including industry leaders Airbus, Trimble, ECA Group and FT Sistemas.

During the most challenging leg of the Solar Impulse 2’s flight around the world—the 5-day and night record-breaking flight from Nagoya, Japan to Hawaii—the Solar Impulse 2’s battery temperature increased due to a different flight profile than the one planned and the over-insulation of the gondolas (engine housings) in relation to the outside temperature. As a result, the Solar Impulse 2’s Ultra High Energy NMC batteries were heated to a temperature close to 50 ˚C for an extended period of time—a temperature higher than the design specifications.

Because it was impossible to rule out capacity loss or other damage to the batteries with the facilities available in Hawaii, for safety reasons the Solar Impulse team decided to replace the batteries with new ones. Later, post flight tests of the original batteries at a facility in Germany determined that the batteries were undamaged, with only a small decrease in the capacity of the batteries compared to their original capacity in November 2013. Given the use of the batteries for two years, this level of capacity loss is normal.

However, to avoid potential overheating of its batteries in the future the Solar Impulse team installed a new cooling system designed to prevent any temperature-related problems if the flight mission profile changes. In addition, in case the cooling system breaks down, a new backup system allows the pilot to manually open the container’s vent, allowing him to use outside air to cool the batteries without letting them get too cold and freeze.

In addition, a few adjustments have been made to the engine housing, which shelters both the battery and engine: an air vent was added to let air flow into the battery’s cooling system. The Solar Impulse team also ensured that future flight plans provided the batteries with sufficient time to cool between flights, and adjusted its flight planning to avoid overheating batteries in tropical climates.

When you are designing an experimental aircraft every additional system is a potential source of failure, and that is why we had not initially integrated a cooling system. As we had the time in Hawaii to replace the batteries, we decided to integrate the cooling system to give the airplane more flexibility, especially in very high temperature environments. The overheating problem was in no way related to any issue with Kokam’s batteries, which have delivered excellent performance for Solar Impulse 1 and on every leg of the flight with Solar Impulse 2, supporting our record- breaking circumnavigation of the globe.

—André Borschberg

In the production of its cells, Kokam uses its patented Z-folding manufacturing technique and advanced Lithium Polymer and thin film laminations. Z-folding is a “zig-zag” type folding technique for Li-ion polymer batteries; other techniques include the conventional flat-wound jelly roll, and plain-stacked electrode structures.

Source: Kokam. Click to enlarge.

Kokam says that its Z-fold cell’s parallel pairs of electrodes offer unmatched low internal resistance, which results in less energy loss in high temperature heat as the cell charges and discharges.

The very large surface area and thin cross-section of Kokam’s polymerized aluminum pouch construction allows much more efficient thermal transfer than do cylindrical or thick, plastic coated prismatic cells. The heat dissipation is also correlated with safety.


Kokam offers its Ultra High Energy NMC cells in 12, 26 and 150 Ah configurations. In addition to the Ultra High Energy NMC cells, Kokam offers High Power NMC cells, Ultra High Power NMC cells and Lithium Titanate cells.

Kokam Co., Ltd has provided a wide range of lithium ion/polymer battery solutions to customers in more than 50 countries and many different industries, including the military, aerospace, marine, Electric Vehicle (EV), Energy Storage System (ESS) and industrial markets.


Big Al

I wondered why it to so long to replace the batteries. It seams to me that with the power to weight ratio of the electric system, an electric dirigible, how ever strange that sounds, would make sense. In due time the battery weight will drop and the solar panels strong enough to make an airplane like this practical.

Henry Gibson

Yes for lightweight uses such as solar airplanes such batteries are needed. Solar airplanes are not needed except for artistic purposes. They make people believe falsely that solar energy can replace less expensive forms of power for the majority the uses of our civilization. No one comments on the cost of the electricity per kilowatt hour including the storage devices for this solar airplane. Boeing could have saved much time and money by using similar capacity ZEBRA batteries in their aircraft rater than Lithium based units.

Most of the people of the world cannot afford a ride in any airplane, and most travelers want high speed. Almost No one who is publicly promoting solar energy mentions the cost of real estate for solar, and does not mention that a total solar home does not cover the energy requirements of the owners of such homes who live in industrialized nations which require factories, farms, roads, street-lighting, railroads, gas stations, pipelines, ships etc.

Half of fuel consumption for automobiles can be eliminated by use of hydraulic hybrids and not a single drive battery as demonstrated greatly by Artemis intelligent power. Wrightspeed does the same for lorries or trucks with low emissions of turbines and batteries. INNAS NOAX free piston technology can be combined with Artemis to eliminate the engine crankshaft and increase efficiency and life.

Batteries sitting on the ground do not need to be lightweight and can cost less for the service provided and can have a very long life and zero maintenance such as ZEBRA sodium-nickel chloride or sodium sulphur. Many automobiles have been tested successfully with ZEBRA batteries. Inefficient tiny range extenders should be required for all electric automobiles, including TESLAs, to get the last few miles home at lower speeds. Such machines can be small and cheap; think model airplane engines, and they can be ethanol fueled. Coates limited and others have rotating valves which allow for high speeds and power density. OPOC and RCV could provide the same. Use gasoline or diesel jet cars for long trips. Turbines from Capstone and Bladon and others can burn diesel in a clean way and diesel requires less CO2 release to produce, and such machines can burn BIODIESEL, AUTOGAS, LPG, CNG and LNG equally well. Artemis Digital Displacement and other hydraulic technology can be used to cut out half the fuel use as mentioned when optimized. Not another automobile should be allowed to be designed without a hybrid transmission Much as low flow shower heads are required. Artemis showed that that they can be retrofitted into existing automobiles. ..HG..

HG> Solar airplanes are not needed except for artistic purposes. They make people believe falsely that solar energy can replace less expensive forms of power for the majority the uses of our civilization.

Henry, by this reckoning, the Wright Brothers flight was just performance art. No sir. It was a scientific and engineering demonstration so all the naysayers could be silenced and more investment and development could be pursued in earnest.

Solar Impulse 2 should inspire us all. Next comes votes, investment, promotion and finally, electric personal aircraft.

The day it is available for sale, you can be sure I will be replacing my last fossil-fuel burning vehicle.

By the way, I expect to put the solar panels on the roof of my hangar, not the wings. Small lightweight panels like those that Solar Impulse 2 uses may provide extra power for the avionics.


HG> it was interesting to read about your opinion on ZEBRA batteries.
However, Zebra batteries has to be maintained at above 270 degrees celcius to operate therefore when the load isn't operating it has to use energy from other source to keep it hot. I wonder how it is a better solution like you said, correct me if I'm wrong, for vehicles or any other application to maintain it at such high temperature.

Big Al> The batteries used in solar impulse had no trouble with batteries if you look at their blog. And yes it is getting lighter by increasing the energy density.

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