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.