Kokam Co., Ltd, has announced that the Solar Impulse 2 used batteries based on its advanced Ultra High Energy Lithium Nickel Manganese Cobalt (NMC) Oxide (Ultra High Energy NMC) battery technology to power the zero-fuel solar airplane’s record-breaking flight around the world. On July 26 the Solar Impulse completed the final leg of this trip, taking off from Cairo, Egypt and landing 48 hours later in Abu Dhabi, United Arab Emirates.
The Solar Impulse uses four 38.5 kilowatt hour (kWh) Kokam Ultra High Energy NMC battery packs 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 solar cells 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.
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 percent 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,” said André Borschberg, co-founder, CEO and pilot of Solar Impulse. “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.”
“The Solar Impulse team and Kokam share a common vision — to create a world powered by non-polluting renewable energy,” said Ike Hong, vice president of Kokam’s Power Solutions Division. “By choosing to use our advanced Ultra High Energy NMC battery technology, the Solar Impulse validates Kokam’s belief that, by continually working to develop more powerful, efficient and cost-effective battery technologies, Kokam can play an important role in accelerating the transition to a clean, electrified global economy.”
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 degrees Celsius 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 (or gondola), 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,” said Borschberg. “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.”