Solar Impulse 2

For those unfamiliar, the Solar Impulse 2 is an electric aircraft, which recently completed a round-the-world flight (recap/videos of accomplishment here); however during the night without the aid of the solar for power, the flight continued with the aid of on board batteries.

As it turns out, those batteries - with very high energy density - was supplied by Kokam.

According to Kokam, their Ultra High Energy Lithium Nickel Manganese Cobalt (NMC) Oxide (Ultra High Energy NMC) technology offers 260 Wh/kg.

Here is some data we though interesting on the battery application for the Solar Impulse 2:

  • four packs - 38.5 kWh - total 154 kWh
  • 150 Ah cells with 96% efficiency
  • total battery weight 633 kg (1,395 lb) - as a comparisonm the original Model S 85 kwh had a pack weighing in at ~544 kg (1,199 lbs)
  • batteries are installed in each motor housing (17.5 HP or about 13 kW and total 50 kW output)
  • 17,248 mono-crystalline silicon solar cells produced 11,000 kWh of electricity over 26,744 miles (43,041 kilometers)
  • 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 described also what happened with the batteries during a 5-day and night flight from Japan to Hawaii, which forced the team to play it safe and replace the batteries for new ones, and redesign the cooling:

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."

Kokam batteries
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