{"id":11077,"date":"2016-08-22T23:18:28","date_gmt":"2016-08-23T06:18:28","guid":{"rendered":"http:\/\/cafe.foundation\/blog\/?p=11077"},"modified":"2016-08-22T23:18:28","modified_gmt":"2016-08-23T06:18:28","slug":"solar-impulse-2-batteries-better-than-we-thought","status":"publish","type":"post","link":"http:\/\/cafe.foundation\/blog\/solar-impulse-2-batteries-better-than-we-thought\/","title":{"rendered":"Solar Impulse 2 Batteries – Better than We Thought"},"content":{"rendered":"

What happened to Solar Impulse 2\u2019s batteries on those long five days and nights over the Pacific?\u00a0 It took months of enforced downtime in Hawaii to have new batteries made, sent from Korea, installed, tested, and flown again.\u00a0 Could the plane have completed the flight on the original batteries?<\/p>\n

\"Full-scale

Full-scale testing of SI2 battteries after addition of cooling system and before its departure from Hawaii<\/p><\/div>\n

Kokam, manufacturer of the airplane\u2019s cells,<\/a> has released new information that provides details of the drive system and relieves a few lingering anxieties.\u00a0 An over-riding concern was that batteries overheated on the Japan to Hawaii part of the mission, topping out at 50 degrees Centigrade (122 degrees Fahrenheit) \u2013 above their design temperature.\u00a0 Your editor has thought deeply about what Andre\u2019 Borschberg must have gone through every day of the five over the Pacific, seeing the temperatures on the four battery packs climbing as he pointed the airplane\u2019s nose up in its saw tooth flight profile.<\/p>\n

Borschberg explained that stress in a comment on SI2\u2019s blog.\u00a0 \u201c”I feel exhilarated by this extraordinary journey. I have climbed the equivalent altitude of Mount Everest five times without much rest. The team at the Mission Control Center in Monaco was my eyes and my ears. The MCC was battling to give me the possibility to rest and recover, but also maximizing the aircraft\u2019s energy levels and sending me trajectories and flight strategies simulated by computer. This success fully validates the vision that my partner Bertrand Piccard had after his round-the-world balloon flight to reach unlimited endurance in an airplane without fuel.”<\/p>\n

\"Kokam's

Kokam’s Z-fold construction would add surface area to allow more interaction of active materials<\/p><\/div>\n

Concern, intense monitoring and precise directions enabled the pilot, craft and the four 38.5 kilowatt-hour battery packs to make it through five days and nights of high tension. \u00a0Kokam\u2019s \u201cUltra High Energy NMC battery packs\u201d one per motor gondola, have a total of 154 kilowatt-hours of energy storage.\u00a0 The 150 Amp-hour cells made 17 flights totaling 26,744 miles (43,041 kilometers), fed by the 17,248 silicon solar cells on the wings, fuselage and horizontal stabilizer.\u00a0 Those cells produced 11,000 kWh of electricity during the around-the-world voyage.<\/p>\n

The lithium nickel manganese cobalt cells produce high energy, sometimes associated with a propensity for thermal runaways.\u00a0 If the cobalt would lead to that, the manganese seems to act as a butter to prevent such problems, and NCM chemistry has a good safety record.\u00a0 The cells\u2019 96-percent efficiency keeps helps reduce temperatures during charging, and the pouch-type configuration provides a larger surface area to disperse heat than that available to prismatic or cylindrical cells..<\/p>\n

Solar Impulse technicians decided to remove the battery packs when the airplane landed in Hawaii.\u00a0 Testing in Germany, where the batteries are packaged for use in SI2, found the cells were undamaged even after exposure to higher than normal temperatures, and had lost only that capacity they would have in more normal circumstances.<\/p>\n

\"Large

Large area presented by pouch cell allows battery to act as large heat sink<\/p><\/div>\n

Andre\u2019 Borschberg explained the SI2 team did not blame the batteries for the mid-journey hiatus.\u00a0 \u201cWhen 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\u2019s 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.\u201d<\/p>\n

SI2\u2019s batteries were subjected to extremes of temperature and multiple days of uninterrupted charging and discharging, and still managed to perform at high levels of performance and reliability.\u00a0 This in itself should provide encouragement to others now designing future electric aircraft.<\/p>\n