Interview: How Solar Impulse's round the world flight took batteries on a journey of discovery


When pv magazine visited Solar Impulse’s vast hangar in Switzerland on a bright fall day back in 2014, a veritable shower of facts and figures poured forth from the presentations given by the aircraft’s pilots, Andre Borschberg and Bertrand Piccard. But amid the eyebrow-raising, record-breaking stats, one seemingly throwaway comment stood out: if the Solar Impulse 2 (si2) aircraft was made totally out of paper, it would actually be heavier than it is. The thousands of new patents filed during the plane’s design and creation was proof if proof were needed: this flight, if it was going to be a success, would be supremely dependent on weight.

Cue a few lighthearted jokes about the pilot’s forthcoming diet regimes, followed by the most obvious question: which components are proving the weightiest? The answer was the four lithium batteries, two to each wing.

Capable of storing 164,580 watts of power, the batteries – with cells developed by South Korean storage specialists Kokam – were key to the success of the operation, storing the solar energy harvested by the 17,000 solar cells to enable the aircraft to fly at night. But with a combined weight of 633 kg, they represented one-quarter of the aircraft’s total bulk – an obvious area for improvement next time out, perhaps?

"Typical lithium-battery sizing is between 10 to 50 AMP hours for a single cell," Ike Hong, Kokam’s VP of power solutions division, told pv magazine. "So what we wanted to do for Solar Impulse was to minimize the number of cells required. We developed a large format battery, so one single cell was 150 AMP hour. That was the single biggest lithium polymer battery ever at the time, back in 2008-9 when we first began working with Solar Impulse. The second approach we took was to increase energy density, meaning that within the small space available the goal was to put much more energy.

"At the time, typical lithium-ion energy density was 150 watt hour (wh) per kilogram (kg), but we were able to provide for Solar Impulse a 260 wh/kg battery – an almost 100% improvement on the industry standard at that time, which by now was 2009-2010.

"This is a high capacity and high energy density battery, which took a few years’ development for the Solar Impulse project, and now we are using this battery for military UAV industry – unmanned aircraft."

How the relationship came about is an interesting, chicken-and-egg story. Did Kokam develop such batteries in response to the demands of the Solar Impulse project, or were Solar Impulse drawn to Kokam after hearing about their work?

"There is now a long history between us," Hong said. "Solar Impulse contacted Kokam back in 2007-8 for the first stage of the project. During this stage, Solar Impulse contacted many different companies to see who could support them in their needs. We worked with Solvay, who did the battery packing, while Kokam produced the cell. This collaboration bore the fruit of the battery system."

And so the relationship was born, the aircraft tweaked and primed, and the take-off date arrived. All was going smoothly until Borschberg touched the plane down in Hawaii last July following the longest leg yet – a five-day, non-stop flight from Japan. Upon landing, the team learned that the batteries had overheated. Decisions had to be made.

"After the Solar Impulse aircraft flew longer than the team had expected, the batteries became a little overheated. We had to discuss with Solar Impulse the state of the batteries, whether they were in a good state or not," revealed Hong. "This was not something that could be quickly assessed and resolved, and so we took the decision to ship the batteries back to Germany to test them. In general, Kokam had some confidence that the batteries would be fit to fly. But obviously, due to the importance of the Solar Impulse flight, and the media attention, even a 1% chance of failure was too much."