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QinetiQ Zephyr Used Sion Li-S Batteries in Record Unmanned Flight

Sion Power’s proprietary lithium sulfur (Li-S) batteries (earlier post) played a critical role in the QinetiQ Zephyr smashing the world record for the longest duration unmanned flight.

A result of an intensive joint development effort between Sion Power and QinetiQ, the Zephyr flight exceeded 336 hours (14 days) of continuous flight, significantly surpassing the previous official record of 30 hours 24 minutes set by Northrop Grumman’s RQ-4A Global Hawk in March 2001. The Zephyr’s world record flight was completed on 23 July 2010 at the US Army’s Yuma Proving Grounds in Yuma, Arizona.

The Zephyr, a solar/battery powered all electric UAV with a wing span of 70 feet (22.5 m) and a weight of just over 110 lbs. (50 kg), achieved this record using a combination of solar power during the day and Sion Power’s Li-S batteries at night. After a ground launch, the Zephyr flew to altitudes of up to 70,000 feet, where the UAV encountered external temperatures as low as minus 75ºC.

The custom built Li-S battery pack was designed and assembled by Sion Power in Tucson, Arizona. The battery utilized Sion’s unique, high specific energy Li-S cells which provide 350 Wh/kg, the highest available for a rechargeable battery. The Li-S battery pack was carefully engineered to minimize total pack weight. Advanced electronic controls maintained the battery condition throughout the flight.

An official from the Federation Aeronautique Internationale (FAI), the world air sports federation, monitored the flight and is in the process of confirming a number of new world records. This includes quadrupling the Zephyr’s previous unofficial world record for longest duration unmanned flight, 82 hours, 37 minutes set in 2008. Zephyr will also have flown longer, non-stop and without refueling, than any other airplane—having significantly passed the Rutan Voyager milestone of 9 days (216 hours) 3 minutes and 44 seconds airborne, set in December 1986.


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14 days in these extreme circumstances (very large temperature and pressure variations) is the best validation yet that Sion’s LiS batteries are working. That should generate more interest with regard to getting these batteries into commercial production for aviation applications.

When compared with Panasonics silicon cells they are not leading in terms of energy density Wh/l. They only lead in terms of specific energy Wh/kg.

Panasonic: 250Wh/kg, 800Wh/l, see

Sion: 350Wh/kg, 320Wh/l, see

Also at the homepage of Sion you can read that they expect future versions of their LiS cells to achieve about 550Wh/kg and 500Wh/l. That is plenty to make a useful Cessna aircraft with electric propulsion but it will still not be able to compete with Panasonic’s silicon batteries that can be expected to reach over 1kWh/l in the not so distant future perhaps by 2016 which will enable automakers to fit a large 72 kWh battery pack under the backseat of a car or enough for 250 miles.

For automobiles high energy density is more important than high specific energy whereas the opposite is true for airplanes. For these reasons we should expect LiS batteries to become the preferred chemistry for airplanes and Silicon based lithium batteries to rule among new EVs in about 6 to 9 years.


A battery pack with 550Wh/Kg and 500W/l (or better) would be enough for a practical highway capable BEV. A gestation period of 6 to 9 years is normal and acceptable.

Light passenger e-aircraft would require better batteries and solar cells. It will come sooner or latter. Meanwhile, very high continuous flying e-planes could become very useful for various purposes.

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Harvey 140wh/kg at the cell level (as is the case with Nissan’s LEAF) is enough for a practical highway capable EVs.

Note that Sion’s LiS batteries can’t fast charge (C/5 = min 5 hours to fully charge) and for all I know they suffer from high self discharge and very short cycle life (probably less than 100 cycles). Moreover, they are not in commercial production. If you are allowed to buy these cells expect to pay 10000 to 30000 USD per kWh as they are currently hand made in a laboratory. For niche military applications that is OK. Sion’s best hope to get these batteries in commercial production is probably to let their partner BASF start producing them for special applications, such as, electric Cessnas, high profile electric sports cars from Lamborghini and Porsche and then of cause an ever expanding fleet of UAVs for civilian and military purposes.

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