Solar Impulse 2: 154 kWh Of High Density NMC Lithium Batteries – Details

SEP 6 2016 BY MARK KANE 21

Solar Impulse 2

Solar Impulse 2

Kokam lithium-ion battery cell

Kokam lithium-ion battery cell

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

Kokam batteries

Categories: Battery Tech

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21 Comments on "Solar Impulse 2: 154 kWh Of High Density NMC Lithium Batteries – Details"

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50C for extended time is a long time to bake the batteries, but probably no worse than what may happen in Arizona garages/parking lot in summer. If that’s done repeatedly (ie, kept plugged in at 100% inside a hot AZ garage in summer with no active cooling), there will probably be noticeable degradation. One time thing is no big deal, especially considering that the batteries were probably not being charged and at 100%.

So roughly the same energy density -on a kwh/unit weight- as Tesla’s batteries only in prismatic form factor.

What are these batteries energy density in kwh/unit volume?

How do you figure? It seems to be a better than 50% improvement, at least from the original Model S. Has Tesla improved that much as well?

Solar Impulse = 154kWh / 633kg = 243Wh/kg
Tesla Model S 85 = 85kWh / 544kg = 156Wh/kg

The cell specific energy looks similar to the ones Tesla is using, but the rest of the battery pack is much lighter. Mostly the cooling system, I’d guess.

@Ambulator @Brian

Yes Ambulator I think that’s probably it.

Brian -I was talking cell only

Pretty amazing kwh/ unit wt on the Kokam pack

Got it. That makes sense.

Don’t ignore the considerable weight of the Model S battery pack’s shell. It’s stiff and strong enough to form part of the structural support for the car.

We can be sure the casing used in the Solar Impulse 2 is much, much lighter!

This question was about the cells and what I have found the energy density in the cells that fitted the 85 kWh pack was 233 Wh/kg so little less but not much. How the Wight if for the new cells in the 90 kWh pack I don’t know.

Solar planes have used Sion lithium sulfur, great energy density but only 50 charge cycles.

“great energy density.”

If I remember correctly the batteries you are referring to have good energy density on a kwh/unit weight basis but poor energy density on a kwh/unit volume basis.

That is why I was asking what the volumetric energy density of the Kokam batteries is.

I would propose that someone could one up Tesla on pack volumetric density if they could come up with a prismatic cell the same volumetric energy density as Tesla simply because prismatics package more efficiently.

I’ll bet Porsche Mission-e will be just the car to do that….ie one up Tesla in pack volumetric density. Especially since price won’t be a factor in that car.

They want lots of energy to weight, they have the room.

The batteries are on the pylon not in the plane structure. I bet this Kokam battery has pretty good kwh/unit volume as well.

Like I said, looks to beat Tesla’s pack.

Prismatics are the way to go when it comes to packing efficiency.

Maybe Kokams are not as good as Tesla on a volumetric basis then. Aren’t Tesla’s cell around 600 wh/liter?

I don’t know, all I know is that you want maximum energy per kilogram when it comes to flight.

The Sion batteries powered an unmanned solar drone for 14 days continuous flight at 70,000 feet altitude, that is performance.

So this article states many of the pluses of this battery chemistry but what are the minuses? IOW, why aren’t EV’s using it?

Cost. Cutting edge prototype EVs (like this EV airplane) generally use more expensive batteries. Tesla uses the cheapest batteries it can get which have sufficient energy and power density for its purposes.

As an example: The SolarTaxi prototype EV used expensive “molten salt” batteries, donated by the manufacturer, Zebra, for its circa 2007 worldwide tour.

Batteries used in prototypes also often have a shorter lifespan; that is, they’re unable to sustain as many charge/discharge cycles. But this article suggests there isn’t much if any reduction in lifespan for these particular batteries.

Possible differences with automotive a grade batteries: short number of recharges before failure, significant degradation curve, not designed for rapid charge or discharge. Price. Battery efficiency in cold temperatures (not a problem, apparently).

I believe the primary problem for lithium-sulphur as mentioned above is that overall number of charge cycles that the battery can handle without substantial capacity loss is too low to be used in daily-driver type vehicles.
For one-off vehicles like the Solar Impulse, the increased energy density is worth the limited recharge cycle counts.

oops, meant to reply to fotomoto.

i wonder what people know about lithium cells. It is common technology widely available in the market cells NCM with more then 265 Wh/kg.
I can ship out illimitate quantities of these cells immediately at a cost of 0,6 USD for 1 Ah.