Solar Impulse 2 Grounded Because of Damaged Lipo Cells

[Jason McNeil]

Not directly EU related, but interesting because such a high profile project is set back by Lipo reliability issues.
http://www.pv-magazine.com/news/details/beitrag/solar-impulse-grounded-following-irreversible-battery-damage_100020176/#axzz3fnwd0h3g
Those touchy Lipo packs became slightly overheated, now the plane is grounded until August. The best Li-ion 18650s possess the same energy density as these, but the project was probably planned years earlier & the design decision made.  

[esaj]

Moved to off-topic, forum rules and all... I think.

The best Li-ion 18650s possess the same energy density as these, but the project was probably planned years earlier & the design decision made.  

The batteries were developed with Solvay, Kokam and Bayer Material Science, and have been optimized to 260 watt hours per kg. 

I know that LiPo-cells can be molded into many different forms, and maybe give out higher discharges than Li-Ions? That could be the reason for selecting them over Li-Ion chemistries... Current NCA (LiNiCoAl) -cells should be able to reach the same around 260Wh/kg -energy density (if not even more) as LiPos, and should be fairly safe (considering they're used in Teslas and what not). Price might have been one factor too, I think (but am not sure) that LiPos are cheaper than NCA, and 633kg of batteries is probably not exactly cheap   

But hard to say really, as it seems there are smaller (and sometimes larger) breakthroughs occurring in lithium-batteries all the time (especially with more exotic chemistries, also "old" chemistries are improved by testing and adjusting the ratios of the different metals). Just looking at specific energy densities reported about 10 years ago (<=170Wh/kg) and nowadays (>200Wh/kg), things are always moving forward. In some article I read a while back (don't remember how new/old it was), they said that even the battery manufacturers admit that we don't know everything about even currently used lithium chemistries and the electrochemical reactions yet...

Edit: I think this could be pretty near current situation (2014-2015?):

Few years back:

 

[Jason McNeil]

What's interesting about these new commercial battery developments, is that historically there was a trade-off between capacity & power, example of the 3,200mAh Panasonic cells that had a very low C rating to extract anything near the nominal capacity of the cell.

With the likes of the LG MJ1/MH1 & the Sanyo GA is that they offer >10Wh at a 10A discharge, which is pretty exciting stuff. Of course in EUs, the continuous power demands isn't anywhere near 10A, but is nice to know it's available on the tap, if necessary. I've read rumours that the new Gigafactory will start producing cells with 4Ah. If true, this will represent a 25% increase over the best 680Wh packs using the same number of cells!

https://www.endless-sphere.com/forums/viewtopic.php?f=14&t=70147

"Todays top 5 ebike performance cells at 10A continuous discharge:
1. Panasonic/Sanyo NCR18650GA, 3.4Ah and 11.2Wh at 10A (down to 2.5V), 47 grams measured. 238Wh/kg at 10A. Measurement is here:https://yunergybattery.files.wordpress. ... lg-mj1.jpg
2. LG INR18650MJ1, 3.3Ah and 10.6Wh at 10A (down to 2.5V), 47 grams. 225 Wh/kg at 10A. Measurement is here:http://d112tss1dzpest.cloudfront.net/wp ... jpg?323d81
3. LG INR18650MH1, 3.2Ah and 10.1 Wh (down to 2.5V), 47 grams. 215Wh/kg at 10A. Measurement is here: download/file.php?id=169435
4. Panasonic NCR18650PF 2.8Ah and 9Wh at 10A (down to 2.5V), 46 grams. 195 Wh/kg at 10A. Measurement is here:http://www.dampfakkus.de/highamps/531-1 ... 8650PF.png

[dmethvin]

Battery is definitely the critical path technology for so many devices nowadays. It seems like every year we get promises of amazing 10x performance gains and instead get minor improvements over time.

Phones and computers have gotten a lot of their performance increases not by better batteries, but better battery management and reduced power consumption. One thing we haven't really determined yet is how efficient the motor and regen system is for all of these wheels. Maybe there is room for improvement there as well?

[esaj]

Battery is definitely the critical path technology for so many devices nowadays. It seems like every year we get promises of amazing 10x performance gains and instead get minor improvements over time.

Phones and computers have gotten a lot of their performance increases not by better batteries, but better battery management and reduced power consumption. One thing we haven't really determined yet is how efficient the motor and regen system is for all of these wheels. Maybe there is room for improvement there as well?

Phones and computers also work very differently, no need to produce motion for large masses... most of the lower energy usage seems to come from being able to manufacture chips with smaller "line/channel widths", currently going somewhere around 14 nanometer level, of course on top of that, you've got all sorts of "intelligent" energy saving measures, like dropping the frequency the processor is running when no high performance is needed, dropping display brightness and frame rate, turning off wireless/dropping transmission power when it's not needed etc.

On a wheel that's not as easy, the displays/battery leds etc. suck so little power compared to the motor itself, that that kind of energy saving is next to useless. AFAIK, DC motors and especially brushless DC motors (BLDC) already have fairly high efficiencies:

In [BLDC] motor, the mechanical "rotating switch" or commutator is replaced by an external electronic switch synchronised to the rotor's position. BLDC motors are typically 85–90% efficient or more. Efficiency for a BLDC motor of up to 96.5% have been reported,^[64] whereas DC motors with brushgear are typically 75–80% efficient.

100% efficiency is probably impossible to reach, and getting some 10% percent more is not likely to lead to very much improvement in terms of battery usage. Of course the wheels could be lighter (think new materials, like nanoceramics or foam metals), but as the most mass usually comes from the rider itself, dieting is probably more efficient.  

I don't think there's THAT much energy available from regenerative braking, although I don't know how efficient it is currently. Unless you brake a lot and often during riding, I don't think it's going to give that much juice. You could also maybe cover the wheel in solar panels, but they would break easily in falls and probably don't provide that much power... direct sunlight gives something like around 1.4kW / square meter (10.76 square feet) at best, but the efficiencies of the available panels are usually far below 20% of that, and they would be partially shaded/covered by your legs while you ride. Just as an example, some random panel I pulled off from Google gives out 15W and is 41x35x2.5cm (about 16" x 13.8" x 1"). 

So that pretty much leaves nothing but the "yet to come" bigger/more efficient (in terms of energy- and power-density) batteries (Lithium-sulphur, lithium-air, zinc-air, supercapacitors, or something else, whatever's the "next big thing"), or just filling up your wheel/device with as many and as large capacity batteries as possible, but in the case of wheels, the space is very limited and the weight may become a problem. Not exactly cheap either. Thorium-plasma batteries are something I haven't looked into a lot, but the idea of putting a small nuclear device on a wheel might not be that good, although they should be really powerful and last for several years if not decades... 

 

 

[Jason McNeil]

Commercial battery innovations seen in the first-half of this year alone is more than a 15% improvement over the previous generation. On the face of it, 15% may not sound like much but look at compounding effects & continued cost reductions!  

Beauty of Electrification is that the wall-to-Wheel efficiency is comparatively very high, probably around 70-80% after the losses of charger, battery, PWM, & BLDC motor are taken into account under a cruising 15-20kph scenario.

The chaps at ebikes.ca have done an exceptional job of putting together a 'simulator' of the type of efficiency one can expect using the similar hub motors found on eBikes, with various diameter Wheels, controllers, & battery combos. All motors have a sweat-spot, where the efficiencies can reach as high as 90% at a particular RPM/speed.