Our article yesterday was about our drive in the Coda Sedan. A reader commented that the lithium iron phosphate chemistry of the battery pack in the Coda was also a lithium ion battery, which prompted some additional investigation on our part.
According to Wikipedia, the lithium ion battery may consist of three different types of cathode construction (currently):
The cathode is generally one of three materials: a layered oxide (such as lithium cobalt oxide), a polyanion (such as lithium iron phosphate), or a spinel (such as lithium manganese oxide). [Source: Wikipedia]
The LEAF battery uses the spinel (lithium manganese) cathode while the Coda uses the polyanion (lithium iron phosphate). Both use a laminate (large rectangular flat panel) cell design, as opposed to the cylindrical cell structure used by Tesla. Our research found that the spinel battery (LEAF) also offers the ability to offer more current (power) in less time than a prismatic battery (Coda) or a cylindrical battery (Tesla). According to DesignNews.com:
What’s more, lithium polymer cells offer higher rates of discharge. The C-rate — a parameter that describes how long it takes to fully discharge a battery — is much higher in polymer batteries than in cylindrical or prismatic lithium-ion batteries. Whereas traditional lithium-ion batteries top out at C-rates of roughly 2, lithium polymer batteries can have C-rates approaching 15 or more. The ability to discharge current at faster rates translates to greater power for the applications. [Source: DesignNews.com]
One might wonder about the importance of the distinction between laminate and cylindrical cells, but there is one significant difference between the two form factors – heat generation.
Image source: Automotive Energy Supply Corporation
If we are reading the referenced information correctly, the image on the left shows a laminate cell with a charge-discharge rate of 26 times its capacity – 100 amps or 26C for a 3.8 amp hour cell in 15 minutes. The change in temperature was 11° celcius, which placed it in roughly the 35° celcius (95° F) range as indicated by the color of the cell’s thermal image. Starting temperature (calculated from the temperature change) would be 24° celcius (75° F) – 35 minus 11. Maximum temperature reached was less than that maintained by the average human body.
The image on the right shows a cylindrical cell with a charge-discharge rate of 10 times its capacity – 8 amps or 10C for a 0.8 amp hour cell in 15 minutes. The change in temperature was 26° celcius, which placed it in roughly the 50° celcius (122° F) range as indicated by the color of the cell’s thermal image. Starting temperature (calculated from the temperature change) would also be 24° celcius (75° F) – 50 minus 26.
If this is how they perform under this charge-discharge scenario, one would think that the laminate cell design would be much less prone to heat generation under normal use than a cylindrical format. So much for Elon Musk’s comment about LEAF having a “much more primitive level of technology”. Interestingly, when that comment was made, many news organizations and blogs wrote about the comment. We have yet to find any that actually researched the validity of the comment. Too, what must be considered is that the Tesla Roadster tightly packs 6,831 of these cylindrical cells together. As we’ve said before, the Tesla design uses an active liquid cooling system because it needs to. Coda uses an air-cooled design to maintain proper battery temperature.
So while the Coda uses different, but similar, technology in its battery pack design as compared to the Nissan LEAF, the Tesla Roadster uses cells which require liquid cooling to maintain a correct operating temperature. Also, worth remembering is that Nissan backs their battery pack with an 8 year/100,000 mile warranty. Tesla Roadster offers 3 year/36,000 miles of warranty coverage. Coda offers a 10 year/100,000 mile battery warranty.
Finally, as we review our recent posts, we see that we neglected to wish you a Merry Christmas before we took our break. Not wishing to slight you yet again, we would like to take this opportunity to thank you for taking your valuable time to read our blog, and to wish you a healthy, prosperous and Happy New Year!