Tesla’s Newest High-Power Batteries

By Jim Harrison, Maxim Guest Blogger

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Let’s take a look at the newest high-power batteries for electric vehicles (EVs) and grid energy storage applications being produced by Tesla at its Gigafactory in Nevada.

21700 Battery Cell

Tesla is upgrading its battery technology with a new 21700 battery cell, which it has been manufacturing since early 2017 at its Gigafactory. CEO Elon Musk said in August of 2017 that the factory is already producing more batteries than any other factory in the world.

Instead of the smaller 18650 cells used in Model S and Model X, the new 21700 Li-ion cell is used in the Model 3. The 21700 (Figure 1) is 21mm x 70mm (this type number says nothing about performance or chemistry, by the way). Tesla calls it their 21-70 battery. The 21-70 cell volume is 24.245mm³, which is 46% more than the 18650 cells. They are also about 15% more ‘energy efficient,’ according to a statement by J. B. Straubel, Tesla’s chief technology officer. Energy density is thought to be 877.5Wh/L and cell capacity is 21.275Wh. Note that energy density here is a very fluid specification. That’s because the depth of charge and discharge has a big effect on lifetime and, thus, charge controller programming has a big say-so about the trade-off between the two.

tesla battery
Fig. 1:  21700, 20700, and 18650 size battery cells

Most Li-ion batteries have a maximum charged voltage of 4.20V/cell, and every reduction in peak charge voltage of 0.10V/cell is said to double the cycle life. For example, a lithium-ion cell charged to 4.20V/cell typically delivers 300–500 cycles. If charged to only 4.10V/cell, the life can be prolonged to 600–1,000 cycles; 4.0V/cell should deliver 1,200–2,000 and 3.90V/cell should provide 2,400–4,000 cycles. A lower peak charge voltage reduces the capacity the battery stores. As a guideline, every 70mV reduction in charge voltage lowers the overall capacity by 10%. For absolute best longevity, the optimal charge voltage is 3.92V/cell (this may vary with chemistry). I’ve seen reports noting that this threshold eliminates all voltage-related stresses; going lower may not gain further benefits, and may induce other symptoms.

The Li-ion 21-70 and the older 18650 cells used in Tesla vehicles have nickel cobalt aluminum (NCA) chemistry. For its stationary energy storage products, like the Powerwall and Powerpack, the company uses nickel manganese cobalt oxide (NMC) cells, which will have a longer cycle life, but less energy density. Tesla hasn’t disclosed what, if any, chemistry changes were designed into the new 21700 size cell. Rumor has it that the company has added more silicon to the anode for higher charge density.

Various battery makers make 21700 Li-ion cells. One manufacturer has a 3700mAh cell with 35A maximum discharge rate. Another offers a cell with 4800mAh and 9.6A maximum discharge rate and has also introduced new integrated battery modules intended for EVs. Detailed information on these products is very hard to find at this point in time.

Although Li-ion batteries are generally referred to by this generic name, there are several different types. They have many similar characteristics, and each has its own – including number of charge cycles, which varies from 1,500 to 5,000+.

Table 1:  Li-Ion Battery Chemistries

Lithium Cobalt Li-CoO2 LCO Cellphones, laptops, and cameras
Lithium Manganese Oxide Li-Mn2O4 LMO Power tools, medical, hobbyist
Lithium Iron Phosphate Li-FePO4 LFP Power tools, medical, hobbyist
Lithium Nickel Manganese Cobalt Oxide Li-NiMnCoO2 NMC Power tools, medical, hobbyist
Lithium Nickel Cobalt Aluminum Oxide Li-NiCoAlO2 NCA Electric vehicles and grid storage
Lithium Titanate Li4Ti5O12 LTO Electric vehicles and grid storage

 

Batteries for Grid Energy Storage Systems

Considering their consumer Powerwall battery and much larger systems for grid optimization, Tesla clearly has big investments in energy storage. The company has unveiled a very large system that uses 21700 size batteries – but it may not be something that you would have expected.

Adelaide is the state capital of South Australia, and 75% of the state’s population lives in that city. Tesla has delivered the world’s largest battery energy storage system (ESS) to Adelaide.

The 100 MW/129MWh Hornsdale Power Reserve is part of South Australia’s plan to integrate renewable energy and provide back-up power services for the 309MW Hornsdale wind farm. The ESS was energized for the first time on the morning of November 24, 2017, and reached 31MW in two minutes, according to Audrey Zibelman, the leader of Australia’s energy market operator. In order to meet strict deadlines, Tesla obtained its 21700 size batteries from an outside manufacturer for the system, rather than use a partner or the Gigafactory – apparently because of heavy commitments for EV production.

The battery energy storage system is the enabling technology for the modern electric grid. These systems improve grid reliability and support the ability of grid operators to manage increasingly variable loads and resources like wind and solar. In addition, microgrid systems using solar and wind generation combined with ESS can provide reliable power in remote areas – a very large market with the potential to have a big impact on world health and well-being.

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