The Lithium-Ion family of battery technologies has quickly become the center stage product in the advanced technology vehicle market. Although Nickel Metal Hydride (NiMH) continues to be a staple product in the Hybrid Electric Vehicle (HEV) market, when it comes to applications that need additional energy storage for extending vehicle range, NiMH is not competitive with the Lithium family of battery technologies. Plug-In Hybrid (PHEV), Battery Electric Vehicle (BEV) and Fuel Cell Electric Vehicle (FCEV) applications need the additional capacity storage capability, coupled with the smaller weight and packaging of Lithium to provide increased vehicle fuel economy and/or range.
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Why use Lithium?
As the advanced technology (electric propulsion) systems continue to populate the vehicle market, the Lithium family of battery products becomes the choice of manufacturers that have entered the market in the past 4-8 years, due to its superior energy storage capability. For the foreseeable future, Toyota/Lexus continue to enjoy the majority of the advanced technology market and the battery technology utilized in their vehicles is predominately NiMH. One of the primary reasons for utilizing NiMH is that it is a very stable chemistry (not prone to thermal events or problems with overcharging), there is a long record of use in industrial applications and it is a known quantity that is predictable in its operating and failure modes.
Lithium products are manufactured with two basic formats – cylindrical 18650 and pouch. The 18650 cell is slightly larger than a AA battery and the pouch style cell can be manufactured in many different size configurations, dependent upon application.
|Lithium 18650 cylindrical battery cell|
|Lithium pouch battery cell|
When compared to NiMH, the Lithium family of products has significant advantages when considering capacity, mass (weight), resistance and size. However, since Lithium products must be controlled within a much narrower charging/discharging voltage range, this becomes a disadvantage for systems due to the cost of adding sophisticated control systems. When considering the cost of the hardware and software systems that are required to monitor and control cell performance, Lithium battery systems cost can be significantly higher than NiMH.
Lithium battery families
Unlike NiMH, Lithium technology has numerous family categories. Each of these categories offer varied energy (capacity) and power characteristics. The primary families utilized in the automotive or medium/heavy duty market as of this printing are:
· Lithium Cobalt Oxide
· Lithium Manganese Oxide
· Lithium Manganese Cobalt Oxide
· Lithium Iron Phosphate
· Lithium Nickel Cobalt Aluminum Oxide
· Lithium Titanate
Each of these chemistries has a different characteristic (power, energy and discharge performance). The electrolytes can be significantly different, although each uses Lithium Salt as a basic element. Each may have different additives in the electrolyte that mitigate aging, permit enhanced performance, fire retardants, etc. Each of these additives will affect how the cell performs and its longevity. Unlike Lead Acid and NiMH batteries, which enjoyed stable and predictable performance metrics, the growing number of Lithium products with widely varying performance metrics will require technicians to more thoroughly understand failure modes and specialized diagnostic techniques. A quick internet search on the Battery University website can provide more detail on each chemistry.