|Fig. 1: Tesla Model S. (Source: Wikimedia Commons)|
Motived by the increasing environmental concerns and also the available resource limitations of oil, the automotive industry has continued to develop various alternative fuel vehicles. Out of all the potential solutions that do not utilize petroleum, battery electric vehicles (BEVs) are among the widespread and most popular option. One of the main advantages of BEVs is that these vehicles have zero emissions (generating no greenhouse gases or pollutants). Hence, BEVs help contribute to cleaner air and they are better for the environment. Also, these vehicles run on electricity, which can be generated through more renewable and environmentally friendly means.
From the more economical Nissan Leaf to the high end Tesla Model S (Figure 1), there are many different models of commercially successful BEVs on the road. Furthermore, as there have been great investments in support of infrastructure, battery electric vehicles have become greatly viable and feasible option in automotive market for consumers. Unlike conventional petroleum driven internal combustion vehicle, BEV is propelled by a large electric motor, which is powered through a rechargeable onboard battery system (as seen in Figure 2). While most of BEVs production models utilize one type of battery, there are several different types of batteries that have been implemented in battery electric vehicles.
|Fig. 2: Chassis of Tesla Model S, displaying Batteries and Motors. (Source: Wikimedia Commons)|
Both lead acid batteries and nickel metal hydride (NiMH) batteries are mature battery technologies. These types of batteries were originally used in early electric vehicles such as General Motor's EV1. However, they are now considered to be obsolete with regards to their uses as the main source of energy storage in BEVs. Lead acid batteries have seen used in conventional petroleum driven vehicles and are relatively inexpensive. However, this type of battery has a poor specific energy (34 Wh/kg).  NiMH batteries are considered to be superior, as they can have up to double the specific energy (68 Wh/kg) compared with lead acid batteries.  This allows electric vehicles that utilize NiMH batteries to be significantly lighter, leading to reduced energy cost for propelling the BEVs. Similarly, NiMH batteries also have greater energy density compared to lead acid batteries, which will allow the battery system to be contained within a smaller space. Though, NiMH batteries do have some drawbacks, such as having lower charging efficiencies than the other batteries. There is also a major issue with self-discharge (up to 12.5% per day under normal room temperature conditions) that is exacerbated when the batteries are in a high temperature environment.  This makes NiMH batteries less ideal for hotter environments. Furthermore, there have been legal controversy regarding large format NiMH batteries, which has affected the use of NiMH batteries in battery electric vehicles.
Lithium ion (Li-ion) batteries are now considered to be the standard for modern battery electric vehicles. There are many types of Li-ion batteries that each have different characteristics, but vehicle manufactures are focused variants that have excellent longevity. Compared to other mature battery technologies, Li-ion offers many benefits. For example, it has excellent specific energy (140 Wh/kg) and energy density, making it ideal for battery electric vehicles.  Li-ion batteries are also excellent in retaining energy, with a self-discharge rate (5% per month) that an order of magnitude lower than NiMH batteries. However, Li-ion batteries also have some drawbacks as well. Comparatively, Li-ion batteries have been a very expensive battery technology. There are also major safety concerns regarding the overcharging and overheating of these batteries. Li-ion can experience a thermal runaway, which can trigger vehicle fires or explosions. There had been serval instances where the Tesla Model S, which utilized Li-ion batteries, had infamously caught on fire due to issues with fluctuating charging or damage to the battery.  However, great efforts have been made to help improve the safety of vehicles that use Li-ion batteries.
Battery electric vehicles have become a significant segment of the automotive market. Having excellent specific energy and low self-discharge rate, it appears that variants of Li-ion batteries are now the dominant type that are currently used in BEVs. Meanwhile, lead acid and NiMH batteries no longer appear to be appropriate for the use, though these batteries are still frequently used in the automotive industry.
© Brian Mok. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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