What You Need to Know About Batteries for Electric Vehicles
Edis Osmanbasic posted on September 25, 2019 |
Explore some key points about electric vehicle’s battery system.
(Image courtesy of BNEF.)
(Image courtesy of BNEF.)

The electric vehicle (EV) as a green energy solution has already become a popular and accepted replacement for the internal combustion engine (ICE) vehicle. Their use is increasing daily because of rising awareness of carbon emissions, government incentives like providing privileges EV drivers, and, of course, increasing oil prices and decreasing reserves. According to the Bloomberg NEF (BNEF) 2019 Electric Vehicle Outlook , EVs will account for 55 percent of all new passenger cars worldwide by 2040. In addition, compared to the ICE vehicles, EV motors are more efficient and react quickly with high torque. They are also cost-efficient because of their lower fuel and maintenance costs. Today, there are different commercially successful models of EVs, from economical models to the powerful sports models.

Annual global vehicle sales. (Image courtesy of BNEF.)
Annual global vehicle sales. (Image courtesy of BNEF.)

The performance of the EV is closely related to the design of the battery pack that powers the vehicle’s engine and must be able to provide enough current for the motor over an extended time. Since one battery cell provides quite low voltage and capacity, in an EV, hundreds of cells are connected in series and in parallel to provide the required voltage and amp hours (Ah). For example, a powerful EV like the Tesla Model S has 7,104 battery cells.

It is wellknown that lithium-ion (Li-Ion) batteries are the most commonly used battery type in EVs. However, several different battery types have also been used in EVs. This article will present the different battery technologies used in EVs and explore their advantages and disadvantages.

Important Battery Parameters

There is specific information available about each battery, but two common ratings are battery voltage and capacity Ah. The nominal voltage of lead-acid batteries is 2V or 12V, while Li-Ion batteries can be in the range of 3.3-3.7V. Nickel-metal hydride (NiMH) batteries have a nominal voltage of 1.2V. Nominal capacity (Ah) rating represents the current value that can be provided by the battery in one hour. This indicates the amount of energy stored in the battery. Additional important information are battery type and the number of cells in the battery string.

In order to select the most suitable battery type for an EV application, the following battery parameters should be considered:

  • Life span—The battery life cycle is influenced by different factors, such as the purpose the battery will be used for, operating conditions, and the depth of battery discharge, but you can generally estimate EV battery life as 8 years or 160,000 km (100,000 miles).
  • Safety—It takes a lot of power to drive an EV, which must be managed properly. A safe operation is assured by a carefully designed battery management system (BMS).
  • Cost—This is a major problem for EVs (compared to ICE vehicles) because an EV’s battery system costs as much as a small ICE vehicle.
  • Performance—This depends mostly on battery operating temperature. High temperature reduces the battery’s life span, while low temperature decreases a battery’s performance.
  • Specific energy—Energy density represents battery capacity in weight (Wh/kg) and the amount of energy stored per unit mass (or by volume). Since the battery system is a significant part of an EV’s weight, the specific energy value is one of the most important parameters for EV batteries. High specific energy is required in applications where a long runtime is required at moderate load.
  • Specific power—Power density represents loading capability. EVs have much better torque than ICE vehicles, and therefore have better acceleration.

Battery Types Used in EVs

EVs are powered by rechargeable batteries. This battery type provides a reversible chemical reaction, allowing both their discharging and charging process. During the battery discharging process, the electrical current flows from cathode (+) to anode (-), while the reverse process occurs during charging.

Battery configuration.
Battery configuration.

Since an ideal universal battery does not exist, different types of batteries are suitable for different applications. The main kinds of rechargeable batteries are lead acid, nickelcadmium (NiCd), nickel-metal hydride (NiMH) and Li-Ion. NiCd batteries are being replaced by more efficient and environmentally friendly batteries such as NiMH and Li-Ion. Although NiCd batteries are robust, less prone to damage and longer lasting, they are an outdated technology and are highly toxic.

Lead-Acid Batteries

Lead-acid battery technology is mature and reliable, but is considered obsolete. Two common lead-acid battery types are the engine starter batteries and deep cycle batteries used in EVs (these days in forkl ifts or golf carts). This battery type requires inspection of electrolyte level and has a short life span, at approximately three years. These batteries have poor specific energy rate (34 Wh/kg). Because they are heavy (remember, it’s made from lead) in order to provide sufficient energy, in an EV application these batteries could represent 25 to 50 percent of the vehicle’s total mass. They also have a negative environmental impact, generate harmful gases, are toxic, and contain concentrated sulfuric acid. This type of battery was used in the early EVs (e.g., General Motors EV1). Taking into consideration all the mentioned disadvantages and the new developments available in other battery types, lead-acid batteries are not used in any new EV designs.

Golf cart batteries. (Image courtesy of bernasjogja.co.)
Golf cart batteries. (Image courtesy of bernasjogja.co.)

NiMH Batteries

Considering the specific energy, NiMH batteries are superior to lead-acid ones in that they have double the value of 68 Wh/kg (with range of 60 to 120 Wh/kg). This feature allows for lower battery weight and reduces the space required for storing the batteries. However, this is still significantly lower compared to the Li-Ion batteries, which have a 40 percent higher value of specific energy. The main advantage of NiMH batteries is their durability. Nickel batteries are well-proven for use in EVs. Many cars with these batteries have been on the road for more than 100,000 miles and have been operating successfully for over 7 years. Basically, this is the only battery type proven to belong lasting (Li-Ion batteries promise a long life, but we’ll have to see if that’s the case after they have had years of real use).

In terms of their use with EVs, NiMH batteries’ disadvantages include low charging efficiency, self-discharge (up to 12.5 percent per day at room temperature, with deteriorating performance at higher temperature). Advantages of this type of battery include that they contain little toxic material and  are recyclable. Another disadvantage of NiMH batteries is also their heat generation rate during fast charging and discharging. This requires a cooling system that consequently increases the weight of the battery, costs and limits the number of batteries that can be used. A number of legal disputes (patent encumbrance) have limited the use of NiMH batteries in EVs, shifting the focus to Li-ion technology.

Li-Ion Batteries

Today, Li-Ion batteries are the most commonly used battery in EVs. According to the Financial Times, Li-Ion batteries will take up to a 90 percent share of the EV battery market by 2025. The cathode of the traditional Li-Ion battery is made of lithium cobalt oxide and the anode involves graphite. This technology provides properties to overcome some of the shortcomings of other battery types. The Li-Ion batteries are lightweight, have a good charge cycle rate (meaning they are capable of being recharged many times), higher energy density, higher cell voltage, and a better self-discharge rate (at only 5 percent per month). An amazing specific energy rate of 140+ Wh/kg is definitely the Li-Ion battery’s main advantage. High energy density allows for a lighter battery weight, which increases an EV’s range and performance. Compared to the lead-acid batteries, the Li-Ion is one-third of the weight, is three times more powerful, and has three times the cycle life.

Li-Ion batteries have a high price, which is their biggest disadvantage. Their production costs can be 40 percent higher than nickel batteries. However, intensive research on Li-ion technology has led to decreased production costs. According to McKinsey, from 2010. to 2016, the cost of Li-Ion batteries decreased by 80 percent. Safety remains a big concern with these batteries, however, as thermal runaway can cause EVs to catch fire or explode if the battery is overcharging and the heat is not dissipated. Also, fluctuating battery charging can be dangerous. Because of this, an advanced battery management system (BMS) is required, which monitors each cell’s voltage and temperature, the state of charge (SoC) and the state of health (SoH), helping to ensure safe and reliable operation, balanced cells for long battery life and an optimized EV performance.

Li-Ion battery pack for a Tesla Model S. (Image courtesy of qnovo.com.)
Li-Ion battery pack for a Tesla Model S. (Image courtesy of qnovo.com.)

Many types of Li batteries are available, such us lithium nickel cobalt aluminum oxide (NCA), lithiummanganese oxide (LMO), lithiumnickel manganese cobalt (NMC), lithium titanate (LTO) and lithium-iron phosphate (LFP). The increasing popularity of EVs has brought battery technology into focus. Studies of new advanced battery types abound. Recent EV battery designers are focusing on providing features like fire resistance, environmental friendliness, fast charging, and long life span. At times, competing requirements have sacrificed specific energy and power properties.

Despite the public perception, the metals in Li-Ion batteries: cobalt, copper, nickel and iron are considered safe for landfills or incinerators. Materials in batteries are nontoxic, including lithium carbonate (e.g., used in ovenware), cobalt oxide (e.g., used in pottery glaze), nontoxic graphite (used in pencils), and a polymer (plastic) membrane. The toxic parts of the battery are the electrolyte and lithium cobalt oxide, which are being replaced by more benign compounds. According to Kate Krebs of the U.S. National Recycling Coalition, “Lithium Ion batteries are classified by the federal (U.S.) government as nonhazardous waste and are safe for disposal in the normal municipal waste stream.” The recycling technology of Li-Ion batteries is constantly under development. Since the availability of the battery material is limited, the recycling not only makes sense environmentally, but also economically. 

As mentioned before, EVs primarily use Li-Ion batteries, but other types of batteries are also in use. The battery types of some popular EV models are presented here:

  • Batteries of a plug-in hybrid electric vehicle (PHEV) can be charged by using an external source of electric power, as well by the vehicle’s onboard engine:
    • The PHEV Toyota Prius uses 4.4 kWh Li-Ion batteries, which provide 11 miles of driving with a charging time of 3 hours (115VAC 15A) and 1.5 hours (230VAC 15A).
    • The Chevy Volt uses 16 kWh Li-manganese/NMC batteries, which weigh 400 lb and provide 40 miles of driving with a charging time of 10 hours (115VAC 15A) and 4 hours (230VAC 15A).
  • Pure electric vehicles include:
    • The Nissan Leaf has 30 kWh Li-Manganese batteries with 192 cells, and weigh of 600 lb, with a driving range of 156 miles and a charging time of 8 hours at 230VAC, 15A, and 4h 30A.
    • The BMW i3 uses 42 kWh LMO/NMC batteries that weigh 595 lb, with a driving range of up to 215 miles and charging time of 4 hours with an 11kW onboard AC charger and 30 minutes with a 50kW DC charger.
    • The Tesla Model S uses a 75kWh battery, has a driving range of 310 miles, with a charging time 9 hours with a 10kW charger and 30 minutes with a 120kW supercharger.

Specifications of the popular battery types used in EVs

Battery type

Life span (cycle)

Nominal voltage (V)

Specific energy (Wh/kg)

Specific power (W/kg)

Charging efficiency

Self-discharge rate (%/month)

Safety

Li-ion

600-3,000

3.2-3.7

100-270

250-680

80-90

3-10

Safe

Lead acid

200-300

2.0

30-50

180

50-95

5

Risky (generate harmful gases)

NiCd

1000

1.2

50-80

150

70-90

20

Risky (highly toxic)

NiMH

300-600

1.2

60-120

250-1,000

65

30

Safe

Source: https://link.springer.com/article/10.1007/s11465-018-0516-8.

Conclusion

The battery system is a significant and important part of an EV. The different varieties of Li-Ion batteries are currently the most dominant battery type used in EVs. As a result of increasing demand, there is a requirement for better performance of batteries in terms of reduced weight, better cycling ability, the use of recyclable materials, and general battery performance and better driving range. The next generation of EV batteries will be solid-state batteries where the liquid electrolyte is replaced with a solid, conductive material. This technology provides a high specific energy rate that will provide an improvement over today’s Li-ion batteries. BMW and Solid Power havealready partnered to develop a new solid-state battery for EVs. In addition, Nexeon is researching new materials based on silicon to replace carbon in the anode, which could double the range of EVs. The combination of silicon and binder significantly increases the battery’s energy density.


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