Battery Technologies for Electric Vehicles and Other Green Industrial Projects

Kevin Yiu

EVB Technology (HK) Limited, Hong Kong E-mail: kevin_yiu@goldpeak.com

Abstract–The hottest debate in the electric transportationindustry has been "which battery technology will be the ultimate winner?" Unlike power for other consumer electronics, batteries for electric vehicles involve green technologies that require high power storage.

The battery with the highest power rating, highest energy density, longest service life, widest operating temperature, best price and a battery management system that makes the battery control logic transparent is the answer. Through an overview of real-life applications, pros and cons of various battery technologies and chemistries will be discussed as well as capabilities of what batteries can attain and the common misunderstanding of batteries.

I. INTRODUCTION

Battery is an energy storage device. It does not generate energy by itself, but rather, it stores energy from an external source and delivers the stored energy to another device.

In this paper, a few common chemical systems of batteries for electric vehicles (“EV”) and industrial applications are discussed. Many factors are affecting the operational characteristics, capacity, energy output and performance of a battery. Different manufacturers have different approaches to address to these issues by different chemical systems, additives, form factors and dimensions, which will have significant effect to different performance and usages of the batteries.

There are basically three major categories of chemical families, namely Lead system, Nickel system and Lithium system. They can be further divided into subcategories. Each system from different manufacturers has its own advantages and disadvantages and may be suitable for specific applications.

Up to now, there is no industrial standard on the dimension or chemical system for electric vehicles and industrial applications. There are, however, some guidelines in the trade that address to the safety and requirements for some specific applications. IEC, ANSI, UN, UL, USABC and a number of other international bodies each provides a range of different requirements for different applications.

II. CHEMICAL SYSTEMS

In general, batteries for EV and industrial applications require long calendar life, long cycle life, light weight, fast charge and discharge rates. For pure EVs, high capacity is also required.

There are basically three families that dominate the EVs and industrial market.

2.1 Lithium Lithium batteries comprise cells that employ lithium intercalation compounds as the positive and negative electrodes. They are further divided into three subfamilies by the positive chemistry and three different form factors:

2.1.1 Lithium Cobalt Lithium Cobalt oxide battery offers good electrical performance and has very high energy density. However, with its low discharge capacity and dangerous behavior on abuse, it is more suitable for consumer applications such as notebook computers, cell phones and other personal electronic devices that require high energy density with low discharge rate. It is not suitable for electric vehicles and industrial applications due to its lack of robustness.

2.1.2 Lithium Manganese Lithium Manganese battery is further divided into sub-catergories of pure oxide and mixed oxide. It has a higher safety factor than Lithium Cobalt and comes at a lower cost. However, it is only suitable for smaller applications such as electric bicycles and scooters until there is a technology breakthrough on its cycle life.

2.1.3 Lithium Ferro Phosphate Lithium Ferro Phosphate battery is the most balanced choice within the lithium family due to its safety factor, high power density, long cycle life and lower cost. Although the energy density is not as good as other lithium batteries, it is the most robust branch within the lithium family.

2.2 Nickel2.2.1 Nickel Metal Hydride Nickel Metal Hydride (“NiMH”) battery has excellent performance in high power density and safety. However, because of volatility of Nickel price and relatively lower energy density than Lithium battery, it loses its competitive advantage when compared with Lithium batteries. Despite this, NiMH is still the most commonly used battery chemistry for electric and hybrid vehicles. The application of NiMH is shifting from EV to heavy EV such as buses and trucks.

2.2.2 Nickel Cadmium Nickel Cadmium battery is arguably the chemical system with most stable performance. However, because of the poisonous Cadmium heavy metal, this system has already

been banned by a lot of countries for EV, industrial and commercial applications.

2.3 Lead Acid Lead Acid battery is the most economical chemical system for EV applications. However, with its limited performance, poor cycle life and heavy weight, it is only limited to proof of concept vehicle and prototype trial.

III. FORM FACTORS

There are three major form factors for batteries in EV and industrial applications:

3.1 Cylindrical This is the most common type for consumer batteries and the most well known design, from AA and AAA to 18650 and 26650 for notebook batteries. They are also called wound type batteries because of the wounded electrode assembly. This form factor has a lower cost because of the short production time. However, the size limits the flexibility and capacity of the battery pack. The capacity is usually limited to less than 10 Ahr per cell.

3.2 Prismatic They are also called stacked batteries as electrode is stacked. This form factor is the best for EV application because it enables a good conduction path and also good air flow for thermal management. However, because of a longer production time and higher production requirement, it tends to be more expensive than cylindrical design. The capacity of prismatic batteries can accommodate batteries up to 100Ahr.

3.3 Polymer This is a form factor especially designed for Lithium batteries. A polymer battery uses aluminum bag as a battery casing and polymer gel as electrolyte. It is excellent for special tailor-made design and very light weight, and therefore suitable for consumer OEM application and e-bike applications. However, with the life of the aluminum bag and sealing, it will need more breakthroughs in the design until it can be widely used in the EV industry.

IV. APPLICATIONS

Cell vs Battery: A cell is the basic electrochemical unit providing a source of electrical energy by direct conversion of chemical energy. The cell consists of an assembly of electrodes, separators, electrolyte, container and terminals. A battery consists of one or more electrochemical cells electrically connected in an appropriate series or parallel arrangement to provide the required operating voltage and current levels, including, if any, monitors, controls and other ancillary components (e.g. fuses, diodes), case, terminals and markings. More often, the electrical data is presented on the basis of a single-cell battery. The performance of a multi-cell battery will usually be different than that of the individual cells or single-cell battery. [1]

Battery packs for electric vehicles or industrial applications are much more complicated than consumer applications. Usually, the system operational voltage ranges from 48V to over 600V, and energy level ranges from 1 kwhr to more than 100 kwhr. The industry has two different approaches in addressing these:

4.1 Small cells with multi-string parallel Many pieces of small cells connected in a parallel string and strings connected in series for voltage. Usually, cell capacity ranges from 1 Ahr to 2 Ahr. Consumer-type cells can be used to enjoy the economy of scale for lower cost. However, the more cells are connected, the higher possibility of failure.

4.2 Large cells with single or minimal string A cell of larger capacity connected in one string or at least a minimum number of strings. There will be fewer cells being connected and thus smaller chance of failure. However, the economy of scale is lower than general consumer cells.

V. BATTERY MANAGEMENT

With large numbers of cells involved, battery management and balancing become a critical issue. For NiMH batteries, the minimum monitoring level is per module or every 10 cells. Lithium batteries require a more sophisticated monitoring system to monitor individual cell’s parameters such as voltage, temperature and internal resistance.

VI. CONCLUSION

A battery pack is only as good as the weakest cell, that is why in a battery pack, the battery monitoring, balancing and maintenance have a crucial role in the performance of the battery. Among all the chemical systems, NiMH batteries and Lithium ferro phosphate are the most popular choices.

REFERENCES

[1] D. Linden, T. B. Reddy, “Handbook of Batteries”, Third edition, McGraw-Hill, 2001, pp. 1.3.

BIOGRAPHY

Kevin Yiu obtained his bachelor’s degree in Engineering from Ryerson University in Canada and an MBA degree from The Chinese University of Hong Kong. He has been working in the electric vehicle industry for more than 10 years, focusing on batteries manufacturing, R&D, and sales and marketing from batteries to energy storage systems for electric vehicles and other industrial applications that require heavy-duty power storage. He is currently Business Development Director of EVB Technology (HK) Limited, a subsidiary of Singapore-listed GP

Batteries International Limited.