31 Battery Basics

The batteries are made of unit cells containing the chemical energy that is convertible to electrical energy. One or more of these electrolytic cells are connected in series to form one battery. The grouped cells are enclosed in a casing to form a battery module. A battery pack is a collection of these individual battery modules connected in a series and parallel combination to deliver the desired voltage and energy to the power electronic drive system.

The energy stored in a battery is the difference in free energy between chemical components in the charged and discharged states. This available chemical energy in a cell is converted into electrical energy only on demand, using the basic components of a unit cell, which are the positive and negative electrodes, the separators, and the electrolytes. The electrochemically active ingredient of the positive or negative electrode is called the active material. Chemical oxidation and reduction processes take place at the two electrodes, thereby bonding and releasing electrons, respectively. The electrodes must be electronically conducting and are located at different sites, separated by a separator, as shown in Figure 3.1. During battery operation, chemical reactions at each of the electrodes cause electrons to flow from one electrode to another; however, the flow of electrons in the cell is sustainable only if electrons generated in the chemical reaction are able to flow through an external electrical circuit that connects the two electrodes. The connection points between the electrodes and the external circuit are called the battery terminals. The external circuit ensures that most of the stored chemical energy is released only on demand and is utilized as electrical energy. It must be mentioned that only in an ideal battery does current flow only when the circuit between the electrodes is completed externally. Unfortunately, many batteries do allow a slow discharge, due to diffusion effects, which is why they are not particularly good for long-term energy storage. This slow discharge with open-circuit terminals is known as self-discharge, which is also used as a descriptor of battery quality.

FIGURE 3.1 Components of a battery cell. (a) Cell circuit symbol; (b) cell cross-section.

The components of the battery cell are described as follows:

1. Positive electrode: The positive electrode is an oxide or sulfide or some other compound that is capable of being reduced during cell discharge. This electrode consumes electrons from the external circuit during cell discharge. Examples of positive electrodes are lead oxide (PbO2) and nickel oxyhydroxide (NiOOH). The electrode materials are in the solid state.

2. Negative electrode: The negative electrode is a metal or an alloy that is capable of being oxidized during cell discharge. This electrode generates electrons in the external circuit during cell discharge. Examples of negative electodes are lead (Pb) and cadmium (Cd). Negative electrode materials are also in the solid state within the battery cell.

3. Electrolyte: The electrolyte is the medium that permits ionic conduction between positive and negative electrodes of a cell. The electrolyte must have high and selective conductivity for the ions that take part in electrode reactions, but it must be a nonconductor for electrons in order to avoid self-discharge of batteries. The electrolyte may be liquid, gel, or solid material. Also, the electrolyte can be acidic or alkaline, depending on the type of battery. Traditional batteries such as lead-acid and nickel-cadmium use liquid electrolytes. In lead-acid batteries, the electrolyte is the aqueous solution of sulfuric acid [H2SO4(aq)]. Advanced batteries currently under development for EVs, such as sealed lead-acid, nickel-metal-hydride (NiMH), and lithium-ion batteries use an electrolyte that is gel, paste, or resin. Lithium-polymer batteries use a solid electrolyte.

4. Separator: The separator is the electrically insulating layer of material that physically separates electrodes of opposite polarity. Separators must be permeable to the ions of the electrolyte and may also have the function of storing or immobilizing the electrolyte. Present day separators are made from synthetic polymers.

There are two basic types of batteries: primary batteries and secondary batteries. Batteries that cannot be recharged and are designed for a single discharge are known as primary batteries. Examples of these are the lithium batteries used in watches, calculators, cameras, etc., and the manganese dioxide batteries used to power toys, radios, torches, etc. Batteries that can be recharged by flowing current in the direction opposite to that during discharge are known as secondary batteries. The chemical reaction process during cell charge operation when electrical energy is converted into chemical energy is the reverse of that during discharge. The batteries needed and used for EVs and HEVs are all secondary batteries, because they are recharged during regeneration cycles of vehicle operation or during the battery recharging cycle in the stopped condition using a charger. All the batteries that will be discussed in the following are examples of secondary batteries.

The maj or types of rechargeable batteries considered for EV and HEV applications are:

• Nickel-metal-hydride (NiMH)

The lead-acid type of battery has the longest development history of all battery technology, particularly for their need and heavy use in industrial EVs, such as for golf carts in sports, passenger cars in airports, and forklifts in storage facilities and supermarkets. Research and development for batteries picked up momentum following the resurgence of interest in EVs and HEVs in the late 1960s and early 1970s. Sodium-sulfur batteries showed great promise in the 1980s, with high energy and power densities, but safety and manufacturing difficulties led to the abandonment of the technology. The development of battery technology for low-power applications, such as cell phones and calculators, opened the possibilities of scaling the energy and power of nickel-cadmium- and lithium-ion-type batteries for EV and HEV applications.

The development of batteries is directed toward overcoming significant practical and manufacturing difficulties. Theoretical predictions are difficult to match in manufactured products due to practical limitations. Theoretical and practical specific energies of several batteries are given in Table 3.2 for comparison.

The characteristics of some of the more important battery technologies mentioned above are given in the following. The theoretical aspects of the lead-acid

TABLE 3.2 Specific Energy of Batteries

Specific Energy (Wh/kg)

Lead-acid Nickel-cadmium

Battery

Theoretical 108

Practical

50 20-30 90

Nickel-zinc Nickel-iron

Zinc-chlorine Silver-zinc Sodium-sulfur Aluminum-air

150-300

battery will be discussed in detail first, followed by shorter descriptions of the other promising technologies.

Lead-acid batteries have been the most popular choice of batteries for EVs. Lead-acid batteries can be designed to be high powered and are inexpensive, safe, and reliable. A recycling infrastructure is in place for them. However, low specific energy, poor cold temperature performance, and short calendar and cycle life are among the obstacles to their use in EVs and HEVs.

The lead-acid battery has a history that dates to the middle of the 19th century, and it is currently a mature technology. The first lead-acid battery was produced as early as in 1859. In the early 1980s, over 100,000,000 lead-acid batteries were produced per year. The long existence of the lead-acid battery is due to the following:

• Relatively low cost

• Easy availability of raw materials (lead, sulfur)

• Ease of manufacture

• Favorable electromechanical characteristics

The battery cell operation consists of a cell discharge operation, when the energy is supplied from the battery to the electric motor to develop propulsion power, and a cell charge operation, when energy is supplied from an external source to store energy in the battery.

DIY Battery Repair

DIY Battery Repair

You can now recondition your old batteries at home and bring them back to 100 percent of their working condition. This guide will enable you to revive All NiCd batteries regardless of brand and battery volt. It will give you the required information on how to re-energize and revive your NiCd batteries through the RVD process, charging method and charging guidelines.

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