Category: 07. Battery Fundamentals

As is well known by any user of rechargeable batteries, performance degrades over time. Although a new battery might last for 3 or 4 hours, with extended use, the working time before recharging may now be just 1 or 2 hours. This reduction in operating time, also known as aging, represents a loss in the…

Charge retention refers to the amount of charge, usually expressed as a percentage of capacity, remaining after a cell is stored for a period of time and not connected to an external circuit. Self-discharge describes the mechanism by which the capacity of the cell is reduced. Rates of self-discharge can vary dramatically—a 10% loss in…

The most basic efficiency of a rechargeable cell is the coulombic efficiency. Note that in contrast to the faradaic and current efficiencies that were defined in Chapters 1 and 3, this coulombic efficiency refers to a battery that undergoes a complete charge–discharge cycle. (7.21) Why would the number of coulombs be different for charge and discharge? The principal…

For a commercial secondary cell, the polarization losses discussed in Section 7.4 are usually quite low when the cell is used as designed. Hence, most of the available chemical energy is converted into electrical work rather than heat. Nonetheless, heat generation plays an important role in cell performance, system design, battery safety, and useable cell life.…

Power and energy are often key design aims of an electrochemical system for energy storage and conversion. The instantaneous power produced by a cell is simply the current multiplied by the potential of the cell. Even if the current is constant, the potential is not generally flat and changes during discharge. Therefore, integration is required…

In order to understand how a battery will perform in service, it is important to understand how the potential of the cell is impacted by factors such as the rate at which the cell is charged or discharged, the cell temperature, and the SOC of the cell. Thermodynamics, electrode kinetics, and transport phenomena all have…

The capacity is a rating of the charge or energy stored in the cell. This value is expressed in either ampere-hours [A·h] or watt-hours [W·h]. The first represents the capacity in terms of coulombs of charge available, the second in terms of energy available. The two are related simply by the average voltage of the…

Batteries are classified as either primary or secondary. The easiest way of thinking about this is that secondary batteries are rechargeable. By reversing the current through the cell, a secondary battery can be recharged and used again. That is, electrical energy is used to restore a secondary cell back to its original state, whereas a…

Figure 7.1 shows a simple electrochemical cell known as the Daniell cell (1839). Here zinc is oxidized at one electrode and copper ions reduced at the other: The main components of a cell are the negative electrode (zinc bar), the positive electrode (copper bar), and the electrolyte. The electrolyte is an ion conductor and an electronic…