Do you know the basics of rechargeable batteries?

Do you know the basics of rechargeable batteries?

A battery includes one or more voltaic batteries. Each voltaic battery contains two and a half batteries. Negatively charged anions migrate to the anode of one half-cell (negative electrode), while positively charged cations migrate to the cathode of the other half-cell (positive electrode). The electrodes are separated by the electrolyte medium, and the electrolyte ionizes to produce anions and cations, and these ions can move.

In some battery designs, half-cells have different electrolytes. In this case, the separator can prevent the electrolyte from mixing, and the ions can pass through many types of batteries, such as carbon-zinc (C-Zn), nickel-cadmium (Ni-Cd) and lithium (Li) batteries. Kind of design. The most critical performance parameters of various rechargeable batteries are mass energy density (expressed in watt·hour/kg, or W·h/kg) and volume energy density (expressed in watt·hour/liter, or W·h/L) ), as shown in Figure 1. The electrolyte medium only acts as a buffer when ions flow between the electrodes. However, in lead-acid batteries that are widely used in automobiles, the electrolyte is part of the electrochemical reaction.

Do you know the basics of rechargeable batteries?
Figure 1 Mass energy density and volume energy density of different rechargeable batteries

The open circuit voltage of a charged battery is the electromotive force (EMF) of the battery, which results from the difference in the reduction potential of the chemical reaction in the half-cell. In the discharge phase, the battery converts the heat released during the chemical reaction between the positive and negative terminals into electrical energy.

In actual use, the battery is equivalent to an equivalent series resistance (ESR). When the battery is used in an external circuit or load, the open circuit voltage will drop a little. As the battery discharges or provides power to an external load, the equivalent series resistance ESR will increase, and the battery’s road terminal voltage will drop under load conditions. In addition, the battery tends to discharge during sleep. Depending on its type, some batteries will discharge more than others. The research conducted by the author shows that nickel-cadmium and nickel-metal hydride (Ni-MH) batteries discharge at a rate of approximately 20% per month. In contrast, lithium batteries discharge 5% to 10% per month, and lead-acid batteries The monthly discharge is 3%~4%, and the monthly discharge of alkaline batteries is less than 0.3%. This clearly shows that the self-discharge rate of alkaline batteries during sleep is low.

Rechargeable batteries are rechargeable through reversible chemical reactions in the discharge process. Charging is a typical redox chemical process. The negative electrode material is reduced, consuming electrons, and the positive electrode material is oxidized and generates electrons. Regardless of the type of battery, the output energy level and power density are key battery characteristics, which are expressed in terms of per unit volume and per unit mass, respectively.

①Key performance characteristics of rechargeable batteries
The two most important performance characteristics of rechargeable batteries are volumetric energy density (W·h/L) and mass energy density (W·h/kg). From these parameters, one can estimate the energy that can be obtained from 1kg of fuel and 1L of fuel. The author has studied several types of rechargeable batteries currently on the market, and can choose a suitable battery according to its specific application3. The mass energy density and volume energy density curves of various rechargeable batteries are shown in Figure 1. The performance evaluation of various rechargeable batteries is summarized in Table 1.

Battery type and classificationMass energy density/(W·h/kg)
Volume energy density/(W·h/L)
Lead acid2570
Prism battery
Type 240160
Nickel Cadmium35240
Nickel Metal Hydrogen62260
lithium ion120300
Lithium Phosphate160290
Lithium polymer (prismatic)188294
Table 1 Performance characteristics of different rechargeable batteries

These are the evaluation values ​​of batteries developed many years ago, accurate to ±5%. The same battery is now increased by 5%~10%. The reliability, efficiency and life of these batteries will be described in detail in later chapters.

②Performance of rechargeable batteries widely used in commercial applications
When choosing a commercial rechargeable power battery, the mass energy density (W·h/kg), volume energy density (W·h/L), life (working time), self-discharge rate (%/month) and specific power (W/ kg) need to be carefully considered. The performance characteristics of commercial batteries are summarized in Table 2.

Battery TypeRated voltage/VMass energy density/(w.h/kg)Volume energy density/(w.h/L)Self-discharge/(%/month)Cycle life/h
Lead acid2.035754~6250~500
lithium ion3.61152605~10500~1000
Lithium polymer3.0100~200150~3500~1200~1000
Nickel Cadmium1.24~6060~10010~20300~700
Nickel Metal Hydrogen1.26022030300~600
Zinc air1.21462040~50~200
Table 2 Performance of widely used commercial batteries

In order to analyze the rechargeable battery more thoroughly, the data of the popular Duracell alkaline battery is also listed. The cycle life is strictly dependent on how the battery is handled or used. Such as lithium batteries, the anode voltage decreases linearly from 4V to 3V. Commercially produced nickel metal hydride (Ni-MH) batteries by Toshiba may have memory loss. Zinc-air batteries were originally installed in electric vehicles and hybrid electric vehicles as battery prototypes. The performance data summarized in Table 2 are not obtained under the same working conditions for all batteries.