Lithium battery basic knowledge, battery classification, working principle, origin

Definition and classification of battery: unnecessarily accompanied by mechanical movement, and the direct conversion of various energy into direct current power generation device

 

Working principle of battery: In the process of discharging, oxidation reaction occurs at the negative electrode, providing electrons to the outside; reduction reaction occurs at the positive electrode, receiving electrons from the external circuit, electrons flow from the negative electrode to the positive electrode, while the current direction is exactly opposite to the direction of electron flow, so the current flows from the positive electrode to the negative electrode through the external circuit

 

Origin of the battery:

The earliest dry cell was invented in 1887 by the Englishman Hererson.

In 1899, Waldmar Jungner invented the nickel-cadmium battery.

1914, Thomas Edison invented the alkaline battery.

In 1976, Philips Research scientists invented the nickel-hydrogen battery.

1991, Sony. rechargeable lithium-ion batteries are commercially produced

 

Lithium Battery Basics - Battery Terminology

Capacity (Capacity) The amount of power that a battery can give under certain discharge conditions is called the capacity of the battery and is expressed by the symbol C. The commonly used unit is ampere-hour, referred to as ampere-hour (Ah) or milliampere hour (mAh).

 

The capacity of a battery can be divided into theoretical capacity, rated capacity and actual capacity.

1) Theoretical capacity is the highest theoretical value obtained by calculating the mass of the active substance according to Faraday's law. In order to compare different series of batteries, the concept of specific capacity is commonly used, i.e. the theoretical power that can be given per unit volume or per unit mass of the battery in Ah/kg (mAh/g) or Ah/L (mAh/cm3).

2) The actual capacity is the amount of electricity that a battery can output under certain conditions. It is equal to the product of discharge current and discharge time in Ah, and its value is less than the theoretical capacity.

3) Rated capacity is also called guaranteed capacity, which is the minimum capacity that the battery should discharge under certain discharge conditions according to the standards promulgated by the state or relevant departments.

 

Internal resistance (Impedance)

1) Definition: The current is resisted when it passes through the interior of the battery, so that the voltage of the battery is reduced, and this resistance is called the internal resistance of the battery.

2) The internal resistance of a battery is not a constant and changes over time during discharge because the composition of the active material, the concentration of the electrolyte and the temperature are constantly changing.

(3) The internal resistance of a battery includes ohmic internal resistance and polarized internal resistance, and polarized internal resistance includes electrochemical polarization and concentration polarization. The existence of internal resistance makes the end voltage of the battery lower than the battery electric potential and open circuit voltage when discharging, and higher than the electric potential and open circuit voltage when charging.

(4) ohmic resistance obeys ohm's law; polarization resistance increases with current density, but not linearly, often increases linearly with the logarithm of current density.

 

Open circuit voltage (OCV)

(1) When the battery is not discharged, the potential difference between the two poles of the battery is called open circuit voltage.

(2) The open circuit voltage of a battery varies according to the material of the positive and negative electrodes and the electrolyte. If the material of the positive and negative electrodes is exactly the same, the open circuit voltage will be the same no matter how large the volume of the battery is and how the geometry changes.

4、Termination voltage (Cut-off discharge voltage)

It means that when the battery is discharged, the voltage drops to the lowest working voltage value that the battery should not continue to discharge. According to different battery types and different discharge conditions, the capacity and life requirements of the battery are also different, so the specified termination voltage of battery discharge is also different.

 

Depth of discharge DOD

(1) In the process of using the battery, the percentage of its rated capacity released by the battery is called the depth of discharge.

(2) The depth of discharge and the charging life of the secondary battery have a deep relationship, when the deeper the depth of discharge of the secondary battery, the shorter its charging life, so in use should try to avoid deep discharge.

 

Over discharge

If the battery is discharged beyond the end voltage value and continues to discharge, it may cause the internal pressure of the battery to rise and the reversibility of the positive and negative active substances to be damaged, resulting in a significant reduction in the capacity of the battery.

 

Energy density

(1) The average volume or mass of electricity released by a battery.

(2) In general, under the same volume, the energy density of lithium-ion batteries is 2.5 times that of nickel-cadmium batteries and 1.8 times that of nickel-metal hydride batteries, so in the case of equal battery capacity, lithium-ion batteries will be smaller and lighter than nickel-cadmium and nickel-metal hydride batteries.

 

Self discharge

This is a phenomenon that occurs when a battery loses its charge for various reasons, whether it is used or not. After the battery has been fully charged, it is left for one month. The battery is then discharged to 3.0V with 1C, the capacity is C2; the initial capacity of the battery is C0; 1-C2/C0 is the monthly self-discharge rate of the battery.

 

Charge cycle life (Cycle life)

(1) The battery is fully charged and then fully discharged, cycling until the capacity decays to 75% of the initial capacity, the number of cycles is the cycle life of the battery

(2) Cycle life is related to the battery charging and discharging conditions, 1C charge/discharge cycle life of lithium-ion batteries at room temperature can be 300-500 times (industry standard), up to 800-1000 times.

 

Memory effect

(1) The memory effect is for NiCd batteries, as the negative electrode in the traditional process is sintered and the cadmium grains are coarse, if NiCd batteries are recharged before they are fully discharged, the cadmium grains tend to agglomerate into a block and form a secondary discharge platform when the battery is discharged. The cell will store this discharge plateau and use it as the end of the discharge in the next cycle, although the capacity of the cell itself will allow the cell to discharge to a lower plateau. The battery will only remember this low capacity during subsequent discharges. Also in each use, any incomplete discharge will deepen this effect, making the battery's capacity even lower.

(2) To eliminate this effect, there are two methods, one is to use a low current deep discharge (such as with 0.1C discharge to 0V) one is to use a high current charge and discharge (such as 1C) several times.

(3) Both NiMH and Li-ion batteries have no memory effect.