How to reduce the internal resistance of lithium batteries to improve battery performance

 

1. Basic concepts and definitions of lithium battery internal resistance:

1. Basic concepts:

The internal resistance of a lithium battery refers to the resistance to current inside the battery. The main sources of internal resistance are the resistance and polarization effects of components such as the electrolyte, positive and negative electrode materials, and diaphragm inside the battery. The size of the internal resistance has a direct impact on the working state of the battery. A smaller internal resistance can enable the battery to output a more stable voltage and current during operation, thereby improving the overall performance of the battery. Conversely, a larger internal resistance will cause the battery to generate more heat during discharge, reducing the efficiency and service life of the battery.

 

2. Classification of lithium battery internal resistance:

The internal resistance of a lithium battery can be divided into ohmic internal resistance and polarization internal resistance.

 

(1) Ohmic internal resistance: Ohmic internal resistance is mainly composed of electrode materials, electrolytes, diaphragm resistance, and contact resistance of various parts, and is related to factors such as the size, structure, and connection method of the battery. Ohmic internal resistance reflects the inherent resistance inside the battery, which always exists during the battery charging and discharging process.

 

(2) Polarization internal resistance: Polarization internal resistance is the resistance that appears at the moment of current loading. It is the sum of the obstacles to the transmission of charged ions caused by various factors inside the battery. Polarization internal resistance can be further divided into electrochemical polarization and concentration polarization. Electrochemical polarization is caused by the limited rate of chemical reactions on the electrode surface, while concentration polarization is caused by the difference in ion concentration in the electrolyte.

 

(3) Calculation of internal resistance: The precise calculation of internal resistance is quite complicated and will continue to change during the use of the battery. The size of the internal resistance is usually estimated through experiments or experience. Generally, the larger the volume of the lithium battery, the smaller its internal resistance. This is because the larger battery volume can provide a wider ion transmission channel, reducing the resistance to ion transmission. The internal resistance of a lithium battery is one of the important indicators of battery performance and is affected by many factors.

 

2. The impact of lithium battery internal resistance on battery performance:
1. Lithium battery discharge:
In terms of discharge performance, the larger the internal resistance of a lithium battery, the more voltage is lost during the discharge process, resulting in a lower output voltage. In addition, a larger internal resistance will also limit the size of the output current. The size of the lithium battery's internal resistance directly affects the discharge capacity of the battery. In scenarios where high power output is required, such as electric vehicle acceleration or high-load operation of electronic equipment, lithium batteries with large internal resistance may not be able to meet the demand, resulting in performance degradation.

 

2. Charging of lithium batteries:
In terms of charging performance, internal resistance also plays a key role. When the internal resistance is large, the charging current will be limited, resulting in slower charging speed. This not only prolongs the charging time, but also may affect the charging efficiency of the battery. Especially with the widespread application of fast charging technology, reducing internal resistance has become the key to improving charging performance.

 

3. Internal resistance also affects the self-discharge rate of lithium batteries:
Self-discharge refers to the loss of power due to internal chemical reactions when the battery is not connected to an external circuit. The larger the internal resistance, the higher the self-discharge rate, which will also cause rapid loss of power when the battery is not in use.

 

4. The impact of internal resistance on the temperature characteristics of lithium batteries:
Lithium batteries generate heat when working, and the larger the internal resistance, the more heat is generated. This may not only cause the battery temperature to rise but also affect the stability and safety of the battery.

 

5. Internal resistance also has an important impact on the cycle life of lithium batteries:
Cycle life refers to the durability of the battery during the charge and discharge cycle. The greater the internal resistance, the greater the loss of the battery during the cycle, resulting in a gradual decline in battery performance, which ultimately affects the battery life.

 

3. Internal resistance standards of lithium batteries:

1. The internal resistance of the currently common and relatively excellent 18650 lithium-ion battery is about 12 milliohms, while the ordinary internal resistance is around 13-15 milliohms. Impedance affects battery performance. Under normal circumstances, 50 milliohms is normal, and battery performance begins to decline at 50-100 milliohms. More than 100 milliohms need to be used in parallel, and more than 200 milliohms are basically unusable.

 

2. Contents that should be indicated in the lithium-ion battery specification table:
The lithium battery specification table contains many key parameters. The following is a simplified example of a lithium-ion battery specification table:
(1) Model: For example, ICR18650 (cylindrical), or ICP103450 (square): Cylindrical lithium battery model: such as ICR18650, where "ICR" indicates a cylindrical lithium battery, "18" indicates a diameter of 18 mm, and "650" indicates a height of 65 mm.

Square lithium battery model: such as ICP103450, where "ICP" may indicate a specific type of square lithium battery, "10" indicates a thickness of approximately 10 mm, "34" indicates a width of approximately 34 mm, and "50" indicates a height of approximately 50 mm. Voltage: nominal voltage of single lithium battery: 3.7V (nominal voltage of single lithium battery: 3.6V, nominal voltage of single lithium iron phosphate battery: 3.2V)

(2) Operating voltage range: for a single lithium battery, it is usually 2.5V-4.2V (2.5V-4.2V for ternary lithium battery, 2.0V-3.65V for lithium iron phosphate battery). For the battery pack, the voltage increases accordingly according to the number of strings.

(3) Capacity: usually expressed in ampere-hour (Ah) or milliampere-hour (mAh). For example, 10Ah, 15Ah, 120mAh, 160mAh, etc. The capacity determines the amount of energy stored in the battery and is usually related to the size and weight of the battery.

(4) Battery type: for example, lithium-ion battery, lithium polymer battery, etc. Different types of batteries have different characteristics and design requirements.

(5) Weight: The weight of lithium batteries varies depending on the model, capacity, and type. For example, a typical lithium battery pack weighs 16-30 kg, while a single lithium battery may weigh between 2.5-3.0 kg.

(6) Internal resistance: includes ohmic internal resistance and polarization internal resistance. Ohmic internal resistance is mainly composed of electrode materials, electrolyte, diaphragm resistance, and contact resistance of current collector and tab. Polarization internal resistance is related to the electrochemical reaction process of the battery.

(7) Other parameters: also include charge and discharge rate, charge cut-off voltage, discharge cut-off voltage, operating temperature range, storage conditions, etc.; this is only a simplified specification sheet example, and the actual product specification sheet may contain more detailed information and parameters. If you need a specific specification sheet, please refer to the manual of the lithium battery you purchased or contact the manufacturer to obtain it.