Definition of DCR
Lithium battery DCR (Direct Current Internal Resistance) is the sum of all internal ohmic resistances of a battery when DC current flows through it, including ohmic internal resistance, charge transfer resistance, and polarization resistance. The DCR test calculates the internal resistance (R = ΔU/I) by applying a constant current and measuring the change in voltage. This reflects the dynamic impedance characteristics of the battery under actual operating conditions. These ohmic resistances include electrode resistance, electrolyte resistance, and separator resistance. When current flows through the battery, this internal resistance causes a voltage drop within the battery, affecting its performance.
DCR measures the role of battery performance
(1) Evaluating Battery State of Health (SOH)
A battery's state of health (SOH) refers to the ratio of its current performance to its performance when brand new. It reflects the battery's aging and remaining service life. DCR is a key indicator for evaluating the SOH of lithium batteries. A new battery has an extremely low DCR. This is because, in the initial stages of battery manufacturing, the electrode material structure is intact, the electrolyte performance is excellent, the ionic conductivity of the separator is optimal, and the internal ohmic resistance is low. As the battery cycles, the electrode material undergoes structural changes, increasing the internal ohmic resistance and gradually increasing the DCR. By monitoring changes in the DCR, the battery's aging can be monitored in real time, providing a basis for replacement and maintenance.
(2) Correlation with Other Battery Performance Parameters
DCR is also closely related to other battery performance parameters. For example, it is related to the battery's capacity retention rate. When the DCR increases, the battery's energy loss during charge and discharge increases, resulting in a decrease in the battery's actual usable capacity. Furthermore, DCR also affects the battery's self-discharge rate. Batteries with higher internal resistance experience increased energy loss during self-discharge, shortening the battery's shelf life.
Effect of DCR on battery discharge capacity
(1) Advantages of a Small DCR
The smaller the DCR, the smaller the battery's voltage drop during high-current discharge. In real-world applications, many devices require batteries capable of high-current discharge, such as mobile phone fast charging and electric vehicle acceleration. For example, when a mobile phone supports fast charging, the charging current increases significantly. If the battery's DCR is small, the internal voltage drop during high-current charging is minimal, ensuring the battery receives energy efficiently and enabling rapid charging. Furthermore, a small DCR ensures more stable battery output power, preventing excessive voltage fluctuations that can affect device operation.
(2) Harms of a Large DCR
When the DCR is excessive, the battery will experience a significant voltage drop during discharge. During electric vehicle acceleration, if the battery's DCR is too high, the motor will require high current, causing the battery voltage to drop rapidly. When the voltage drops below a certain level, the battery management system triggers a protection mechanism to limit the battery's output current, resulting in a decrease in the electric vehicle's power performance and even failure to accelerate properly. In addition, excessive DCR will cause the battery to generate excessive heat during discharge, which will not only reduce the efficiency of the battery, but also accelerate battery aging and shorten the battery life.
The DCR of lithium batteries plays an essential role in measuring battery performance and influencing its discharge capacity. It is not only a key indicator for assessing battery health but also closely correlates with actual performance. Monitoring DCR changes through appropriate testing methods provides timely insight into battery performance, providing a basis for optimized battery design, rational use, and effective management.