5. Cycle Life (unit: times) & Depth of discharge, DoD
Depth of discharge: Indicates the percentage of battery discharge to the rated capacity of the battery. Shallow cycle batteries should not discharge more than 25% of their capacity, while deep cycle batteries can discharge 80% of their capacity. The battery starts discharging at the upper limit voltage and terminates discharging at the lower limit voltage. Define all discharged charge as 100%. Battery standard 80% DOD means to discharge 80% of the charge. For example, if the initial SOC is 100% and I put it at 20% and stop, that’s 80% DOD.
The life of a lithium-ion battery will gradually decay with use and storage, and it will be more obvious. Still take smart phones as an example, after using the phone for a period of time, you can obviously feel the phone battery “not durable”, the beginning may only charge once a day, the back may need to charge twice a day, which is the embodiment of the continuous decline in battery life.
Lithium-ion battery life is divided into two parameters: cycle life and calendar life. Cycle life is generally measured in cycles, which characterizes the number of times a battery can be charged and discharged. Of course, there are conditions here, generally in the ideal temperature and humidity, with the rated charge and discharge current for the depth of charge and discharge (80% DOD), calculate the number of cycles experienced when the battery capacity declines to 20% of the rated capacity.
The definition of calendar life is a bit more complicated, the battery can’t always be charging and discharging, there is storage and shelving, and can’t always be in the ideal environmental conditions, it will go through all kinds of temperature and humidity conditions, and the multiplication rate of charging and discharging is also changing all the time, so the actual service life needs to be simulated and tested. Simply put, the calendar life is the time span for the battery to reach the end-of-life condition (e.g., the capacity decreases to 20%) after a specific use condition under the use environment. Calendar life is closely aligned with specific usage requirements, which usually require the specification of specific usage conditions, environmental conditions, storage intervals, and so on.
6. Internal Resistance (unit: Ω)
Internal Resistance: It refers to the resistance of the current flowing through the battery when the battery is working, which includes ohmic internal resistance and polarization internal resistance, and polarization internal resistance includes electrochemical polarization internal resistance and concentration polarization internal resistance.
Ohmic internal resistance consists of electrode material, electrolyte, diaphragm resistance and contact resistance of each part. Polarization internal resistance refers to the resistance caused by polarization during electrochemical reaction, including the resistance caused by electrochemical polarization and concentration polarization.
The unit of internal resistance is generally milliohm (mΩ). Batteries with large internal resistance have high internal power consumption and serious heat generation during charging and discharging, which will cause accelerated aging and life span degradation of lithium-ion batteries, and at the same time limit the application of charging and discharging with large multiplication rate. Therefore, the smaller the internal resistance is, the better the life and multiplication performance of the lithium-ion battery will be.
Post time: Nov-15-2023