Why is it necessary to learn about DOD of lithium-ion battery?
Do you agree with the point that the life of your battery is related to not only its quality but also how you use it? Hence, the battery jargon: depth of discharge, also known as “DOD”, is something you can take into consideration when you are trying to figure out how to use your battery properly to maintain your battery and prolong its life. In this passage, we will dive into the definition of DOD of lithium-ion batteries and its related concepts to fully understand why is it necessary to learn about the depth of discharge for lithium-ion batteries and tips on how to use your lithium-ion battery in the right way.
What is the DOD of lithium-ion battery?
In order to define the depth of discharge of a lithium battery, we should first define what is battery capacity. Battery capacity is the total electrical energy supply available from the battery, which can be expressed as a unit of power over time, e.g. B. Kilowatt-hours (kWh).
The depth of discharge is the percentage of the battery that has been discharged relative to the total battery capacity of the battery. For instance, with a battery capacity of 10kWH, when you discharge 80% of the capacity, which is 80kWH, the DOD would be 80%. Then, it would be easy to understand, when the DOD of the lithium-ion battery is 80%, it indicates that there is 20% left of the capacity.
What is the influence of DOD on lithium battery?
The effect of the depth of discharge on the battery is: the deeper the depth of discharge, the easier it is to shorten the battery life, on the other hand, the performance on the discharge curve. The deep discharge comes with unstable voltage and current, and under the same discharge system, a small voltage value will deepen the discharge depth.
What can we do to control the influence of DOD?
In practice, the termination voltage value is usually used to control the DOD. When the lithium-ion battery is likely to be over-discharged, the voltage of the lithium battery is lower than the IC over-discharge voltage detection point of 2.75 V (some set to 3.0V). At this time, the discharge protection function is activated, and the MOSFET turns from ON to OFF to cut off the discharge, thus completing the control of the depth of discharge of the lithium-ion battery.
In the lithium battery discharge depth comparison table from 3.0V (termination voltage) to 4.2V (open circuit voltage), the reverse relationship between current and discharge depth is shown: the greater the current, the shallower the DOD of the lithium-ion battery. When the battery is lowered to the same voltage, the charged capacity of the lithium-ion battery discharged at 0.05C at 3.9V is 65.6%, and if the discharge is changed to 0.5C, the charged capacity of the lithium battery at 3.9V is still 74.9%, this comparative explanation: The small current discharge is more thorough, the smaller the current, the longer the working time, the less the charged electricity reaches the same voltage. In conclusion, when it comes to lithium-ion battery discharge, we should consider the discharge system, in which the current is the most important part.
Precautions for Lithium Battery Discharge
Prevent excessive discharge current
Excessive current causes heat inside the lithium battery, which may cause permanent damage. Another thing is that the greater the discharge current of a lithium-ion battery, the smaller the discharge capacity and the faster the voltage drop. And as we’ve mentioned above, and vice versa, the larger the current, the less working time of a lithium battery.
The internal storage of electric energy in lithium-ion batteries is created by a reversible chemical change in electrochemistry. The excessive discharge will cause irreversible reactions to this chemical change. Therefore, over-discharge on lithium batteries should be avoided. If the discharge voltage is lower than 2.7V, it will lead to permanent damage.
Avoid high-rate discharge
When a lithium battery is discharged, Li+ is extracted from the negative electrode and inserted into the positive electrode through the electrolyte. The positive electrode is in a lithium-rich state, that is, the battery will discharge during normal use. The voltage attenuation of lithium batteries decreases as the discharge rate increases, which will reach earlier termination of voltage. Therefore, the characteristics of low-rate discharge are better than high-rate discharge.
The life of your battery has something to do with DOD. The deeper the DOD of the lithium-ion battery, the greater the loss of the battery, the same as the more fully charged lithium-ion battery. The lithium-ion battery should stay in an intermediate state of power supply, in which case the battery life will be the longest.