March 14, 2024
Why depth of discharge of lithium batteries matters? The secret to battery longevity
Lithium batteries, as the core energy source of modern electronic devices, electric two-wheeled vehicles and new energy vehicles, their performance and life are directly related to our daily life and technological development. What is less known is that the depth of discharge of lithium batteries has a huge impact on lithium battery life, which can be called the “invisible killer” that determines battery life. This article will explore the relationship between life and depth of discharge of lithium batteries in depth, and provide practical suggestions to help you better manage and maintain lithium batteries.
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What is the depth of discharge lithium batteries?
The depth of discharge (DOD) of lithium batteries refers to the percentage of the battery’s discharged power to its rated capacity. For example, a lithium battery with a rated capacity of 100mAh is discharged by 50mAh, so its depth of discharge is 50%.
The higher the DOD value, the more the battery is discharged and the less power is left. 0% DOD means that the battery is fully charged, and 100% DOD means that the battery is fully discharged. It should be noted that not all lithium batteries allow 100% DOD, and excessive discharge can cause permanent damage to the battery.
How does the depth of discharge affect the life of lithium batteries?
Deep discharge is one of the main factors that shorten the life of lithium batteries, and its negative effects are mainly reflected in the following aspects:
Accelerate the aging of electrode materials
The charging and discharging process of lithium batteries involves the insertion and deintercalation of lithium ions. Deep discharge means that lithium ions need to undergo more intense migration, which will accelerate the structural changes and losses of electrode materials.
In particular, positive electrode materials are prone to lattice collapse and surface structure rupture during deep charge and discharge cycles, resulting in capacity decay. Negative electrode materials are also more prone to lithium dendrite growth during deep discharge, which eventually causes internal short circuits in the battery.
Increase battery internal resistance
Deep discharge increases the internal resistance of the battery (explore the lithium ion battery internal resistance). The increase in internal resistance will not only reduce the output power of the battery, but also cause the battery to generate more heat during the charging and discharging process. Excessive temperature will accelerate the decomposition of the electrolyte and the thickening of the SEI film, further aggravating the aging of the battery and even causing safety accidents.
Reduce cycle life
Lithium battery cycle life refers to the number of charge and discharge cycles that a battery can complete under specific charge and discharge conditions. Studies have shown that the cycle life of lithium batteries is negatively correlated with the depth of discharge. The higher the DOD, the shorter the cycle life. Frequent deep discharges can significantly shorten the battery’s service life and cause its capacity to drop rapidly. For example, a battery that can cycle 1,500 times at 50% DOD may only cycle 500 times at 80% DOD.
Electrolyte decomposition and SEI film thickening
When the DOD is too high, the battery voltage decreases, which can easily lead to electrolyte decomposition and generate some byproducts that consume lithium ions and form a thicker SEI (solid electrolyte interface) film. An overly thick SEI film will increase the battery’s internal resistance and reduce the ion conduction efficiency, thereby affecting the battery’s performance and life.
Different types of lithium batteries have different tolerances to depth of discharge
Not all lithium batteries are equally sensitive to deep discharge. Different types of lithium batteries have different tolerances to depth of discharge due to their different positive and negative electrode materials and electrolytes.
| Depth of discharge | Ternary lithium battery cycle number | Lithium iron phosphate battery cycle number |
|---|---|---|
| 20% DOD | more than 5000 times | more than 8000 times |
| 80% DOD | 1000 times | 2000 times |
| 100% DOD | 300 times | 500 times |
- Ternary lithium batteries: They are sensitive to deep discharge, and it is recommended that the DOD be controlled below 80% to extend their life. Frequent deep discharges will accelerate their capacity decay.
- Lithium iron phosphate battery: Compared with ternary lithium battery, lithium iron phosphate battery has stronger tolerance to deep discharge and longer cycle life. Even at 100% DOD, it can withstand a certain number of cycles, but frequent deep discharge will still affect its life.
How to extend the life of lithium batteries ?
The key to extending the life of lithium batteries is to control the depth of discharge of lithium batteries and avoid deep discharge. Here are some practical suggestions:
Shallow charge and shallow discharge strategy
Try to avoid completely exhausting the battery power. It is recommended to use the battery in the range of 20%-80% power, which is the best working range for most lithium batteries.
Regularly fully charge and discharge
Performing a full charge and discharge cycle from 0% to 100% once a month can help calibrate the battery management system (BMS) and improve the accuracy of power metering. But do not do this operation frequently.
Avoid high and low temperature environments
High and low temperatures will accelerate battery aging and reduce battery performance. When using the battery in a high temperature environment, pay attention to heat dissipation; when using the battery in a low temperature environment, the depth of discharge of lithium batteries should be appropriately reduced.
Use a battery management system (BMS)
BMS can effectively monitor battery voltage, current, temperature and other parameters, and take corresponding measures to protect the battery from over-discharge and overcharging.
Choose a suitable charger
Using a suitable charger can ensure that the battery is charged at the best charging rate to avoid overcharging or undercharging.
Power management for long-term storage
For lithium batteries that are not used for a long time, it is recommended to keep the power level at 50%-60% to avoid full discharge or full charge.
Discharge depth recommendations for different application scenarios
Different devices have different requirements for battery life, so the control of discharge depth should also be different:
- Mobile phones, laptops and other portable electronic devices
It is recommended to control DOD between 20%-80% and turn on the “Optimize Battery Charging” function of the device.
- Electric vehicles (electric cars, two- and three wheelers)
The battery management system of electric vehicles usually limits DOD to ensure the safety and life of the battery. It is recommended to avoid running out of battery power and charge in time when the driving range is sufficient.
- Energy storage battery
The DOD control strategy of energy storage battery depends on the specific application scenario and requirements. Normally, in order to extend the life, a lower DOD is selected.
- Small electronic devices (such as toys, remote controls)
This type of device has relatively low requirements for battery life, and the DOD limit can be appropriately relaxed, but the battery over-discharge must still be avoided.
Conclusion
Controlling the depth of discharge of lithium batteries is the key to extending battery life. By adopting reasonable charging and discharging strategies and avoiding over-discharge, we can effectively extend the service life of lithium batteries, improve their cost-effectiveness, and provide more reliable energy security for our lives and technological development.
In the future, with the continuous development of battery technology, it is believed that more efficient and longer-life lithium batteries will be available, further improving our lives and energy efficiency. But at present, mastering and applying reasonable DOD management strategies is still an effective way to extend the life of our existing lithium batteries.
FAQ
1.What is the appropriate depth of discharge of lithium batteries?
For most lithium batteries, the recommended DOD for daily use is between 20% and 80%. This can strike a balance between capacity and life. Maintaining 100% DOD for a long time will significantly accelerate battery degradation.
2.Can lithium batteries be fully discharged to 0%?
It is not recommended to fully discharge. Although some devices show that the power is 0%, in fact, the BMS will retain a part of the "protection capacity". If the voltage is artificially discharged to too low, it may cause the battery to be over-discharged, locked or even scrapped.
3.How much impact does a 100% discharge depth have on the battery?
Long-term 100% DOD will cause structural damage to the battery and rapid capacity decay, especially for ternary lithium batteries. It is okay once in a while, but frequent deep discharge will seriously shorten the service life.
4.Is it also important to control the discharge depth of lifepo4 batteries?
Although lithium iron phosphate (LFP) has strong deep discharge resistance and DOD can reach 100%, appropriately reducing DOD can still significantly extend the life. Especially in energy storage systems, it is recommended to control DOD within 80%.
5.How to determine whether the battery of your device is over-discharged?
If the battery power drops rapidly, the device automatically shuts down early, the battery swells or cannot be charged, it may be caused by over-discharge. For some devices, you can view the historical lowest voltage and usage status through the battery health detection tool.
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Willow
Willow is a materials engineer with a Master’s degree in Materials Science and Engineering, specializing in lithium-ion battery materials and energy storage technologies.
Her work focuses on EV battery swapping solutions, battery innovation, and new energy industry trends, aiming to translate research insights into practical applications for sustainable transportation.
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