The cycle life of NCM lithium battery pack is getting shorter and shorter. What's the reason?

The cycle life of NCM lithium battery cell is about 500 cycles, while the cycle life of lithium iron phosphate battery cell is about 2000 cycles. The test conditions are: 1C charge and discharge, 100% DOD, 25 ℃ environment. (Remarks: 1C charge and discharge means magnification. Assuming that the capacity of the battery pack is 10Ah, 1C charge and discharge is 10A charge and discharge. 100% DOD is the discharge depth, and the battery is discharged from a fully charged state to the lower limit voltage, which belongs to 100% DOD discharge.)

The cycle life of a lithium battery pack is related to the life of individual lithium cells, to the consistency between cells, to the magnitude of the charging and discharging current, and to the small details in daily use.

1. The size of the charge and discharge current affects the cycle life of lithium batteries

The standard charge and discharge current of a lithium battery is 0.2C, that is to say, for a lithium battery pack with a capacity of 10Ah, the standard charge and discharge current is 2A. Such a current is the best for the life of the lithium battery pack.

Some high-rate lithium batteries have a shorter cycle life than low-rate lithium batteries. The ternary lithium battery of the small rate type (below 3C) generally has a cycle life of 500 times, while the ternary lithium battery of the high rate type (above 5C/10C) generally has a cycle life of about 300 times.

2. Depth of discharge (DOD), which affects the cycle life of lithium batteries

Shallow charge and shallow discharge are more beneficial to the life of lithium batteries. Lithium batteries will always be charged, which is more beneficial to the activity of materials.

3. Lithium batteries are often overcharged or overdischarged, which affects the cycle life of lithium batteries

Lithium batteries are often overcharged or overdischarged, which will also shorten the cycle life of lithium batteries. Even though the lithium battery BMS protection board has overcharge and overdischarge protection functions, overcharging or overdischarging the lithium battery will cause certain damage to the lithium battery material, thereby affecting the cycle life of the lithium battery.

How to prevent lithium batteries from being overcharged or overdischarged?

The overcharge protection value of the BMS protection board can be appropriately lowered, and the overdischarge protection value of the BMS protection board can be appropriately increased. The lithium battery cannot be charged too much, nor can it be placed in space. Considering the cycle life of the lithium battery, the overcharge protection value of the ternary lithium battery can be set to 4.2V, and the overdischarge protection value to 3.0V. Of course, the capacity discharged in this way will be less, but it is conducive to the long cycle life of lithium batteries.

4. The temperature is too high or too low, which affects the cycle life of lithium batteries

Lithium batteries are best charged and discharged at an ambient temperature of 25°C. High or low temperatures tend to accelerate the decay rate of lithium batteries, resulting in shortened cycle life.

When the temperature of the lithium battery is too high or too low, it is necessary to use the lithium battery BMS to control and manage the temperature of the lithium battery pack.

5. The storage method affects the cycle life of lithium batteries

When the lithium battery pack is not used for a long time, it is recommended to store more than 50% of the power, and it cannot be stored empty. It is recommended to perform a charge-discharge cycle within 3 months, otherwise the lithium battery material will be damaged and the cycle life will be shortened accordingly.

6. The environment of use affects the cycle life of lithium batteries

For example, areas with high humidity and high salt spray concentration will accelerate the rusting of lithium batteries and reduce the cycle life of lithium batteries.

When external destructive interference such as excessive vibration, excessive collision, high magnetic field, etc., the cycle life of the lithium battery will also be affected to a certain extent.