Lithium ion batteries have the advantages of large capacity, high specific energy, good cycle life and no memory effect, etc., and have developed rapidly. Capacity, as the most critical performance index, has also attracted much attention from researchers.Accordingly, lithium battery PACK is developing towards the direction of large capacity, fast charging, long life and high safety, and new requirements have been put forward for its manufacturing process.
Lithium ion battery PACK mainly tests the electrical performance of the battery cell after screening, allocation, package and assembly to determine whether the capacity and pressure difference are qualified.
Battery series-parallel monomer is the consistency between need special consideration in the battery PACK, only have good capacity, charged state, such as internal resistance, self-discharge consistency can be achieved to play and release, battery capacity if bad consistency can seriously affect the whole battery performance, even cause of charging or discharging that they cause safe hidden trouble.A good combination scheme is an effective way to improve monomer consistency.
LiFepo4 battery is restricted by the influence of ambient temperature. Too high temperature or too low temperature will affect the battery capacity.If the battery works for a long time under high temperature, its cycle life may be affected. If the temperature is too low, the capacity is difficult to play.
The discharge rate reflects the charging and discharging capacity of the battery with large current.If the multiplier is too large, the capacity will be reduced due to the polarization effect and thermal effect of the battery, so it is necessary to choose an appropriate charge and discharge ratio.
1. Pack consistency
Good configuration can not only improve the utilization rate of the cell, but also control the consistency of the monomer.However, the dispersion degree of ac impedance of single capacity of poorly packaged battery will be intensified, which will weaken the cycle performance and usable capacity of battery pack.
The proper charging system has an important effect on the discharge capacity of the battery.If the charging depth is shallow, the discharge capacity will be correspondingly reduced. If the overcharge will affect the chemical active substances of the battery and cause irreversible damage, which will reduce the capacity and life of the battery.Therefore, it is necessary to choose the appropriate charging ratio, upper limit voltage and constant voltage cut-off current, so as to ensure that the charging efficiency and safety stability can be optimized while the charging capacity is realized
Discharge rate is an important index for power – type power battery.The large discharge rate of the battery is a test for the positive and negative electrode materials and electrolyte.For the positive electrode material lithium iron phosphate, its structure is stable, the charging and discharging process strain is small, has the large current discharge basic conditions, but the disadvantage is that lithium iron phosphate has the poor conductivity.The diffusion velocity of lithium ions in electrolyte is the main factor affecting the discharge rate of battery, and the diffusion of ions in battery is closely related to the structure and electrolyte concentration of battery.
4. Temperature conditions
Temperature mainly affects the activity and electrolyte performance of the electrode material in the battery.High temperature and low temperature have great influence on the capacity of the battery.
At low temperature, the battery’s activity significantly decreases, the ability of lithium to embed and disengage decreases, the battery’s internal resistance and polarization voltage increase, the actual usable capacity decreases, the battery’s discharge capacity decreases, the discharge platform is low, and the battery is easier to reach the discharge cut-off voltage, which is manifested as the battery’s usable capacity decreases and the battery’s energy utilization efficiency decreases.
At higher temperatures, between is the cathode of lithium ion embedded emerge and become active, battery internal resistance is reduced, thereby slow resistance stability, makes in the external circuit, electronic shift increases, the capacity is more effective.However, if the battery works in a high temperature environment for a long time, the stability of the positive lattice structure will become worse, the safety of the battery will be reduced, and the battery life will be significantly shortened.
In this article, the factors affecting discharge capacity of LiFepo4 battery PACK are analyzed and discussed.Good consistency of battery pack is the premise to realize discharge performance and level of battery pack.It is recommended to use balanced charging method to ensure that each single SOC platform is similar before discharge.It is necessary to choose an appropriate discharge rate, and to give consideration to both capacity and test efficiency.The environment has a great influence on the battery test, so the temperature condition should be controlled well.