Factors affecting PACK discharge capacity of lithium ion battery

Factors affecting PACK discharge capacity of lithium ion battery

Lithium-ion battery PACK is important to perform electrical energy test after filtering, group, assembly, assembly, and assembly, determining the capacity, the differential pressure is qualified. The consistency between the battery string parallel monomer is particularly considered in the battery pack, only with good capacity, charged state, internal resistance, self-discharge consistency, etc., can achieve the performance of the battery pack capacity, if consistent Sexuality will seriously affect the overall performance of the battery pack, and even trigger over charge or over-discharge to cause safety hazards.

. The good matching method is an effective way to improve monomer consistency. Lithium-ion batteries are affected by ambient temperature, and the temperature is too high or too low will affect battery capacity.

. Battery for a long time in high temperature conditions, its cycle life can be affected, the temperature is too low, the capacity is difficult to play. The discharge magnification reflects that the battery’s large current charge charging capacity, the magnification is too small, the charge and discharge speed is slow, affecting the test efficiency; the magnification is too large, due to the polarization effect and thermal effect of the battery, the capacity will decrease, so choose appropriate Charge and discharge.

First, the group consistency can improve the battery utilization, but also control the consistency of the monomer, which is the basis for achieving a battery pack to obtain good discharge capacity and cycle stability.. The degree of dispersion of the unpacking battery monomer capacity AC impedance will exacerbate, then weaken the battery pack cycle performance and the available capacity.

Some people have proposed a method of making a battery or from a battery-based vector. This feature vector reflects the degree of similarity of the charge and discharge voltage of the monomer battery and the standard battery. The closer the charging and discharge curve of the battery, the higher the similarity, the closer the correlation coefficient is 1.

This grouping method is based on the correlation coefficient of monomer voltage, and then combines other parameters to obtain a better matching effect.. This method is difficult to supply standard battery characteristics.

Due to the limitation of production level, there is certain difference between the batteries produced by each batch, and it is very difficult to get a set of batteries.. Analysis of differential evaluation methods between monomer batteries by quantitative analysis.

First, the mathematical approach is extracted by the key point affecting the performance of the battery, then mathematical abstraction, the comprehensive evaluation and comparison of battery performance will be transformed into quantitative analysis, and the comprehensive performance of the battery pack A simple way to actually operate. The comprehensive performance evaluation system based on the battery screen selection group is proposed, and the subjective Delphi score and objective gray correlation degree are set, and the multi-parameter gray association model of the battery is established, and the single indicator is used as a single indicator as a piece of evaluation standard. The performance evaluation of the power dynamic lithium ion battery is realized, and the degree of association obtained by the evaluation results is a reliable theoretical basis for the post-supply of the battery.

. The dynamic characteristic layer method is important to achieve the matching function according to the charge and discharge curve of the battery. The specific implementation step is first to extract the characteristics of the curve, form a characteristic vector, depending on the distance between the characteristic vectors between each curve.

The groung indicator, by selecting the appropriate algorithm to achieve the classification of the curve, thereby completing the battery preparation process. This method of performance changes in the performance of the battery at work..

On this basis, choose other suitable parameters to perform battery or group, which can select a battery that is more consistent performance.. Second, the appropriate charging system of the charging method is important about battery discharge capacity.

The charging depth is shallow, and the discharge capacity will be reduced accordingly. If over, it will affect the chemical active substance of the battery, reducing the capacity and life of the battery..

Therefore, it is necessary to select the appropriate charging magnification, the upper limit voltage, and the constant voltage cutoff current to ensure that the optimization of charging efficiency and safety stability are taken into account in the event that the charging capacity is achieved.. At present, power lithium-ion battery uses constant current-constant voltage charging mode.

By analyzing the lithium iron phosphate system and the three-dimensional battery battery at different charging current and different cutoff voltage charging results, it is understood that: (1) When the charging cutout voltage is pressed, the charging current is increased, and the constant current is reduced. The charging time is shortened, but the energy consumption is added; (2) When the charging current is pressed, with the decrease in the charge cutout voltage, the constant current is reduced, and the capacity and energy are reduced. In order to ensure the battery capacity, iron phosphate The charging cutoff voltage of the lithium ion battery cannot be less than 3.

4V.. To balance the charging time and energy loss, select the appropriate charging current and deadline.

Each monomer SOC consistency determines the battery pack discharge capacity, and the balanced charging is possible to provide similar supply to the initial SOC platform of each monomer discharge, which can improve discharge capacity and discharge efficiency (discharge capacity / grouped capacity). The equalization mode of charging refers to the balance between the power lithium ion battery during charging, which is generally balanced when the battery pack monomer voltage reaches or higher than the set voltage, by reducing the charging current from prevention over charge..

Depending on the different state of the unit cell in the battery pack, the charging current of the battery is balanced by the battery pack, and the charging current of the single circuit is fine-tuned. It is proposed that a battery pack can be quickly charged, and the monomer battery is inconsistent. Balance charging control strategy affecting battery pack cycle life.

Specifically, the monomer battery is supplemented by the switch signal, or the monomer battery is supplemented, or the unit cell energy is energy conversion to the overall battery pack.. During the battery pack charging process, by detecting the voltage value of the respective monomer batteries, the equalization module starts working when the unit cell voltage reaches a certain value.

. Diversify the charging current in the unit cell to reduce the charging voltage, and the current flow is converted back to the charging bus to achieve a balanced purpose by module conversion..

Some people have proposed a variable ratio charging equalization solution. The balanced idea of ​​this method is to add energy replenishment only to the low energy cells, preventing the process of removing energy of a single cell energy, which is greatly simplified. Topological structure of equalization circuit.

That is, a different energy state is charged with a different energy state, thereby achieving a good balance effect.. Third, discharge magnification discharge magnification related to power dynamic lithium ion batteries is a crucial indicator.

Battery’s large rate discharge, a test of positive, negative electrode materials and electrolyte, etc.. For the lithium phosphate of the positive electrode material, the structure is stable, the charge and discharge process should be small, and the basic conditions of high current discharge, but the unfavorable factor is the conductive capacity of lithium iron phosphate.

. The lithium ion diffusion velocity in the electrolyte is an important factor affecting the battery discharge ratio, and the diffusion of the internal ions of the battery is closely related to the structure of the battery and the electrolyte concentration..

Therefore, different discharge magnification results in the time of discharge of the battery, and the discharge voltage platform is different, which in turn leads to different discharge capacity, which is particularly obvious.. Therefore, choose the appropriate discharge rate.

The available capacity of the battery decreases with the increase of the discharge current.. Jiang Tan et al, the discharge rate of the discharge rate on the ionized ion battery monomer can be discharged, and the cells that are preferably initial in the same model are charged to 3.

8V at 1C current, and then 0.1, 0.2, respectively.

The discharge magnification of 0.5, 1, 2, 3c is discharged to 2.5 V, the relationship curve between the voltage and the discharge power, as shown in FIG.

. The experimental results show that the capacity of 1,2C is 97.8% and 96.

5% of C / 3 release capacity, respectively, and the energy of the released energy is 97.2% and 94.3% of the energy of C / 3, thereby visible as discharge current Increased, the capacity of the lithium ion battery is significantly reduced.

When the lithium ion battery is discharged, the national standard 1C is generally selected, and the maximum discharge current is usually limited to 2 ~ 3C.. When the large current is discharged, there will be a large temperature rise and cause loss of energy.

. Therefore, you need to monitor the temperature of the battery pack in real time, prevent the temperature of the temperature to cause damage to the battery, reduce the battery life. Fourth, temperature conditions Temperature Important affect the activity and electrolyte properties of the internal pole substances in the battery.

The temperature is too high and the low capacity of the battery has a large impact. At low temperature, the activity is significantly reduced, the ability to embedding and extraction of lithium decreases, the internal resistance of the battery, the polarization voltage, the actual available capacity, the battery discharge ability, low, battery easier to achieve discharge cutout voltage, performance Decreased for battery available capacity, and decreased battery energy utilization efficiency. When the temperature rises, the lithium ion discharge and embedding between the positive and negative electrodes become active, so that the internal resistance of the battery is small, the internal resistance is stable, so that the amount of electrons in the outer circuit increases, and the capacity is more effective.

Play. However, the battery is working for a long time in a high temperature environment, and the stability of the positive electrode lattice structure is deteriorated. The safety of the battery will decrease, and the battery life will be significantly shortened.

. Li Zhe has studied the effects of temperature on the actual discharge capacity of the battery, records the ratio of the actual discharge of the battery and the standard can release capacity (25 ° C 1C discharge) at different temperatures. Fitting the capacity of the battery with the temperature to give: C: C is battery capacity; T is temperature; R2 is the correlation coefficient of the fit.

Experiments have shown that the battery capacity at low temperature is extremely fast, and the capacity of about temperature is rising with temperature increases.. When the -40 ° C is only 1/3 of the nominal value, while 0 ° C to 60 ° C, the battery capacity rises from 80% of the nominal capacity to 100%.

. Analysis believes that the change rate of ohm internal resistance is greater than the rate of change at high temperatures under low temperatures, which indicates that the effect of low temperature on the activity of the battery is more obvious, which affects the capacity of the battery..

As the temperature rise, the ohmic internal resistance of charging and discharging, the polarization internal resistance decreases. However, at a higher temperature, it will damage the chemical reaction balance and material stability in the battery, resulting in a side reaction, thereby affecting the battery capacity and internal resistance, resulting in a shortement of cycle life or even reduced safety..

Therefore, high temperature and low temperatures will affect the performance and service life of the lithium phosphate ion battery. In the actual work process, the battery should be ensured that the battery works under suitable temperature conditions..

Cost temperature detection chamber at 25 ° C can be established in the battery pack Pack test link. .

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