What impact does charge and discharge current have on lithium battery performance?

As we all know, as the number of charges and discharges of lithium batteries increases, the battery capacity SOH will become less and less, which directly reflects the performance of lithium batteries getting worse and worse. There are many factors that affect the capacity of lithium batteries. Factors such as operating temperature, charge and discharge current (charge and discharge rate), charge and discharge cut-off voltage, etc. will all affect the decay rate of lithium-ion batteries. The mechanisms causing the capacity attenuation of lithium batteries can be divided into three categories: increase in internal resistance and polarization, loss of positive and negative active materials, and loss of Li. This time we mainly share the impact of charge and discharge current (charge and discharge rate) on the performance of lithium batteries.

Before that, let’s first understand how to calculate the charge and discharge rate of lithium batteries? The charge-discharge rate refers to the ratio of the current endured by the battery during the charge-discharge process to its rated capacity. The unit is C and the dimension is 1/h, which is the reciprocal of “hour”. This parameter indicates the battery’s charge and discharge capacity and charge and discharge speed. The calculation formula is as follows:

Charge/discharge rate = charge/discharge current (A)/battery rated capacity (Ah)

Take EITAI ELESHELL-10.2K and ET-HV32S-5K as examples for calculation.

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From the parameters in the above figure, we can see that the ELESHELL-10.2K system is equipped with a PCS with a maximum power of 5.12kW and a battery with a maximum energy storage of 10.24kWh.

PCS power: P=U·I
Battery energy storage: W=U·Q

Since the PCS DC side working voltage is the battery system working voltage during charging and discharging, the more intuitive calculation method for judging the maximum charge and discharge rate of the energy storage system is P/W=5.12kW/10.24kWh=0.5, taking into account actual conditions such as battery life , generally the maximum depth of discharge is 90% DOD, which is 0.5C in parameters.

The calculation method of ET-HV32S-5K is similar. The maximum charge and discharge rate is calculated as P/W=5.12kW/5.12kWh=1. Combined with the discharge depth, the maximum charge and discharge rate is 1C.

In practical applications, the greater the charge and discharge rate, the stronger the battery’s charge and discharge capability and the faster the charge and discharge speed. However, high rate charge and discharge will have a certain impact on lithium batteries, that is, the greater the charge and discharge rate, the battery performance degradation rate The faster, mainly in the following aspects:

  • Battery capacity fading: During frequent high-rate charging and discharging, the chemical reactions inside the lithium battery will be affected, resulting in battery capacity fading. This attenuation is usually irreversible, reducing battery life;
  • Increased temperature and reduced charging efficiency: During high-rate charging and discharging, due to excessive current, the heat inside the battery will increase, which will increase the power loss of the lithium battery, reduce the charging efficiency, and make the battery charging time longer;
  • Shortened battery life: During high-rate charging and discharging, lithium ions move quickly and materials migrate frequently, exacerbating the internal loss and material fatigue of the battery. Long-term high-rate charging and discharging will shorten the life of lithium batteries and reduce the number of times they can be recycled

The impact of charge and discharge rates on batteries can also be known from the relevant graphical data provided by battery manufacturers.

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The BMS battery management system independently developed by EITAI supports the SOX function. It balances the SOC, SOH, SOP, and SOE of single cells and battery packs to achieve battery balance management, which can effectively improve battery consistency; it supports thermal management functions, and according to the battery Temperature status, active hot and cold management of the battery, to achieve control functions of cooling, heating, and temperature consistency improvement; supports charge and discharge control functions, sending battery SOP, current, voltage and other information to the PCS in real time to ensure that the PCS is at the highest level of the battery Charging and discharging are carried out under optimal conditions. In addition, EMS can reasonably adjust the PCS charging and discharging power within the maximum charging and discharging rate of the energy storage system according to the on-site application conditions to achieve the purpose of controlling the charging and discharging rate.

Therefore, when using lithium batteries, a reasonable charge and discharge strategy is an effective means to control battery attenuation, extend battery life, improve capacity utilization, and ensure the safe operation of the battery pack.

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