Why do lithium-ion batteries need to be pre charged

Some lithium battery peers do not fully understand why lithium-ion batteries need to be pre charged before charging. This is mainly because lithium-ion batteries have a high energy ratio. If they enter the fast charging mode directly, it will cause damage to the battery, affect its service life, and may pose safety hazards.

Although lithium-ion batteries are currently designed with built-in circuit protection boards, the probability of overcharging is usually low. However, if the battery is not pre charged initially, it can bring many hidden dangers, such as protection board failure, long-term self discharge rate, etc. When the battery is over discharged, it will cause difficulty in recovering the active substance. Therefore, it is necessary to first charge it to 2.5V to 3.0V with a small current, and then switch to fast charging. Pre charged small batteries can effectively solve the charging problem of over discharged batteries.

The voltage of lithium-ion batteries during operation ranges from 2.5V to 4.2V. When the voltage is less than 2.5V, the battery discharge terminates. At the same time, the current loss of the internal protection circuit is minimized due to the closure of the discharge circuit. Of course, in practical applications, the discharge termination voltage can be in the range of 2.5V-3.0V due to different internal materials. When the voltage exceeds 4.2V, the charging circuit terminates to protect the safety of the battery; When the working voltage of a single battery drops below 3.0V, we can consider it to be in an over discharge state and terminate the discharge circuit to protect the safety of the battery.

Some peers believe that choosing high current fast charging for fast charging is a misconception. The higher the charging current, the better. Taking single lithium-ion batteries as an example, their charging methods all include constant current and constant voltage charging processes. The constant voltage is usually 4.2V, and the constant current setting is 0.1C-1C. Although high current charging can shorten the charging time, it can also cause a shortened battery life cycle and a decrease in capacity. Therefore, it is important to correctly understand the importance of battery pre charging. The reason why lithium-ion batteries have increased pre charging function in the design process is because of this. Therefore, batteries must be pre charged and choose the appropriate constant current value for charging.

The charging process of lithium-ion batteries can be divided into four stages: trickle charging (low-voltage pre charging), constant current charging, constant voltage charging, and charging termination.

The basic requirements for lithium-ion battery chargers are specific charging currents and voltages to ensure safe battery charging. The addition of other charging assistance functions is to improve battery life and simplify the operation of the charger, including using trickle charging for over discharged batteries, battery voltage detection, input current limitation, turning off the charger after charging is completed, and automatically starting charging after partial discharge of the battery.

The charging method of lithium-ion batteries is voltage limited and constant current, which is controlled by the IC chip. A typical charging method is to first detect the voltage of the battery to be charged. If the voltage is below 3V, pre charging should be carried out first. The charging current is 1/10 of the set current. After the voltage rises to 3V, it enters the standard charging process. The standard charging process is to perform constant current charging at the set current. When the battery voltage rises to 4.20V, it is changed to constant voltage charging and the charging voltage is maintained at 4.20V. At this point, the charging current gradually decreases, and when the current drops to 1/10 of the set charging current, the charging ends.
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