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Requirements for the design of lithium-ion battery charging circuit

by:dcfpower     2021-03-18

Charging procedure of lithium ion battery: trickle charge →constant current charge > Constant voltage charging>Stop charging

To make full use of lithium-ion battery capacity or extend battery life, charging parameters must be extremely strictly controlled. The key to prolonging the life of lithium-ion batteries is to select charging parameters such as current, voltage and temperature. During the charging process, the accuracy of the applied voltage plays a very important role in improving the efficiency of the battery and extending the life of the battery. Exceeding the charge termination voltage will lead to overcharging, which will increase the battery's power supply in the short term, but will cause the battery to fail and cause safety problems in the long term.

For every 1% increase in the end-of-charge voltage of lithium-ion batteries, the initial capacity of the battery will increase by approximately 5%. This obvious short-term gain effect will have serious consequences for the charge/discharge times of lithium-ion batteries. Overcharging leads to a reduction in the number of charging cycles. On the other hand, although undercharging will not cause safety problems, it will significantly reduce the capacity of the battery.

When charging a lithium-ion battery, the voltage at both ends of the battery will rise, and the charging current will gradually decrease. When the charging current drops below 0.1C, it can be considered that the battery has been fully charged. Because trickle charg is not advocated, the charger must be completely shut down or disconnected when charging is over. A faster charging method is constant current/constant voltage (CC/CV) charging. When charging starts, the CC/Cv charger first applies a constant current equivalent to the battery capacity C. To prevent overcharging during the constant current charging cycle, the voltage across the battery package needs to be monitored. When the voltage rises to a given end voltage, the circuit switches to the constant voltage source operating mode. Even if the voltage across the battery package reaches the termination voltage, the actual battery voltage will be lower than the termination voltage because there is a voltage drop on ESR.

During constant current charging, the battery can be charged at a high current rate close to its termination voltage, and there should be no risk of being applied high voltage and overcharging.

After constant current charging, the battery's capacity will reach about 85% of its rated value. After the constant current period ends, the charger switches to the constant voltage period. In the constant voltage cycle, the charger determines whether to end the charging by monitoring the charging current. The charging cycle ends when the charging current drops below 0.1C of the battery.

Protection features include preventing over-discharge, over-charging, excessive charge/discharge current and avoid high voltage applied to the battery.

During the charging or discharging of the battery, if any parameter exceeds the limit value set by the specific battery, the connection between the battery cell and the battery terminal will be disconnected.

In addition to electronic protection, the battery also contains mechanical secondary overcurrent protection devices. A polymer positive temperature coefficient (PPTC) overcurrent protection device is connected in series between the battery package and the cell terminal.

When an overcurrent occurs, the PPIC device switches from a low-impedance state to a high-impedance state, thereby protecting the circuit. The device's temperature rises due to the heat generated by the 12R heating effect, and the above changes are the result of the rapid increase in device temperature.

The charging cycle begins with plugging in the power transformer. At this stage, a current source applies 50mA to the battery terminal while monitoring the voltage. If the voltage across the battery is higher than 3.0V, indicating that the battery is in good condition, the constant current charging cycle will start. The magnitude of the current is a function of the battery capacity, which can refer to the value recommended by the battery manufacturer. The typical charging current is 1C, but some batteries require a lower charging current.

During the constant current cycle, the lithium battery pack is charged to about 80-85 capacity. After the constant current period ends, the constant voltage period starts. In this cycle, the charging current of the battery is lcmargeru003d(Vau-kl)/ESR. During the charging process, Vcl keeps increasing and the charging current drops accordingly. When the current drops below the preset end of charge (EOC) current, charging is terminated. Generally, the recommended EOC current levels are 0.1C, 0.15C, and 0.2C. Once the EOC current is detected, the charging cycle ends. At this time, the charger circuit is closed

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