Charging characteristics of lithium batteries

Charging characteristics of lithium batteries
Charging method

There are many charging methods for lithium batteries, which can be divided into conventional charging and fast charging according to charging efficiency. The conventional charging methods include constant current charging, constant voltage charging, stage charging, and intermittent charging, while fast charging includes pulse charging and Reflex charging. Finally, an analysis was conducted on intelligent charging.

1. Constant current charging

According to the magnitude of the charging current, constant current charging can be divided into fast charging, standard charging, and trickle charging. During the entire charging process, it is generally necessary to adjust the charging voltage of the power supply or change the resistance value in series with the battery to maintain the constant charging current of the battery. The advantage of this method is that it is easy to control and suitable for charging battery packs with multiple batteries connected in series. The disadvantage is that the acceptable charging capacity of lithium batteries will gradually decrease as charging progresses, and excessive charging current in the later stage of charging will cause bubbles to form inside the battery, causing damage to the battery. Therefore, constant current charging is often a part of stage charging.

2. Constant voltage charging

Constant voltage charging refers to the process where the charging voltage remains constant throughout the entire charging process, and the magnitude of the charging current automatically adjusts with changes in the battery state. As charging progresses, the charging current gradually decreases. Compared with constant current charging, its charging process is closer to the optimal charging curve, with simple control and low cost. The disadvantage is that the charging time is relatively long, and the charging current of the battery is too high in the early stage of charging, which directly affects the life and quality of use of lithium batteries. So the constant voltage charging method is rarely used alone, only when the charging power supply voltage is low and the current is high.

3. Constant current and constant voltage charging

The constant current and constant voltage charging curve is shown in Figure 2. Before starting charging, first check the battery voltage. If the battery voltage is below the threshold voltage (around 2.5V), then trickle charge the battery with a small current of C/10 to slowly increase the battery voltage; When the battery voltage reaches the threshold voltage, it enters constant current charging. During this stage, the battery is rapidly charged with a large current intensity (0.5C~1C), and the battery voltage rises rapidly. The battery capacity will reach about 85% of its rated value; After the battery voltage rises to the upper limit voltage (4.2V), the circuit switches to constant voltage charging mode, and the battery voltage is basically maintained at 4.2V. The charging current gradually decreases and the charging speed slows down. This stage is mainly to ensure that the battery is fully charged. When the charging current drops to 0.1C or 0.05C, it is judged that the battery is fully charged.