How to Charge Lithium ion Batteries
The charging process of lithium-ion batteries can be divided into four stages: trickle charge (low-voltage precharge), constant current charge, constant voltage charge, and charge termination.
The basic requirement of a lithium battery charger is a specific charging current and charging voltage to ensure safe battery charging. Other charging auxiliary functions are added to improve battery life and simplify the operation of the charger, including trickle charging for over-discharged batteries, battery voltage detection, input current limit, turn off the charger after charging is complete, and automatic start charging battery after partial discharge, etc.
The charging method of lithium battery is limited voltage and constant current, which is controlled by IC chip. The typical charging method is: first detect the voltage of the battery to be charged, if the voltage is lower than 3V, pre-charge first, and the charging current is set 1/10 of the current, after the voltage rises to 3V, it enters the standard charging process. The standard charging process is: constant current charging with the set current, when the battery voltage rises to 4.20V, change to constant voltage charging and keep the charging voltage at 4.20V. At this time, the charging current gradually decreases, and when the current drops to 1/10 of the set charging current, charging termination.
Stage 1: Trickle charge
Trickle charge is used to precharge (recovery charge) the fully discharged battery cell first. When the battery voltage is lower than about 3V, trickle charging is used. The trickle charging current is one tenth of the constant current charging current, which is 0.1c (taking a constant charging current of 1A for example, and the trickle charging current is 100mA).
Stage 2: Constant current charging
When the battery voltage rises above the trickle charge threshold, increase the charging current for constant current charging. The current for constant current charging is between 0.2C and 1.0C. The battery voltage gradually increases with the constant current charging process. Generally, the voltage set for a single battery is 3.0-4.2V.
Stage 3: Constant voltage charging
When the battery voltage rises to 4.2V, the constant current charging ends and the constant voltage charging phase begins. According to the saturation of the battery cell, the charging current gradually decreases from the maximum value as the charging process continues. When it decreases to 0.01C, the charging is considered to be terminated. (C is a way to express the nominal capacity of the battery against the current. For example, if the battery has a capacity of 1000mAh, 1C is the charging current of 1000mA.)
Stage 4: Charge termination
There are two typical charging termination methods: use the minimum charging current to determine or use a timer (or a combination of the two). The minimum current method monitors the charging current in the constant voltage charging stage, and terminates the charging when the charging current decreases to the range of 0.02C to 0.07C. The second method starts timing at the beginning of the constant voltage charging phase, and terminates the charging process after two hours of continuous charging.
The above-mentioned four-stage charging method takes about 2.5 to 3 hours to complete the fully discharged battery. Advanced chargers also adopt more safety measures. For example, if the battery temperature exceeds the specified window (usually 0°C to 45°C), then charging will be suspended.After the charge is terminated, if the battery voltage is detected to be lower than 3.89V, it will recharge.
The remaining capacity of the battery can be estimated by measuring the voltage roughly like below:
　　4.20V—-100%
　　3.95V—-75%
　　3.85V—-50%
　　3.73V—-25%
　　3.50V—-5%
　　2.75V—-0%
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