Unique explanation of lithium battery charging method, 3.7V lithium battery charging method involved in the four stages

For the lithium battery charging method, has brought a large number of related articles, such as mobile phone lithium battery charging method, iron phosphate lithium battery charging method, electric vehicle lithium battery charging method. In the previous article, xiaobian on the lithium battery charging method are based on the actual matters needing attention. In this article, I will cover the four stages of charging 3.7V lithium batteries to help you better understand how to charge lithium batteries.

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

The basic requirements of lithium battery chargers are specific charging current and charging voltage, so as to ensure the safe charging of batteries. Other charging AIDS are added to improve battery life and simplify the operation of the charger, including trickle charging for overdischarging batteries, battery voltage detection, input current limitation, turning off the charger after charging, and automatic charging after partial battery discharge.

The charging mode of lithium battery is voltage limited and constant current, which is controlled by IC chip. The typical charging mode is: first test the voltage of the battery to be charged. If the voltage is lower than 3V, pre-charge should be carried out first. The standard charging process is: constant current charging at 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 point, the charge current gradually drops, when the current drops to the set charge current 1/10, the charge is finished. The following figure shows the charging curve.

Stage 1: Trickle charge -- The trickle charge is used to precharge a fully discharged battery unit (recovery charge). When the battery voltage is lower than 3V or so, the trickling current charge is used. The trickling current charge current is one tenth of the constant current charge current, that is, 0.1c(take the constant charging current for 1A, then the trickling current charge current is 100mA).

Stage 2: Constant current charge - When the battery voltage rises above the trickle-current charge threshold, the charge current is increased for constant current charge. The current of a constant current charge 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 degree of the cell, the current gradually decreases from the maximum value as the charging process continues. When the current decreases to 0.01c, the charging is considered to terminate. (C is a representation of the nominal capacity of the battery compared with the current. For example, the capacity of the battery is 1000mAh, and 1C is the charging current of 1000mA.)

Stage 4: Charge termination -- There are two typical methods of charging termination: using a minimum charge current judgment or using a timer (or a combination of both). The minimum current method monitors the charge current during the constant voltage charging phase and terminates the charge when the charge current decreases to the range of 0.02C to 0.07C. The second method starts at the beginning of the constant voltage charging phase and stops the charging process after two hours of continuous charging.

It takes about 2.5 to 3 hours for a fully discharged battery to be charged by the above four-stage charging method. Advanced chargers also come with additional safety measures. For example, if the battery temperature exceeds the specified window (usually 0 ° C to 45 ° C), then charging will be suspended.

After charging, if the battery voltage is lower than 3.89V, the battery will be recharged.

The workflow of mobile phone charger is generally as follows:

1. Check the battery voltage. If the voltage is below a threshold, trickle charge is required.

2. When the battery is charged to a certain voltage (generally set to 2.9V), the battery is charged at full current;

3. When the battery voltage reaches the preset voltage (generally 4.2V for lithium-ion batteries), the constant voltage charging starts, and the charging current decreases at the same time;

4. When the current gradually decreases to the specified value, the charging process ends.

When the battery voltage is lower than 2.5V(Vshort), the lithium ion battery charger is precharged with 25mA current to prevent the deep discharge lithium ion battery from being damaged or even dangerous during fast charging.

When the battery voltage is lower than 2.5V(Vshort), the lithium ion battery charger is precharged with 25mA current to prevent deep discharge lithium ion batteries from being damaged or even dangerous during fast charging.

In addition to voltage detection, use other auxiliary methods to prevent overcharge. For example, use a battery temperature sensor to continuously check the battery temperature. When the temperature of the battery exceeds the specified range, shut down battery charging.

Limit charging time to provide additional battery protection.

In addition to the process described above, it also has the characteristics of automatic recharge, minimum current terminating charge, etc. In general, the small current charging process at the end of the constant voltage charge, the size of the current is generally one tenth of the current when the constant current charge. Current lithium-ion battery chargers are designed to detect when the phone is discharged after charging for some reason. Once the battery voltage is detected to be low, the charging process is restarted (see picture above).

Software of work to do is to set up into the quick charge voltage threshold, into the constant-voltage charging voltage threshold, charging timeout, constant current charging current value, constant voltage charging voltage, charging current threshold, the end of the interrupt handling, provide sys interface to the upper state of charge, including the type of battery, battery high voltage, minimum voltage battery, battery voltage, current Percentage of battery power, battery state, charging current and battery temperature, etc.

The remaining battery capacity can be estimated by measuring the voltage:

4.20 V - 100%

3.95 V - 75%

3.85 V - 50%

3.73 V - 25%

3.50 V - 5%

2.75 V - 0%

1, lithium battery charging

Due to the structural characteristics of lithium battery, the highest charging termination voltage should be 4.2V, not overcharging, otherwise the battery will be scrapped because too much lithium ion is taken away from the positive electrode. Its charge and discharge requirements are high, can use the special constant current, constant voltage charger for charging. Usually the constant current charge to 4.2V/ node after the constant voltage charge, when the constant voltage charge current drops to less than 100mA, should stop charging. Charging current (mA)= 0.1~1.5 times the battery capacity (for example, 1000mAh battery, its charging current can be controlled between 100~1500mA). The conventional charging current can be selected at about 0.5 times the battery capacity, and the charging time is about 2~3 hours.

2, lithium battery discharge

Due to the internal structure of the lithium battery, the lithium battery cannot all move to the positive electrode when discharging, and a part of the lithium ion must be retained in the negative electrode to ensure that the lithium ion can be unblocked into the channel when the next charge. Otherwise, the battery life will be shortened accordingly. In order to ensure that some lithium ions remain in the graphite layer after discharge, the minimum voltage of discharge termination should be strictly limited, that is to say, the lithium ion battery can not overdischarge. The discharge termination voltage is usually 3.0V/ section, and the minimum voltage cannot be lower than 2.5V/ section. The battery discharge time is related to the battery capacity and discharge current. Battery discharge time (hours)= Battery capacity/discharge current. Lithium battery discharge current (mA) should not exceed 3 times the battery capacity. (For example, 1000mAh battery, the discharge current should be strictly controlled within 3A) otherwise the battery will be damaged.

The following figure shows the charging circuit of TP4056

The following picture shows the selection circuit of USB and battery as input power

A. When there is only battery, Q1 conducts, and the battery supplies power to the VCC;

B. When only USB is available, supply power to VCC directly through D3;

C. When both battery and USB are present at the same time, USB directly supplies power to VCC through D3 until Q1.