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Nov 21 2019

Lithium Battery Technology: Charging Analysis

Ⅰ. Overview

Lithium battery charging mode is voltage limit and constant current, which is controlled by IC chip. The typical charging method is: detect the voltage of the battery to be charged firstly, if its voltage is lower than 3V, pre-charge is required necessarily, and the the charging current is 1 ≤ 10 of the set current. After the voltage rises to 3V, then transferring into the standard charging process. The standard charging process is: having constant current charging with set current. When the battery voltage rises to 4.20V, it is changed to constant voltage charging mode, and the charging voltage is kept at 4.20V. At this time, the charging current gradually decreases till the current drops to 1/10 of the set charging current, the charging ends.

The charging process of a Li-ion battery can be divided into three processes: trickle charging (low voltage precharging), constant current charge, and constant voltage charge.


Ⅱ. Charging Analysis

Charging characteristics of lithium battery charge IC

Typical Charge Profile 

Figure 1. Typical Charge Profile

As can be seen from the above figure, the charging current and voltage of the lithium battery are dynamically changed, which is determined by the chemical content of the lithium battery itself. Therefore, it is necessary to configure the performance of the charging IC according to the charging characteristics of the lithium battery itself to achieve a correct, safe and efficient use of the lithium battery. The "lithium battery charging current" in the daily expression is for the charging current of the lithium battery in the fast charging phase during charging. As a dynamic process, the optimal charging current of the lithium battery is actually divided into three stages.

 2.1 Charging States

Performance description of several different charging states

1) On standby

The standby state is handled in the following cases:

a. The input voltage is lower than the minimum operating voltage of the circuit.

b. After the battery voltage is approach to the limit.

c. Using external switch to turn off IC to stop charge.

Voltage and current characteristics in standby mode: The charging IC has no charging voltage output, and the IC input current is in the uA level, which can reduce power loss.

2) Precharging

As shown in above figure. Optimal current during precharging: that is, when the initial/no-load voltage of the lithium battery is lower than the pre-charge threshold, it needs a pre-charging stage. For a single lithium-ion battery, this threshold is generally 3.0V, in the phase, the precharge current is about 10% of the current in the constant current charging phase.

3) Constant charge

As shown in the figure above, when the battery voltage is greater than the preset voltage threshold and less than the maximum voltage of 4.2V, the IC will charge the battery with the maximum charging current set by the external resistor. When the battery voltage is equal to the maximum charging voltage (approach 4.2V), the charge stop.

The best current for constant current charging: when stay in constant current stage, the voltage gradually rises, then enter the fast charging phase. Most of the constant current charging current is set between 0.5 and 0.8C, and the best set is 0.7C, because the battery can be full charged about two hours without consider other factors. The case is a good balance between charging time and charging safety.

Li-ion batteries

Several problems that should be paid attention to when at constant current charging:

(1) In this state, the IC is in the state of maximum charging current, and the loss at this time is also the largest. The linear voltage drop loss calculation is L = (VIN-VOUT) × IOUT, it is necessary to pay attention to the maximum operating temperature of the IC.

(2) The increasing temperature due to the highest charging current, the IC will automatically reduce the maximum charge current, and this is why the charging current drops during overheating.

Constant voltage charging state: The maximum charging voltage portion shown in the above figure, when it is detected that the battery voltage is equal to or close to the battery charging voltage, at this time, the charging mode will be stepped down with a constant charging voltage of 4.2V. When it is detected that the charging current is less than 1/10 of the maximum set current, charging will stop. Charging current during constant voltage charging: In the case of a single-cell lithium-ion battery, when the battery reaches a certain voltage value, it enters a constant voltage charging, in addition, this voltage value is generally 4.2V.

At this stage, the voltage is keeping constant and the current is reduced, and this current reduction is a sequential decrement process. Most lithium battery protection selects 0.1C as the termination current, which means that the charging process enters the end state. Once charging is finished, the charging current drops to zero. The problem to be noted in this state is that the battery can be automatically turned off when the battery is charged to the highest setting voltage. At the same time, when the overvoltage protection of the IC is in the abnormal battery state, it can be automatically locked.

2.2 Summary

The core of the best charging current of lithium battery is the current design of constant current charging. It should be emphasized that most portable lithium batteries should be designed to charge 0.5C~0.8C. For example 1400mAh capacity of iPhone battery(capacity mAh= current mA × time h), choosing 0.7C, that is, Apple's charging current is about 1A, so that most of the batteries between 0.5C--0.8C you can choose.

When charging, the voltage of the battery should be detected first. If the voltage is lower than 3V, pre-charging should be performed first. When the charging current is 1/10 of the set current, 0.05C is selected generally. After the voltage rises to 3V, it enters the standard charging process. The standard charging process is constant current charging with set current. Till the battery voltage rises to 4.20V, it is changed to constant voltage charging, and the charging voltage is kept at 4.20V. At this time, the charging current gradually decreases, and when the current drops to 1/10 of the set charging current, the charging ends.

Generally, the charging current of the lithium battery is set between 0.2C and 1C. The larger the current, the faster the charging, and the greater the heat of the battery. Moreover, when lies in excessive current charging, the capacity is not full, because the electro-chemical reaction inside the battery takes time.

 Lithium-ion rechargeable battery LI18650

Ⅲ. Charging Process Analysis

a. High voltage constant current mode

In general, the charging process of the mobile phone is to first reduce the 220V charging voltage to the 5V charger voltage, and the 5V charger voltage reduce to the 4.2V battery voltage. During the entire charging process, if the voltage is increased, heat is generated, therefore, the charger will heat up and the phone will heat up. Moreover, the greater the power consumption, the greater the damage to the battery.

b. Low voltage high current mode

When the voltage is constant, the current can be increased by using a parallel circuit. Under this situation, the smaller the volume shared by each circuit after parallel shunting, each circuit has the smaller load damage, so as to the phones charging process.

c. High voltage high current mode

This method increases the current and voltage at the same time, so that from the previous formula P=UI, we can know that this method is the best way to increase the power, but it will generate more heat when the voltage is increased. In this way, the more energy is consumed, but the voltage and current are not freely increased without limitation.

The maximum charging current of a lithium battery is strictly determined by the structure of the battery. Therefore, the specifications of the lithium battery manufacturers are not consistent, some are set to 0.6C, and the highest current specification for portable lithium batteries is 1C.  

Of course, the current design of pre-charging and constant voltage charging cannot be ignored. In the two processes, if the initial voltage is not lower than the pre-charge threshold of 3.0V, there is no pre-charging process. In general, there is a process to check batteries charging voltage that is beneficial to keep the long-term use of lithium batteriess.

 8V 150Ah Lithium battery

Ⅳ. Note: Common Charging Sense

Common sense in the daily use of lithium batteries

1) Misunderstanding: “battery activation, charging for more than 12 hours in the first three times."

For the “activation” problem of lithium batteries, many sayings are: charging time must be more than 12 hours, and repeat three times in order to activate the battery. This statement that “the first three charges have to be charged for more than 12 hours” is obviously a continuation of nickel batteries (such as nickel cadmium and nickel hydride), in other words, this kind of statement can be said to be misinformation of the other batteries. After a sample survey, it conformed that a considerable number of people have confused the charging methods of the two batteries.

The charge and discharge characteristics of lithium and nickel batteries are very different. All the professional technical data reviewed emphasize that overcharge and overdischarge can cause huge damage to lithium batteries, especially liquid Li-ion batteries. Therefore, charging is preferably performed in accordance with standard methods, especially for ultra-long charging of more than 12 hours. For example, the charging method described in the mobile phone manual is a standard charging method suitable for the mobile phone.

2) It is not suitable to charge for a long time, also the battery is completely discharged and then recharged.

The lithium battery phone or charger will automatically stop charging when the battery is fully charged. There is no so-called “turbulent” charging over 10 hours for nickel battery chargers. If the lithium battery is fully charged, it will not be charged anymore continuously.

Over-time charging and power off completely will cause over-charging and over-discharging, which will cause permanent damage to the positive and negative electrodes of lithium-ion batteries. At the molecular level, over-discharge will cause the anode carbon to release lithium ions excessively causing the layer structure collapses, and overcharging will hardly plug too much lithium ions into the negative carbon structure, and some of the lithium ions will no longer be released.

3) Regular deep charge and discharge for battery calibration

Li-ion batteries generally have a management chip and a charge control chip. The management chip has a series of registers, which contain values such as capacity, temperature, ID, state of charge, and discharge times. These values will gradually change during use, so the main function of the “The batteries should be fully charged and discharged when used once a month or so” is to correct the improper values in these registers.

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