What is the charging and discharging principle of lithium batteries?

First, lithium battery

1. Briefly describe the lithium battery and how it works.

Since its inception in 1990, it has developed rapidly due to its superior performance and is widely used in society. Lithium-ion batteries quickly occupy many fields with the incomparable advantages of other batteries, such as mobile phones, notebook computers, small cameras, and so on.

The basic principle recognized by lithium batteries at present is the so-called "rocking chair theory". The discharge and discharge of a lithium battery is not a transfer of electrons in a conventional manner, but an energy change occurs by the passage of lithium ions in the crystal of the layered substance. In the case of normal rushing and discharging, the lithium ion is generally only caused by the change of the interlayer spacing without causing the destruction of the crystal structure. Therefore, the lithium ion battery is an ideal reversible battery. During the discharge and discharge, lithium ions are turned in and out of the positive and negative electrodes of the battery, and are shaken between the positive and negative electrodes like a rocking chair. Therefore, the image of the lithium ion battery is called a rocking chair battery.

The advantages of lithium-ion batteries we often say are for traditional nickel-cadmium batteries (Ni/CD) and nickel-hydrogen batteries (Ni/MH). It has the advantages of high working voltage, high energy ratio, long cycle life, low self-discharge rate and no memory effect.

2. Common sense during the daily use of lithium batteries

(1) Misunderstanding: "Battery activated, charging for more than 12 hours in the first three times"

For the "activation" problem of lithium batteries, many claims are: charging time must be more than 12 hours, repeated 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). So this kind of statement can be said to be misinformation at the beginning. After a sample survey, it can be seen 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 serious formal technical data reviewed emphasize that overcharge and over discharge can cause huge damage to lithium batteries, especially liquid lithium ion batteries. Therefore, charging is preferably done in accordance with standard time and standard methods, especially for ultra-long charging of more than 12 hours. Usually, the charging method described in the mobile phone manual is a standard charging method suitable for the mobile phone.

(2), do not benefit from charging for a long time, the battery is completely used up and recharge

The battery or charger of the lithium battery will automatically stop charging when the battery is full. There is no so-called “turbulent” charging that the nickel battery charger has for 10 hours. If the lithium battery is fully charged, it will not be charged on the charger.

Over-time charging and full emptying 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. 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), battery life

For the experimental table of charge and discharge cycles for lithium-ion batteries, the data on cycle life are listed below (DOD is the abbreviation for depth of discharge):

Cycle life (10% DOD): >1000 times

Cycle life (100% DOD): >200 times

As can be seen from the above data, the number of chargeable times is related to the depth of discharge, and the cycle life at 10% DOD is much longer than that of 100% DOD. Of course, if the relative total capacity of the actual charge is 10%*1000=100, 100%*200=200, the full charge and discharge of the latter is better. However, the life of the lithium battery is mainly reflected in the charge and discharge cycle. This cycle is an absolute concept. The last time 30% of the electricity was used, it was fully charged, and the next time it used 70% of the power, it was fully charged. This is just a charge. cycle. Therefore, the lithium battery is still used in accordance with the slogan of the inventor of the lithium battery, "ready to use, that is, charge and use".

(4), regular deep charge and discharge for battery calibration

Lithium-ion batteries typically come with a management chip and a charging control chip. In the management chip, there are a series of registers, which contain the capacity, temperature, ID, charging state, discharge times and other values. These values vary over time. The main function of the instruction "after use for a month or so, it should be fully charged and discharged" should be to correct the improper values in these registers.

Second, the charging method of lithium battery is limited pressure cross flow

Mainly completed in three steps:

The first step: to determine the voltage <3V, you must first pre-charge, 0.05C current;

The second step: judge 3V < voltage <4.2V, constant current charging 0.2C ~ 1C current;

The third step: judge the voltage > 4.2V, constant voltage charging, the voltage is 4.20V, the current decreases with the increase of the voltage until it is full.

In fact, today I just don't understand this point. I checked it online and then I used it as a common sense.

When charging starts, the voltage of the battery to be charged should be detected first. If the voltage is lower than 3V, pre-charging should be performed first. The charging current is 1/10 of the set current, generally about 0.05C. After the voltage rises to 3V, it enters the standard charging process. The standard charging process is: constant current charging with set current. When 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, excessive current charging, the capacity is not full, because the electrochemical reaction inside the battery takes time. Just like pouring beer, if it is too fast, it will produce bubbles, but it will not be full.

Explanation of terms: The charge and discharge current is generally referred to as C, and C is the value corresponding to the battery capacity. Battery capacity is generally expressed in Ah, MAH, such as M8 battery capacity 1200mAh, the corresponding C is 1200mA. 0.2C is equal to 240mA.

The following is a typical charging curve for a lithium battery:

Third, the discharge of the lithium battery, for the battery, the normal use is the discharge process

Lithium battery discharge only needs to pay attention to a few points:

1. The discharge current should not be too large. Excessive current will cause the internal heat of the battery, which may cause permanent damage.

2. Absolutely cannot be over-discharged! Lithium batteries are most afraid of over discharge. Once the discharge voltage is lower than 2.7V, the battery may be scrapped.

The following is a typical discharge curve for a typical lithium battery:

It can be seen from the typical discharge graph that the larger the discharge current of the battery, the smaller the discharge capacity and the faster the voltage drop.

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