What is the principle of lithium battery charging and discharging
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2022/05/17 09:24:39
First, lithium batteries
1. Brief introduction of lithium battery and its working principle
Since its invention in 1990, because of its excellent performance, it has been rapidly developed and widely used in society. Lithium-ion batteries have rapidly occupied many fields, such as mobile phones, laptop computers, small video cameras and so on, with advantages unmatched by other batteries.
The accepted rationale for lithium-ion batteries is the so-called "rocking chair theory". The charge and discharge of lithium battery is not through the traditional way of electron transfer, but through the lithium ion in and out of the crystal of solid material, energy change. Under normal charge and discharge conditions, lithium ion in and out only causes the change of layer spacing, but does not cause the destruction of crystal structure. Therefore, from the point of charge and discharge reflection, lithium ion battery is an ideal reversible battery. In the discharge of lithium ions in the battery positive and negative poles back and forth, just like rocking chair in the positive and negative poles to shake, so some people will be lithium ion battery image called rocking chair battery.
The advantages of lithium-ion batteries we often talk about are based on the traditional nickel-cadmium (Ni/Cd) and nickel-metal hydride (Ni/MH) batteries. It has the advantages of high voltage, high energy ratio, long cycle life, low self-discharge rate and no memory effect.
2. Common sense in the daily use of lithium batteries
(1) misunderstanding: "Battery activation, more than 12 hours after the first three charges"
For lithium battery "activation" problem, many say: charging time must be more than 12 hours, repeated three times, in order to activate the battery. This "the first three charge to charge more than 12 hours", apparently from nickel batteries (such as nickel cadmium and nickel hydrogen) continued to say. So this argument, it can be said, was misrepresented from the very beginning. After a sample survey, it can be seen that a significant number of people confuse the two battery charging methods.
The charge and discharge characteristics of lithium battery and nickel battery are very different, and all the serious and formal technical data I have consulted emphasize that over charge and over discharge will cause great harm to lithium battery, especially liquid lithium ion battery. Therefore, it is best to charge in accordance with the standard time and standard method, especially do not charge for more than 12 hours. Usually, the charging method introduced in the mobile phone manual is the standard charging method suitable for the mobile phone.
2), not for a long time to charge, the battery is completely used up before charging
Lithium-ion phones or chargers automatically stop charging when the battery is fully charged, and there is no such thing as a "trickle" charge that lasts 10 hours or so for nickel chargers. If the lithium battery is fully charged, it will no longer charge on the charger.
Supernormal charging time and completely empty battery may result in excessive charging and discharging, to permanent damage across the electrodes of lithium ion battery, from molecular level, discharge will cause excessive release of lithium ion cathode carbon makes its layer structure appeared to collapse, excessive charging will take too much lithium ion shoehorned into the structure of cathode carbon, And some of those lithium ions are never released.
(3) Battery life
As for the experimental table of charge and discharge cycle of lithium ion battery, the data about cycle life are listed as follows (DOD is the Abbreviation of discharge depth) :
Cycle life (10% DOD) : >1000 times
Cycle life (100% DOD) : >200 times
As can be seen from the above data, the charging times are related to the discharge depth, and the cycle life of 10% DOD is much longer than that of 100% DOD. Of course, if converted to the relative total capacity of the actual charge 10% *1000= 100,100 % *200=200, the latter is still better to fully charge and discharge. But the life of lithium battery is mainly reflected in the charge and discharge cycle, this cycle is an absolute concept, the last use of 30% power, fully charged, the next use of 70% power, fully charged, this is just a charge cycle. Therefore, we still follow the slogan of the lithium battery inventor: "Charge when you use, charge when you use".
(4) Conduct battery calibration with regular deep charge and discharge
Lithium-ion batteries generally have management chips and charging control chips. The management chip has a series of registers, holding capacity, temperature, ID, charging state, discharge times and other numerical values. These values change gradually over time. The main function of the instructions for "charging and discharging once a month or so" should be to correct improper values in these registers.
Second, the charging mode of lithium battery is the cross-current mode of limited pressure
It is mainly accomplished in three steps:
The first step: to judge the voltage <3V, pre-charge and 0.05c current should be carried out first;
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 charge, voltage is 4.20V, the current decreases with the increase of voltage, until full.
In fact, today I am this point some do not understand, looked up on the Internet, and then those as a common sense understanding.
Before charging, check the voltage of the battery to be charged. If the voltage is lower than 3V, precharge the battery. The charging current is 1/10 of the set current, which is about 0.05c. After the voltage rises to 3V, the standard charging process is entered. The standard charging process is as follows: constant current charging at 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 charging current gradually decreases, when the current drops to the set charging current 1/10, the charging end.
The general lithium battery charging current is set between 0.2C and 1C, the greater the current, the faster the charging, and the greater the battery heating. What's more, too much current can't charge to full capacity, because the electrochemical reactions inside the cell take time. Just like pouring beer, if you pour it too fast, you'll get frothy and dissatisfied.
The charge and discharge current is usually referred to as C, which is the value corresponding to the battery capacity. Battery capacity is generally expressed in Ah, mAh, such as M8 battery capacity of 1200mAh, the corresponding C is 1200mA. 0.2C is equal to 240mA.
Below is the typical charging curve of lithium batteries:
Three, the discharge of lithium battery, for the battery, normal use is the discharge process
There are few points to pay attention to when discharging lithium batteries:
1. Discharge current should not be too large, because excessive current may cause heat inside the battery, which may cause permanent damage;
2, absolutely not over discharge! Lithium battery is most afraid of overdischarge, once the discharge voltage is lower than 2.7V, it may cause the battery to scrap.
Below is a typical discharge curve of a typical lithium battery:
As can be seen from the typical discharge curve, the higher the discharge current, the lower the discharge capacity and the faster the voltage drop.
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