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1、 Lithium batteries

1. Briefly describe lithium batteries and their working principles

Since its inception in 1990, it has experienced rapid development due to its outstanding performance and has been widely applied in society. Lithium ion batteries have rapidly occupied many fields with advantages that are unmatched by other batteries, such as well-known mobile phones, laptops, small cameras, and so on.

The current recognized basic principle of lithium batteries is the so-called "rocking chair theory". The charging and discharging of lithium batteries are not achieved through traditional methods of electron transfer, but through the entry and exit of lithium ions in the crystal of layered materials, resulting in energy changes. Under normal charging and discharging conditions, the entry and exit of lithium ions generally only cause changes in interlayer spacing, without causing damage to crystal structure. Therefore, from the perspective of charging and discharging reflection, lithium-ion batteries are an ideal reversible battery. During charging and discharging, lithium-ion batteries move back and forth between the positive and negative poles of the battery, shaking back and forth like a rocking chair, hence some people refer to lithium-ion batteries as rocking chair batteries.

The superiority of lithium-ion batteries that we often talk about is related to traditional nickel cadmium batteries (Ni/Cd) and nickel hydrogen batteries (Ni/MH). It has the advantages of high working voltage, high specific energy, long cycle life, low self discharge rate, and no memory effect.

2. Common knowledge in the daily use of lithium batteries

(1) Misconception: "Battery activation, first three charges for more than 12 hours"

There are many sayings about the "activation" issue of lithium batteries: the charging time must exceed 12 hours and be repeated three times in order to activate the battery. The statement that the first three charges require more than 12 hours of charging is clearly a continuation of nickel batteries (such as nickel cadmium and nickel hydrogen). So this statement can be said to have been a misunderstanding from the beginning. After a sampling survey, it can be seen that a considerable number of people have confused the charging methods of the two types of batteries.

There is a significant difference in the charging and discharging characteristics between lithium batteries and nickel batteries, and all the serious formal technical materials consulted emphasize that overcharging and overcharging can cause significant harm to lithium batteries, especially liquid lithium-ion batteries. Therefore, it is best to charge according to standard time and methods, especially do not charge for more than 12 hours. Usually, the charging method introduced in the phone manual is the standard charging method suitable for the phone.

(2) Not conducive to long-term charging, fully depleted battery before recharging

Lithium ion phones or chargers will automatically stop charging after the battery is fully charged, and there is no "trickle" charging that lasts for more than 10 hours as claimed by nickel chargers. If the lithium battery is fully charged and placed on the charger, it will no longer charge.

Excessive time charging and complete depletion of electricity can cause overcharging and overdischarge, which will cause permanent damage to the positive and negative electrodes of lithium-ion batteries. From a molecular perspective, overdischarge will lead to excessive release of lithium ions from the negative electrode carbon, causing its layered structure to collapse. Overcharging will force too many lithium ions into the negative electrode carbon structure, making it impossible for some of them to be released again.

(3) Battery life

The experimental table for lithium-ion battery charging and discharging cycles includes the following data on cycle life (DOD stands for discharge depth):

Cycle life (10% DOD):>1000 cycles

Cycle life (100% DOD):>200 cycles

From the above data, it can be seen that the number of rechargeable times is related to the depth of discharge, and the cycle life at 10% DOD is much longer than that at 100% DOD. Of course, if it is equivalent to the actual total charging capacity of 10% * 1000=100100% * 200=200, the complete charging and discharging of the latter is still better. However, the lifespan of lithium batteries is mainly reflected in the charging and discharging cycle, which is an absolute concept. The last time 30% of the electricity was used and fully charged, and the next time 70% of the electricity was used and fully charged, which happens to be one charging cycle. So we still follow the slogan of the inventor of lithium batteries, which is "use and charge, charge and use" when using lithium batteries.

(4) Regular deep charging and discharging for battery calibration

Lithium ion batteries generally come with management chips and charging control chips. There are a series of registers in the management chip, which store values such as capacity, temperature, ID, charging status, and number of discharges. These values will gradually change during use. The main purpose of the practice of "charging and discharging all registers once every month or so" in the user manual should be to correct improper values in these registers.

2、 The charging method of lithium batteries is voltage limited transverse current mode

It is mainly completed in three steps:

Step 1: To determine if the voltage is less than 3V, pre charge with a current of 0.05C;

Step 2: Determine that 3V<voltage<4.2V, constant current charging 0.2C~1C current;

Step 3: Determine if the voltage is greater than 4.2V, charge at a constant voltage of 4.20V, and the current decreases with the increase of voltage until it is fully charged.

Actually, I didn't understand this point today. I looked it up online and learned about it as common sense.

At the beginning of charging, the voltage of the battery to be charged should be checked first. If the voltage is below 3V, pre charging should be carried out first. The charging current is 1/10 of the set current, usually around 0.05C. After the voltage rises to 3V, it enters the standard charging process. The standard charging process is to perform constant current charging at the set current. When the battery voltage rises to 4.20V, it is changed to constant voltage charging and the charging voltage is maintained at 4.20V. At this point, 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 lithium batteries is set between 0.2C and 1C. The higher the current, the faster the charging, and at the same time, the greater the heat generated by the battery. Moreover, excessive current charging results in insufficient capacity, as the electrochemical reactions inside the battery require time. Just like pouring beer, pouring too fast will produce foam, but dissatisfaction.

Terminology explanation: The charging and discharging current is generally referred to as C, which is the numerical value corresponding to the battery capacity. The battery capacity is generally expressed in Ah and mAh, for example, for an M8 battery with a capacity of 1200mAh, the corresponding C is 1200mA. 0.2C is equal to 240mA.

The following is a typical charging curve diagram for lithium batteries:

3、 The discharge of lithium batteries, for batteries, normal use is the discharge process

The discharge of lithium batteries only requires a few precautions:

1. The discharge current should not be too large, as excessive current can cause internal heating of the battery, which may cause permanent damage;

2. Never over discharge! Lithium batteries are most afraid of over discharge, and once the discharge voltage is below 2.7V, it may lead to the battery being scrapped.

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

From the typical discharge curve, it can be seen that the larger the discharge current of the battery, the smaller the discharge capacity, and the faster the voltage drop.

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