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Use And Maintenance Of Lithium Ion Batteries
- Sep 05, 2018 -

A lithium ion battery is a secondary battery (rechargeable battery) that mainly relies on lithium ions moving between a positive electrode and a negative electrode to operate. During charge and discharge, Li+ is intercalated and deintercalated between the two electrodes: when charging, Li+ is deintercalated from the positive electrode, and the electrolyte is embedded in the negative electrode, and the negative electrode is in a lithium-rich state; The battery is generally made of a material containing lithium as an electrode, and is a representative of modern high-performance batteries. Lithium batteries are classified into lithium batteries and lithium ion batteries. Both mobile phones and laptops use lithium-ion batteries, which are commonly referred to as lithium batteries. True lithium batteries are rarely used in everyday electronics because of their high risk.

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Lithium Ion Battery (Li-ion, Lithium Ion Battery): Lithium-ion batteries are widely used because of their light weight, large capacity, and no memory effect. Many digital devices now use lithium-ion batteries as power sources. Although its price is relatively expensive. Lithium-ion batteries have a high energy density, and their capacity is 1.5 to 2 times that of nickel-hydrogen batteries of the same weight, and has a very low self-discharge rate. In addition, the lithium-ion battery has almost no "memory effect" and does not contain toxic substances, which is also an important reason for its wide application.

Use and maintenance of lithium ion batteries:

Lithium-ion battery charging

In the use of lithium batteries, it should be noted that after the battery is placed for a period of time, it enters a sleep state. At this time, the capacity is lower than the normal value, and the use time is also shortened. However, the lithium battery is easy to activate, as long as the battery is activated after 3 to 5 normal charge and discharge cycles, and the normal capacity is restored. Due to the characteristics of the lithium battery itself, it is determined that it has almost no memory effect. Therefore, the new lithium battery in the user's mobile phone does not require special methods and equipment during the activation process. To charge a Li-Ion battery, use a dedicated Li-Ion battery charger. Lithium-ion battery charging adopts “constant current/constant voltage” mode, first constant current charging, and changing to constant voltage charging when it is close to the termination voltage. For example, a battery of 800 mA.h capacity has a termination charging voltage of 4.2V. The battery is charged at 800mA (charge rate 1C) with constant current. At the beginning, the battery voltage rises with a large slope. When the battery voltage is close to 4.2V, it is changed to 4.2V constant voltage charge. The lithium battery current gradually decreases, and the voltage does not change much. When the charging current drops to 1/10C (about 80mA), it is considered to be nearly full, and the charging can be terminated (some chargers start the timer after 10C, and end charging after a certain time). It is not possible to charge a lithium-ion battery with a charger filled with nickel-cadmium batteries (three-cell nickel-cadmium batteries) (although the rated voltage is 3.6V), it is easy to cause overcharging due to different charging methods.

(1) Charging voltage: The charging voltage at the time of full charge is related to the negative electrode material of the battery. The coke is 4.1V, and the graphite is 4.2V. It is generally called 4.1V lithium ion battery and 4.2V lithium ion battery. When charging, be aware that the 4.1V battery cannot be charged with the 4.2V charger, otherwise there is a danger of overcharging (4.1V is different from the charger IC used by the 4.2V charger). Lithium-ion batteries have high requirements for charging, and they require sophisticated charging circuits to ensure safe charging. The termination charge voltage accuracy tolerance is ±1% of the rated value (for example, a 4.2V lithium-ion battery with a tolerance of ±0.042V), and over-voltage charging can cause permanent damage to the lithium-ion battery.

(2) Charging current: The charging current of the lithium ion battery should be based on the recommendations of the battery manufacturer and require a limited current circuit to avoid overcurrent (overheating). The commonly used charging rate is 0.25~1C, and the recommended charging current is 0.5C (C is the capacity of the battery, such as the battery with the nominal capacity of 1500mA.h, the charging current is 0.5*1500=750mA). It is often necessary to detect the battery temperature during high current charging to prevent damage to the battery or explosion due to overheating.

(3) Charging temperature: When charging the battery, its ambient temperature should not exceed the temperature range listed in the product characteristics table. The battery should be charged within the temperature range of 0 to 45 ° C, away from high temperature (above 60 ° C) and low temperature (-20 ° C) environment.

If the lithium ion battery is overcharged, overdischarged or overcurrent during charging or discharging, it may cause damage to the battery or reduce the service life. To this end, various protection components and protection circuits consisting of protection ICs have been developed, which are installed in the battery or battery pack to provide complete protection of the battery. However, overcharge and overdischarge should be prevented as much as possible during use. For example, when the mobile phone battery is charging, it should be separated from the charger in time when it is fully charged. When the depth of discharge is shallow, the cycle life will be significantly improved. Therefore, when using, do not wait until the battery is low on the phone to charge, and do not continue to use this signal, although some residual capacity is available when this signal appears.

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Lithium-ion battery discharge

Discharge termination voltage: The rated voltage of the lithium ion battery is 3.6V (some products are 3.7V), and the termination discharge voltage is 2.5-2.75V (the battery factory gives the working voltage range or gives the termination discharge voltage, and the parameters are slightly different. ). The discharge termination voltage of the battery should not be less than 2.5 (n is the number of batteries in series). The discharge below the termination discharge voltage is called overdischarge. Overdischarge will shorten the battery life and cause battery failure in severe cases. When the battery is not in use, the battery should be charged to 20% of the capacity, and then stored in a moisture-proof package. The voltage is detected once every 3 to 6 months, and the battery is charged to ensure that the battery voltage is within the safe voltage (3V or more).

Discharge current: Lithium-ion batteries are not suitable for high-current discharge. When excessive current is discharged, high internal temperature will be generated to lose energy, which will reduce the discharge time. If there is no protection element in the battery, it will overheat and damage the battery. Therefore, the battery production plant gives the maximum discharge current and cannot exceed the maximum discharge current given in the product characteristics table during use.

Discharge temperature: The discharge curves at different temperatures are different. At different temperatures, the discharge voltage and discharge time of the lithium-ion battery are also different. The battery should be discharged (operating) within the temperature range of -20 ° C to +60 ° C.

Storage and transportation

Lithium-ion batteries can be stored in a clean, dry and ventilated environment with a temperature of -5 to 35 ° C and a relative humidity of not more than 75%. Avoid contact with corrosive substances, keep away from fire and heat sources, and do not expose to direct sunlight. Place, you can't disassemble the battery at will. If the battery is stored for a long time, the battery should maintain 30% to 50% of the nominal capacity. The recommended battery should be charged every 6 months.

The battery should be packed into boxes for transportation. During transportation, it should be protected from severe vibration, impact or extrusion to prevent sun and rain. It can be transported by means of vehicles, trains, ships, airplanes and other means of transportation.

Short circuit

Regarding the safety of lithium-ion batteries, please pay attention to your friends. Lithium-ion batteries are prone to short-circuit conditions during charging. Although most lithium-ion batteries have protection circuits that are short-circuit proof, there are also explosion-proof lines. But in many cases, this circuit does not necessarily work in various situations. Explosion-proof lines can also play a limited role.

Overcharge

All lithium-ion batteries, including polymer lithium-ion batteries, lithium-iron batteries, etc., are very afraid of overcharging. If the lithium-ion battery is charged for too long, the possibility of an explosion will increase. The chemical properties of lithium are very active and easy to burn. When the battery is charged and discharged, the inside of the battery continues to heat up, and the gas generated during the activation process expands, causing the internal pressure of the battery to increase. If the pressure reaches a certain level, if the outer casing is scratched, it will rupture, causing liquid leakage, fire, and even explosion. Therefore, everyone should pay great attention to safety when using lithium-ion batteries.

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Charging precautions

Excessive charging and discharging of lithium-ion batteries can cause permanent damage to the positive and negative electrodes. Over-discharge leads to collapse of the negative carbon sheet structure, and collapse can cause lithium ions to be inserted during charging; excessive charging causes excessive lithium ions to be embedded in the negative carbon structure, and some of the lithium ions can no longer be released. The amount of charge is equal to the charging current multiplied by the charging time. When the charging control voltage is constant, the charging current is larger (the charging speed is faster), and the charging amount is smaller. If the battery charging speed is too fast and the termination voltage control point is improper, the battery capacity will be insufficient. Actually, some of the electrode active materials of the battery are not fully reacted and the charging is stopped. This phenomenon of insufficient charging is aggravated as the number of cycles increases.

 

Safety features of lithium-ion batteries:

For the safety performance evaluation indicators of lithium-ion batteries, very strict standards have been set internationally. A qualified lithium-ion battery should meet the following conditions in terms of safety performance:

(1) Short circuit: no fire, no explosion

(2) Overcharge: no fire, no explosion

(3) Hot box test: no fire, no explosion (150 °C constant temperature 10 min)

(4) Acupuncture: no explosion (penetrate the battery with Ф3mm nail)

(5) Flat impact: no fire, no explosion (10kg weight from 1M high to battery)

Advantages of lithium-ion batteries:

(1) The operating voltage of a high-voltage single cell is as high as 3.7-3.8V (3.2V for lithium iron phosphate), which is three times that of Ni-Cd and Ni-MH batteries.

(2) The actual specific energy achieved by the specific energy is about 555Wh/kg, that is, the material can reach a specific capacity of 150 mAh/g or more (3--4 times Ni-Cd, 2--3 times Ni-MH) , is close to about 88% of its theoretical value.

(3) The cycle life can generally reach more than 500 times, or even more than 1000 times, and lithium iron phosphate can reach more than 2000 times. For small current discharge appliances, the life of the battery will double the competitiveness of the appliance.

(4) Good safety performance, no pollution, no memory effect. As a lithium-ion battery of Li-ion's predecessor, a new type of lithium-ion battery, the metal lithium is liable to form a short circuit of dendrites, which reduces its application: Li-ion does not contain cadmium, lead, mercury and other environmentally polluting elements: part of the process A major drawback of Ni-Cd batteries (such as sintered) is the "memory effect", which severely constrains the use of batteries, but Li-ion does not have this problem at all.

(5) Self-discharge The self-discharge rate of Li-ion fully charged at room temperature for 1 month is about 2%, which is much lower than 25-30% of Ni-Cd and 30-35% of Ni-MH.

(6) Fast charging 1C charging 30 minutes capacity can reach more than 80% of the nominal capacity, and the ferrophosphorus battery can reach 10% of the nominal capacity in 10 minutes.

(7) Working temperature The working temperature is -25~45 °C. With the improvement of electrolyte and positive electrode, it is expected to be widened to -40~70 °C.

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Disadvantages of lithium-ion batteries:

Aging: Unlike other rechargeable batteries, the capacity of lithium-ion batteries will slowly decline, depending on the number of uses and temperature. This phenomenon of decay can be expressed as a decrease in capacity or as an increase in internal resistance. Because of the temperature, electronic products with high operating currents are easier to embody. Replacing graphite with lithium titanate seems to extend life. The relationship between storage temperature and the rate of permanent loss of capacity:

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(2) Recovery rate: About 1% of new products are required to be recycled for various reasons.

(3) Intolerance overcharge: When overcharged, excessively embedded lithium ions are permanently fixed in the crystal lattice and cannot be released again, which may result in short battery life.

(4) Intolerance overdischarge: When overdischarged, the electrode deintercalates excessive lithium ions, which may cause the lattice to collapse and shorten the life.


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