Wireless charging design must know: a thorough understanding of lithium battery technology　

　The purpose of wireless charging is to charge batteries. If you don't understand batteries, don't talk about charging, let alone wireless charging. If you want to understand wireless charging, you must first understand batteries. The batteries mentioned here mainly refer to lithium batteries. This article only introduces the basic knowledge of batteries, which is suitable for all practitioners in the wireless charging industry.

　　Too many people have misunderstandings about batteries, etc.:

　　Misconception 1: Does the battery need to be activated? Does a new battery need to be charged and discharged repeatedly three times, each time for 12 hours?

　　Myth 2: Charge the battery as little as possible so that it can last longer.

　　Misconception 3: The battery will explode if charged for too long, so you should unplug the power cord when it is fully charged.

　　Myth 4: The faster the charging, the better the battery.

　　Myth 5: The greater the charging current of the charger, the faster it charges.

　　Myth 6: Wireless charging is not as fast as wired charging.

　　Let’s not discuss the reasons for these misunderstandings first. Let’s analyze the characteristics of lithium batteries and then we will naturally understand the specific reasons.

　　The full text is divided into three major parts: battery basics, battery protection, and battery charging.

　　(I) Lithium battery basics

　　Composition of lithium battery

　　The mobile phone batteries we usually see are all lithium batteries. The industry is more about battery packs. Lithium battery packs are mainly composed of two major parts: battery cells and protection boards. Battery cells are mainly composed of five parts: positive electrode materials, negative electrode materials, electrolytes, diaphragms and shells. The protection board is mainly composed of protection chips, MOS tubes, resistors and capacitors, and PCB boards.

　　Classification of lithium batteries

　　Lithium batteries are divided into cylindrical batteries and square batteries according to their shapes. Cylindrical batteries include 18650 batteries used in some mobile power supplies and power tools, and square batteries include mobile phone batteries.

　　According to the electrolyte, they are divided into lithium-ion batteries and polymer batteries. The electrolyte of lithium-ion batteries is liquid, and the shell is generally hard, usually aluminum or steel. For example, Samsung's traditional machines are all lithium-ion batteries. The electrolyte of polymer batteries is solid, such as the battery of Apple mobile phones.

　　Common terms for lithium batteries

　　1>Battery capacity

　　The capacity of a battery is also called the amount of electricity in the battery, and its unit is Ah (ampere-hour) or mAh (milliampere-hour). Ideally, a battery with a capacity of 1A can be discharged continuously for 1 hour when fully charged with a current of 1A.

　　2>Charge/discharge rate

　　The charge/discharge rate indicates the current used for charging/discharging. It is usually calculated as a multiple of the nominal capacity of the battery, and is generally referred to as "how many C". For a 2500mAh lithium battery, if the discharge rate is 1C, it means the discharge current is 2500mA, and if the charge current is 0.2C, it means the charge current is 500mA.

　　3> Nominal voltage

　　The nominal voltage of a lithium battery is also called the rated voltage. The nominal voltage of an ordinary mobile phone battery is generally 3.7V. The voltage when the battery is fully charged is generally around 4.2V, and the voltage when the battery is just discharged is generally around 2.5V.

　　4> Nominal power/nominal energy

　　According to the physics textbook, P=U*I, W=U*I*t, the unit of power is W (watt), and the unit of energy is Wh (watt-hour). A battery with a capacity of 1500mAh has a nominal voltage of 3.7V, a nominal power of 5.55W, and a nominal energy of 5.55Wh.

　　5>Internal resistance

　　The battery can be equivalent to a voltage source with a certain internal resistance. For the battery, the smaller the internal resistance, the better, but the smaller the internal resistance, the more expensive it is.

　　The unit of the internal resistance of a battery is Ω (ohm) or mΩ (milliohm). The internal resistance of a battery is mainly affected by the material and manufacturing process of the battery.

　　6>Cycle life

　　The cycle life of a battery generally refers to the cycle of being fully charged and discharged. When the battery reaches the end of its cycle life, it ages severely and the battery capacity will decrease a lot. The cycle life is also an important indicator for measuring batteries.

　　The IEC standard stipulates that the battery's cycle life should maintain 60% of its initial capacity after 500 cycles, and the national standard stipulates that the capacity should maintain 70% of its initial capacity after 300 cycles.

　　7>Remaining power (SoC)

　　The remaining capacity of the battery refers to the percentage of the current battery power to the total available power, 0%~100%, reflecting how much power is left.