Do you know power banks well

A portable power bank is usually composed of shell + battery + circuit board.

The shell is usually plastic or metal, which makes powerbank achieves a beautiful appearance, protection, etc.

The circuit board is mainly used to realize voltage, current control, input and output control, and other functions.
Batteries are the most costly component of power banks. 18650 batteries and polymer batteries are the two most common ones.
* Battery cell

The current mobile power batteries generally use lithium-ion batteries, but the performance is different, and it is completely invisible from the appearance, and the general specifications cannot be reflected.

1. Battery materials
Lithium-ion batteries can be divided into liquid lithium-ion batteries (LIB) and polymer lithium-ion batteries (LIP) according to the different electrolyte materials used. The positive and negative materials used in the two are the same. The positive electrode material includes three materials: lithium cobalt oxide, nickel cobalt manganese, and lithium iron phosphate. The negative electrode is graphite, and the working principle of the battery is the same.

2. Electrolyte
The main difference between lithium-ion batteries is the difference in electrolytes. Liquid lithium-ion batteries use liquid electrolytes, while polymer lithium-ion batteries use solid polymer electrolytes instead. This polymer can be in a "dry state" or a "colloid state". At present, most of the polymer gel electrolytes are used.

① lithium cobalt oxide (LiCoO2)
Lithium cobalt oxide is commonly known as a liquid lithium-ion battery, and its common shapes are 18650 and square shape. The 18650 battery is a cylindrical battery with a diameter of 18 mm and a height of 65 mm (it looks like an enlarged version of the No. 5 battery) and is widely used in notebook batteries. Because it is compatible with laptop batteries, more than 60% of mobile power sources on the market now use 18650 batteries.

② nickel cobalt manganese (LiNiCoMnO2)
Nickel-cobalt-manganese, also known as ternary material (LiNiCoMnO2), is a type of polymer lithium-ion battery, and its common form is a square soft pack shape. Note that lithium cobalt oxide can also be made into a square shape, but it is hard after molding, and it can be distinguished by pinching it with your hands.
With the popularity of smartphones, ternary materials have developed rapidly in the past two years and are used in more and more fields. It uses nickel salt, cobalt salt, and manganese salt as raw materials, and the ratio of nickel, cobalt, and manganese can be adjusted according to actual needs.
Compared with lithium cobalt oxide batteries, the battery with ternary material as the positive electrode has higher safety, and has a higher service life than lithium cobalt oxide, reaching a service life of 500 cycles.

Main advantages: Diversified volume, very wide range of use, not easy to explode, and high safety factor.

Main disadvantages: high price, environmental pollution after being abandoned, weak charging and discharging performance with high current.

③Lithium iron phosphate (LiFePO4)
The scientific name of lithium iron phosphate is ferroelectric. The biggest difference between the previous two types of batteries is that iron is added to the positive electrode of the battery. Lithium iron has only just started in recent years. It is a material with great potential. Its safety performance and cycle life cannot be compared with other materials. These are also the most important technical indicators of power batteries. The charging and discharging cycle life is up to 2000 times, and the single-cell battery will not burn or explode if the overcharge voltage is 30V.
The large-capacity battery pack made of lithium iron phosphate cathode material is easier to use in series to meet the needs of frequent charging and discharging of electric vehicles and has the advantages of non-toxic, non-polluting, good safety performance, wide source of raw materials, low price, and long life. It is an ideal cathode material for a new generation of lithium-ion batteries.
At present, ferroelectrics are mainly used for large-capacity electric buses, signal base station energy storage, and large-scale UPS applications. Among them, mobile power and AA batteries have just begun to test water and mass production, which makes ferroelectrics gradually use medium and large-capacity UPSs and small energy storage batteries. , Lawn lights and power tools are widely used.

Main advantages: 2000 cycles service life, high current charge and discharge, low internal resistance, low heat generation, safety, environmental protection, and non-toxic.

The main disadvantages: the price is expensive, the digital product field has not yet been used on a large scale, and the awareness of consumers is low.

* Circuit board

In addition to the battery, the circuit board in the mobile power supply is also very important. For rechargeable batteries, the specification has a safe charge cut-off voltage and a safe discharge cut-off voltage, and a calibrated rated maximum operating current. The design of the mobile power supply must first charge the polymer battery safely, because the battery cost is relatively high, and for the safe and reliable operation of the system, there must be a charging management system. When the portable device needs to be charged, the polymer battery is discharged to the outside. Because the portable device generally has an input voltage of 5V, there is a 5V boost system. Regardless of whether it is a charging management system or a boost system, a circuit board is required to provide it, so the design of the internal circuit board of the mobile power supply determines the intelligence of the product.

After a large number of safety accidents are exposed by the media, the safety performance of mobile power has become an important factor that people consider. Therefore, engineers should spend more effort on protecting modules when designing mobile power products. What protection modules does it include when it comes to the inside of the product?

1. Overcharge protection

Lithium battery overcharge protection is realized by using a power management chip to detect the voltage, and the chip is in the reference setting state (the mobile phone lithium battery is generally 3.5V). When the reference rises slowly when it rises to the VSS-VDD design value, the voltage at this time is the protection overcharge shutdown voltage, and the external control circuit is controlled to achieve overcharge protection by outputting a low or high level through a logical relationship. When the voltage drops slowly, set the VSS-VDD voltage value to take the reference value. When the reference detects that it is below the setting, it is a logical relationship to release the overcharge protection.

2. Over-discharge protection

The over-discharge protection voltage refers to the lowest voltage that protects the battery during the discharge transition. When the discharge reaches this voltage point, the protection circuit cuts off the circuit to achieve the purpose of protecting the battery. According to the relationship between battery life and depth of discharge and the relationship between battery voltage and discharge rate and depth, combined with the actual load of the equipment, determine the battery discharge end voltage, and design the battery discharge protection circuit.

3. Short circuit protection
The loop current caused by a short circuit is generally more than 10 times the rated operating current, and overcurrent protection needs to be delayed by about tens of milliseconds. The dozens of times the rated current caused by a direct short circuit will also affect the performance of the battery pack within tens of milliseconds. The existing protection method is the PPTC method, which cuts off the circuit by heat generated by the current, and also requires a response time of milliseconds, and at the same time increases the impedance in the circuit. There are also short-circuit integrated chips dedicated to battery packs. This chip has a narrow application range and high cost.

4. PTC introduction

PTC positive temperature coefficient thermistor is also called poly switch, polymer resettable fuse (polymer resettable fuse) The polymer resettable fuse is composed of a polymer matrix and carbon black particles that make it conductive. Since the polymer resettable fuse is a conductor, current will flow through it. When an overcurrent flows through the polymer resettable fuse, the generated heat (I2R) will cause it to expand. As a result, the carbon black particles will separate and the resistance of the polymer resettable fuse will increase. This will promote the polymer resettable fuse to generate heat faster, expand more, and further increase the resistance. When the temperature reaches 125°C, the resistance changes significantly, so that the current is significantly reduced. At this time, the small current flowing through the polymer resettable fuse is enough to keep it at this temperature and in a high resistance state. When the fault is cleared, the polymer resettable fuse shrinks to its original shape to reconnect the carbon black particles, thereby reducing the resistance to the level of the specified holding current.

5. Other protection circuits, etc.

* The main chip on the circuit board

1. Lithium battery management IC
At present, the domestic charging management system is relatively mature, and the intelligent IC monitors the entire charging process and performs the three-stage charging function of lithium battery pre-charge, constant current, and constant voltage. Mainstream management IC

2.MCU
The intelligent control system on the PCB board prevents the device from being damaged by unstable current and voltage impacts during charging; it can control the charge and discharge of the product, and provide charge protection, discharge protection, temperature protection, leakage protection, overload protection, short circuit protection, etc. Multiple protections make the product performance safer and more stable, and the product itself has a longer service life, while also avoiding unstable output from causing damage to the mobile phone; the MCU can also prevent power loss when the output is not charged. It solves the user's worries; it automatically recognizes mobile phones and a variety of digital products, supports charging of smartphones and various tablet computers of various brands and is compatible with charging other digital electronic products with USB 5V input.

3. SMD boost IC
The voltage of the mobile power battery is 3.7V, and the output voltage is 5.0V. The power needs to be output through a booster circuit. In the process of boosting, part of the power is lost due to heat on the circuit, so that there is a certain difference between the actual output power and the power output of the battery. The ratio of the two is called the conversion rate of the mobile power supply. At present, the conversion efficiency of domestic technology varies, generally between 75-85%. Some powerful manufacturers adopt higher-cost solutions and independently research and develop circuit designs. The actual conversion rate can reach more than 90%. Of course, with the development of technology, This conversion rate will get higher and higher. There are also two batteries connected in series to 8.4V and then adopt a step-down method. The efficiency can reach about 95%, but the consistency of the battery core is high, and the safety is relatively low. If it fails, it is easy to burn the user's mobile phone and other digital devices. Product, so few manufacturers use it.

Mobile power system vendors are accelerating the introduction of high-current direct current to direct current (DC-DC) converters. Aiming at the huge mobile power market business opportunities, power IC suppliers such as Zha Energy, Yuanxiang, O2Micro, MPS, Richtek, and Zhixin have begun to develop or launch high-current DC-DC converters to help system manufacturers develop charging speeds Faster mobile power products.