Lithium battery overcharge, over discharge, short circuit protection circuit design

The circuit is mainly composed of lithium battery protection ASIC DW01, charge and discharge control MOSFET1 (containing two N-channel MOSFETs), etc. The single lithium battery is connected between B+ and B-, and the battery pack outputs voltage from P+ and P-. When charging, the charger output voltage is connected between P+ and P-, and the current flows from P+ to B+ and B- of the single battery, and then passes through the charge control MOSFET to P-. During the charging process, when the voltage of the single battery exceeds 4.35V, the OC pin of the ASIC DW01 outputs a signal to turn off the charge control MOSFET, and the lithium battery stops charging immediately, thereby preventing the lithium battery from being damaged due to overcharging. During the discharge process, when the voltage of the single cell drops to 2.30V, the OD pin of DW01 outputs a signal to turn off the discharge control MOSFET, and the lithium battery stops discharging immediately, thereby preventing the lithium battery from being damaged due to over-discharge. The CS pin of DW01 is a current detection pin. When the output is short-circuited, the on-state voltage drop of the charge and discharge control MOSFET increases sharply, and the CS pin voltage rises rapidly. The output signal of DW01 turns off the charge and discharge control MOSFET quickly, thereby achieving over-current or short-circuit protection.

What are the advantages of secondary lithium batteries?

1. High energy density

2. High operating voltage

3. No memory effect

4. Long cycle life

5. No pollution

6. Lightweight

7. Low self-discharge

What are the advantages of lithium polymer batteries?

1. No battery leakage problem, the battery does not contain liquid electrolyte, but uses colloidal solid.

2. Can be made into a thin battery: with a capacity of 3.6V400mAh, its thickness can be as thin as 0.5mm.

3. Batteries can be designed into a variety of shapes

4. The battery can be bent and deformed: the polymer battery can be bent up to about 900 degrees

5. Can be made into a single high voltage: Liquid electrolyte batteries can only achieve high voltage by connecting several batteries in series. Since polymer batteries do not contain liquid, they can be made into multiple layers in a single battery to achieve high voltage.

7. The capacity will be twice that of a lithium-ion battery of the same size

IEC stipulates that the standard cycle life test for lithium batteries is:

After the battery is discharged at 0.2C to 3.0V/piece

1. 1C constant current and constant voltage charge to 4.2V, cut-off current 20mA, leave for 1 hour, then discharge at 0.2C to 3.0V (one cycle)

After 500 cycles, the capacity should be above 60% of the initial capacity.

The national standard stipulates that the standard charge retention test for lithium batteries is (IEC has no relevant standards).

The battery is discharged to 3.0V at 0.2C at 25 degrees Celsius, then charged to 4.2V at 1C constant current and constant voltage, with a cut-off current of 10mA. After being stored at 20+_5 for 28 days, the discharge capacity is calculated by discharging at 0.2C to 2.75V.

What is the self-discharge of secondary batteries? What are the self-discharge rates of different types of batteries?

Self-discharge is also known as charge retention capacity, which refers to the ability of the battery to retain the amount of electricity stored in the open circuit under certain environmental conditions. Generally speaking, self-discharge is mainly affected by manufacturing process, materials, and storage conditions. Self-discharge is one of the main parameters for measuring battery performance. Generally speaking, the lower the battery storage temperature, the lower the self-discharge rate. However, it should also be noted that too low or too high a temperature may cause the battery to be damaged and unusable. BYD conventional batteries require a storage temperature range of -20~45. After the battery is fully charged and left open for a period of time, a certain degree of self-discharge is normal. The IEC standard stipulates that after nickel-cadmium and nickel-metal hydride batteries are fully charged, they are left open for 28 days at a temperature of 20 degrees and a humidity of 65%. The 0.2C discharge time is greater than 3 hours and 3 hours and 15 minutes respectively, which is up to standard.

Compared with other rechargeable battery systems, the self-discharge rate of solar cells containing liquid electrolytes is significantly lower, about 10%/month at 25°C.

What is the internal resistance of a battery and how to measure it?

The internal resistance of a battery refers to the resistance encountered by the current flowing through the battery when the battery is working. It is generally divided into AC internal resistance and DC internal resistance. Since the internal resistance of a rechargeable battery is very small, when measuring the DC internal resistance, the electrode capacity is polarized, resulting in polarized internal resistance, so its true value cannot be measured. However, measuring its AC internal resistance can eliminate the influence of polarized internal resistance and obtain the true internal value.

The AC internal resistance test method is: using the characteristic that the battery is equivalent to an active resistor, giving the battery a constant current of 1000HZ, 50mA, and performing a series of processing such as voltage sampling, rectification and filtering to accurately measure its resistance value.

What is the internal pressure of a battery? What is the normal internal pressure of a battery?

The internal pressure of the battery is the pressure formed by the gas generated during the charging and discharging process. It is mainly affected by the battery material manufacturing process, structure and other usage process factors. Generally, the internal pressure of the battery is maintained at a normal level. In the case of overcharging or over-discharging, the internal pressure of the battery may increase:

If the rate of the recombination reaction is lower than the rate of the decomposition reaction, the generated gas will not be consumed in time, which will cause the internal pressure of the battery to increase.

What is internal pressure testing?

Lithium battery internal pressure test: (UL standard)

Simulate the battery at an altitude of 15240m (low air pressure 11.6kPa) to check whether the battery leaks or bulges.

Specific steps: Charge the battery 1C with constant current and constant voltage to 4.2V, with a cut-off current of 10mA, and then store it in a low-pressure box with an air pressure of 11.6Kpa and a temperature of (20+_3) for 6 hours. The battery will not explode, catch fire, crack, or leak.

How does ambient temperature affect battery performance?

Among all environmental factors, temperature has the greatest impact on the battery's charge and discharge performance. The electrochemical reaction at the electrode/electrolyte interface is related to the ambient temperature, and the electrode/electrolyte interface is considered the heart of the battery. If the temperature drops, the reaction rate of the electrode also decreases. Assuming that the battery voltage remains constant and the discharge current decreases, the battery's power output will also decrease. If the temperature rises, the opposite is true, that is, the battery's output power will increase. Temperature also affects the transmission speed of the electrolyte. The transmission speed increases when the temperature rises, and slows down when the temperature drops. The battery's charge and discharge performance will also be affected. However, if the temperature is too high, exceeding 45, it will destroy the chemical balance in the battery and lead to side reactions.

What are the methods for controlling overcharging?

In order to prevent the battery from being overcharged, the charging endpoint needs to be controlled. When the battery is fully charged, there will be some special information available to determine whether the charging has reached the end point. Generally, there are six ways to prevent the battery from being overcharged:

1. Peak voltage control: Determine the end point of charging by detecting the peak voltage of the battery;

2. dT/dt control: Determine the end point of charging by detecting the rate of change of battery peak temperature;

3. T control: When the battery is fully charged, the difference between the temperature and the ambient temperature will reach the maximum;

4. -V control: When the battery is fully charged and reaches a peak voltage, the voltage will drop to a certain value

5. Timing control: Control the charging endpoint by setting a certain charging time. Generally, the time required to charge to 130% of the nominal capacity is set;

6. TCO control: Considering the safety and characteristics of the battery, charging at high temperature (except for high-temperature batteries) should be avoided. Therefore, charging should be stopped when the battery temperature rises by 60.

What is overcharging and how does it affect battery performance?

Overcharging refers to the act of continuing to charge a battery after it has been fully charged through a certain charging process.

Since the negative electrode capacity is higher than the positive electrode capacity during design, the gas generated by the positive electrode will pass through the diaphragm paper and compound with the cadmium generated by the negative electrode. Therefore, under normal circumstances, the internal pressure of the battery will not increase significantly. However, if the charging current is too large or the charging time is too long, the generated oxygen will not be consumed in time, which may cause the internal pressure to increase, the battery to deform, leak, and other undesirable phenomena. At the same time, its electrical performance will also be significantly reduced.

What is over-discharge and what effect does it have on battery performance?

When the battery has discharged all the electricity stored in it and the voltage reaches a certain value, continuing to discharge will cause over-discharge. The discharge cut-off voltage is usually determined based on the discharge current. 0.2C-2C discharge is generally set at 1.0V/cell, and 3C or above, such as 5C or 10C, is set at 0.8V/cell. Over-discharge of the battery may have catastrophic consequences for the battery, especially high-current over-discharge or repeated over-discharge. Generally speaking, over-discharge will increase the internal pressure of the battery, and the reversibility of the positive and negative active materials will be destroyed. Even if charged, it can only be partially restored, and the capacity will also be significantly attenuated.

What problems will occur if batteries of different capacities are used together?

If batteries of different capacities or new and old batteries are mixed together, leakage and zero voltage may occur. This is because during the charging process, the capacity difference causes some batteries to be overcharged and some batteries to be undercharged. During discharge, some batteries with high capacity are not fully discharged, while those with low capacity are over-discharged. This vicious cycle causes the battery to be damaged and leak or have low (zero) voltage.

What is battery explosion? How to prevent battery explosion? Any solid matter in the battery is instantly discharged and pushed to a distance of more than 25cm from the battery, which is called explosion. To determine whether the battery explodes or not, use the following conditions to test. Cover the experimental battery with a net, with the battery in the middle and 25cm away from any side of the net. The density of the net is 6-7 wires/cm, and the net wire is made of soft aluminum wire with a diameter of 0.25mm. If no solid part passes through the net in the experiment, it proves that the battery has not exploded.

Lithium battery series connection problem

Since the battery has to go through many processes from coating to finished products during the production process. Even after a strict testing procedure, the voltage, resistance and capacity of each power supply group are consistent, but after a period of use, there will be differences of one kind or another. Just like twins born to a mother, they may look exactly the same when they are just born, and it is difficult for the mother to tell them apart. However, as the two children continue to grow, there will be differences of one kind or another. The same is true for lithium-powered batteries. After a period of use, the overall voltage control method is difficult to apply to lithium-powered batteries. For example, a 36V battery stack must use 10 batteries in series. The overall charge control voltage is 42V, and the discharge control voltage is 26V. With the overall voltage control method, there may be no problems in the initial use stage due to the particularly good battery consistency. After a period of use, the internal resistance and voltage of the battery fluctuate, forming an inconsistent state (inconsistency is absolute, consistency is relative). At this time, the overall voltage control still cannot achieve its purpose. For example, when 10 batteries are discharged, the voltage of two of them is 2.8V, the voltage of four batteries is 3.2V, and the voltage of four batteries is 3.4V. Now the overall voltage is 32V. We let it continue to discharge until it reaches 26V. In this way, the voltage of the two 2.8V batteries is lower than 2.6V and is in an over-discharge state. Lithium batteries are scrapped after several over-discharges. On the contrary, charging with the overall voltage control charging method will also cause overcharging. For example, charging with the voltage state of the above 10 batteries at that time. When the overall voltage reaches 42V, the two 2.8V batteries are in a "hungry" state, and quickly absorb electricity, which will exceed 4.2V. Batteries that are overcharged and exceed 4.2V will not only be scrapped due to excessive voltage, but may even be dangerous. This is the characteristic of lithium power batteries.

The rated voltage of lithium-ion batteries is 3.6V (some products are 3.7V). The termination charging voltage when fully charged is related to the battery anode material: 4.2V for graphite anode material; 4.1V for coke anode material. The internal resistance of different anode materials is also different. The internal resistance of coke anode is slightly larger, and its discharge curve is also slightly different, as shown in Figure 1. They are generally called 4.1V lithium-ion batteries and 4.2V lithium-ion batteries. Most of the ones used now are 4.2V, and the termination discharge voltage of lithium-ion batteries is 2.5V~2.75V (the battery factory gives the operating voltage range or the termination discharge voltage, and the parameters are slightly different). Continuing to discharge below the termination discharge voltage is called over-discharge, which will damage the battery.

Portable electronic products use batteries as power sources. With the rapid development of portable products, the use of various batteries has increased significantly, and many new types of batteries have been developed. In addition to the high-performance alkaline batteries, rechargeable nickel-cadmium batteries, and nickel-metal hydride batteries that everyone is familiar with, there are also lithium batteries developed in recent years. This article mainly introduces the basic knowledge about lithium batteries. This includes its characteristics, main parameters, the meaning of models, application range, and precautions for use.

Lithium is a metal element with the chemical symbol Li (its English name is lithium). It is a silvery white, very soft, chemically active metal and the lightest metal. In addition to being used in the atomic energy industry, it can be used to make special alloys, special glass (fluorescent screen glass used on televisions) and lithium batteries. In lithium batteries, it is used as the anode of the battery.

Lithium batteries are also divided into two categories: non-rechargeable and rechargeable. Non-rechargeable batteries are called disposable batteries, which can only convert chemical energy into electrical energy once and cannot convert electrical energy back to chemical energy (or the conversion performance is extremely poor). Rechargeable batteries are called secondary batteries (also called storage batteries). They can convert electrical energy into chemical energy and store it, and then convert chemical energy into electrical energy when in use. It is reversible, such as the main characteristics of electrical energy and chemical energy lithium batteries.

Smart portable electronic products require small size and light weight, but the size and weight of batteries are often the largest and heaviest compared to other electronic components. For example, the "big brother" in the past was quite "big and bulky", while today's mobile phones are so light. The improvement of batteries has played an important role in this: in the past, it was nickel-cadmium batteries, and now it is lithium-ion batteries.

The biggest feature of lithium batteries is their high specific energy. What is specific energy? Specific energy refers to the energy per unit weight or unit volume. Specific energy is expressed in Wh/kg or Wh/L. Wh is the unit of energy, W is watt, h is hour; kg is kilogram (weight unit), L is liter (volume unit). Here is an example to illustrate: the rated voltage of a No. 5 nickel-cadmium battery is 1.2V, and its capacity is 800mAh, so its energy is 0.96Wh (1.2V×0.8Ah). The rated voltage of a No. 5 lithium-manganese dioxide battery of the same size is 3V, and its capacity is 1200mAh, so its energy is 3.6Wh. The volumes of these two batteries are the same, so the specific energy of a lithium-manganese dioxide battery is 375 times that of a nickel-cadmium battery!

One AA nickel-cadmium battery weighs about 23g, while one AA lithium-manganese dioxide battery weighs about 18g. One lithium-manganese dioxide battery is 3V, while two nickel-cadmium batteries are only 24V. Therefore, when lithium batteries are used, the number of batteries is small (making portable electronic products smaller and lighter), and the battery life is long.

In addition, lithium batteries have the advantages of stable discharge voltage, wide operating temperature range, low self-discharge rate, long storage life, no memory effect and no pollution.

The disadvantage of lithium batteries is that they are expensive, so they are not widely used at present. They are mainly used in handheld computers, PDAs, communication equipment, cameras, satellites, missiles, torpedoes, instruments, etc. With the development of technology, improvement of processes and increase in production volume, the price of lithium batteries will continue to decline and their application will become more common.

Non-rechargeable lithium battery

There are many types of non-rechargeable lithium batteries, the most commonly used ones are lithium-manganese dioxide batteries, lithium-thionyl chloride batteries, and lithium and other compound batteries. This article only introduces the first two most commonly used ones.

1. Lithium-manganese dioxide battery (Li MnO2)

Lithium-manganese dioxide battery is a disposable battery with lithium as anode, manganese dioxide as cathode, and organic electrolyte. The main features of this battery are high battery voltage, rated voltage of 3V (twice that of ordinary alkaline batteries); termination discharge voltage of 2V; large specific energy (see the example above); stable and reliable discharge voltage; good storage performance (storage time of more than 3 years), low self-discharge rate (annual self-discharge rate ≤2%); operating temperature range of -20℃～+60℃.

The battery can be made into different shapes to meet different requirements. It has rectangular, cylindrical and button shapes. The cylindrical ones also have different diameters and heights. Here are the main parameters of the more familiar 1# (size code D), 2# (size code C) and 5# (size code AA) batteries.

CR stands for cylindrical lithium-manganese dioxide battery; the first two digits represent the diameter of the battery, and the last three digits represent the height with one decimal. For example, CR14505 has a diameter of 14mm and a height of 505mm (this model is universal).

It should be pointed out here that the parameters of the same type of batteries produced by different factories may be slightly different. In addition, the standard discharge current value is relatively small, the actual discharge current can be greater than the standard discharge current, and the allowable discharge current for continuous discharge and pulse discharge is also different, and the battery factory provides relevant data. For example, the CR14505 produced by Lixing Power Supply Company gives a maximum continuous discharge current of 1000mA, and the maximum pulse discharge current can reach 2500mA.

Most of the lithium batteries used in cameras are lithium-manganese dioxide batteries. Here, the lithium-manganese dioxide batteries commonly used in cameras are listed in Table 2 for reference.

Button batteries are smaller in size, with a diameter of 12.5 to 24.5 mm and a height of 16 to 50 mm. Several commonly used button batteries are shown in Table 3.

CR is a cylindrical lithium-manganese dioxide battery. The first two digits of the last four digits are the diameter of the battery, and the last two digits are the height with a decimal point. For example, the diameter of CR1220 is 125mm (excluding the number after the decimal point) and its height is 20mm. This model representation method is internationally accepted.

This button battery is commonly used in clocks, calculators, electronic notebooks, cameras, hearing aids, electronic game consoles, IC cards, backup power supplies, etc.

2. Lithium-thionyl chloride battery (Li SOCl2)

Lithium-thionyl chloride battery has the highest specific energy, which can currently reach 500Wh/kg or 1000Wh/L. Its rated voltage is 3.6V, and it has an extremely flat 3.4V discharge characteristic when discharged with a medium current (it can discharge flatly within the 90% capacity range, maintaining little change). The battery can operate in the range of -40℃ to +85℃, but the capacity at -40℃ is about 50% of the capacity at room temperature. The self-discharge rate is low (annual self-discharge rate ≤1%) and the storage life is more than 10 years.

Let's compare the specific energy of 1# (size code D) nickel-cadmium battery and 1# lithium-thionyl chloride battery: the rated voltage of 1# nickel-cadmium battery is 1.2V and the capacity is 5000mAh; the rated voltage of 1# lithium-thionyl chloride is 3.6V and the capacity is 10000mAh. The specific energy of the latter is 6 times greater than that of the former!

Application Notes

The above two types of lithium batteries are disposable batteries and cannot be recharged (it is dangerous when charging!); the positive and negative poles of the battery cannot be short-circuited; excessive current cannot be discharged (discharge exceeding the maximum discharge current); when the battery is used to the termination discharge voltage, it should be removed from the electronic product in time; used batteries cannot be squeezed, incinerated or disassembled; and they cannot be used beyond the specified temperature range.

Since the voltage of lithium batteries is higher than that of ordinary batteries or nickel-cadmium batteries, be careful not to make mistakes when using them to avoid damaging the circuit. By being familiar with the CR and ER in the model number, you can know its type and rated voltage. When buying a new battery, be sure to buy it according to the original model, otherwise it will affect the performance of electronic products.