The charging principle of lithium-ion batteries: lithium and appropriate charging voltage and current

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Starting from the principle of the charging circuit of lithium batteries, this article introduces the design of the charging circuit of the charging pool on the basis of a deep understanding of the principle of lithium batteries, and then analyzes in detail how to choose the appropriate charging voltage and charging current. It is hoped that this can deepen everyone's understanding of the basic knowledge of lithium batteries that cannot be avoided in daily life.

Principle of lithium battery charging circuit

1、 Lithium batteries and nickel cadmium, nickel hydrogen rechargeable batteries:

The negative electrode of a lithium-ion battery is graphite crystal, and the positive electrode is usually lithium dioxide. During charging, lithium ions move from the positive electrode to the negative electrode and are embedded in the graphite layer. During discharge, lithium ions detach from the negative electrode surface inside the graphite crystal and move towards the positive electrode. So, during the charging and discharging process of the battery, lithium always appears in the form of lithium ions, rather than in the form of metallic lithium. Therefore, this type of battery is called a lithium-ion battery, abbreviated as a lithium battery.

Lithium batteries have advantages such as small size, large capacity, light weight, no pollution, high single cell voltage, low self discharge rate, and multiple battery cycles, but they are relatively expensive. Nickel cadmium batteries are gradually being phased out due to their low capacity, severe self discharge, and environmental pollution. Nickel hydrogen batteries have a high cost performance ratio and do not pollute the environment, but their individual voltage is only 1.2V, which limits their range of use.

2、 The characteristics of lithium batteries:

1. Has a higher weight to energy ratio and volume to energy ratio;

2. The voltage is high, with a single lithium battery voltage of 3.6V, which is equal to the series voltage of three nickel cadmium or nickel hydrogen rechargeable batteries;

3. The most prominent advantage of this battery is that it can be stored for a long time due to its small self discharge capacity;

4. No memory effect. Lithium batteries do not have the so-called memory effect of nickel cadmium batteries, so they do not need to be discharged before charging;

5. Long lifespan. Under normal working conditions, the number of charging/discharging cycles for lithium batteries is much greater than 500;

6. It can be charged quickly. Lithium batteries can usually be charged with a current of 0.5-1 times the capacity, reducing the charging time to 1-2 hours;

7. Can be used in parallel at will;

8. Due to the absence of heavy metal elements such as cadmium, lead, and mercury in batteries, they are non polluting to the environment and are the most advanced green batteries of our time;

9. High cost. Compared to other rechargeable batteries, lithium batteries are more expensive.

3、 Internal structure of lithium batteries:

Lithium batteries typically come in two shapes: cylindrical and rectangular.

The internal structure of the battery adopts a spiral winding structure, which is separated by a very fine and highly permeable polyethylene film isolation material between the positive and negative electrodes. The positive electrode includes a lithium ion collector composed of lithium and cobalt dioxide, and a current collector composed of aluminum film. The negative electrode is composed of a lithium ion collector made of sheet-like carbon material and a current collector made of copper thin film. The battery is filled with an organic electrolyte solution. In addition, it is equipped with safety valves and PTC components to protect the battery from damage in abnormal states and output short circuits.

The voltage of a single lithium battery is 3.6V, and its capacity cannot be infinitely large. Therefore, single lithium batteries are often processed in series or parallel to meet the requirements of different occasions. String 5

4、 Charging and discharging requirements for lithium batteries;

1. Charging of lithium batteries: According to the structural characteristics of lithium batteries, the maximum charging termination voltage should be 4.2V, and overcharging is not allowed, otherwise the battery will be scrapped due to the removal of too much lithium ions from the positive electrode. Its charging and discharging requirements are high, and dedicated constant current and constant voltage chargers can be used for charging. Usually, constant current charging is switched to constant voltage charging after reaching 4.2V/kWh. When the constant voltage charging current drops to within 100mA, charging should be stopped.

Charging current (mA)=0.1~1.5 times the battery capacity (such as a 1350mAh battery, its charging current can be controlled between 135~2025mA). The conventional charging current can be selected at around 0.5 times the battery capacity, and the charging time is about 2-3 hours.

2. Discharge of lithium batteries: Due to the internal structure of lithium batteries, during discharge, lithium ions cannot all move towards the positive electrode, and a portion of lithium ions must be retained at the negative electrode to ensure smooth insertion of lithium ions into the channel during the next charge. Otherwise, the battery life will be correspondingly shortened. In order to ensure that there are some lithium ions left in the graphite layer after discharge, it is necessary to strictly limit the minimum voltage for discharge termination, which means that lithium batteries cannot be over discharged. The discharge termination voltage is usually 3.0V/knot, and the minimum cannot be lower than 2.5V/knot. The length of battery discharge time is related to battery capacity and discharge current. Battery discharge time (hours)=battery capacity/discharge current. The discharge current (mA) of lithium batteries should not exceed three times the battery capacity. If a 1000mAH battery is used, the discharge current should be strictly controlled within 3A, otherwise the battery may be damaged.

At present, all lithium battery packs sold on the market are equipped with matching charging and discharging protection plates inside. As long as the external charging and discharging current is well controlled.

5、 Protection circuit for lithium batteries:

The charging and discharging protection circuit of two lithium batteries is shown in Figure 1. Composed of two field-effect transistors and a dedicated protection integrated block S-8232, the overcharge control transistor FET2 and the overcharge control transistor FET1 are connected in series in the circuit. The protection IC monitors the battery voltage and controls it. When the battery voltage rises to 4.2V, the overcharge protection transistor FET1 stops charging. To prevent misoperation, a delay capacitor is generally added to the external circuit. When the battery is in a discharge state and the battery voltage drops to 2.55V, the over discharge control tube FET1 cuts off and stops supplying power to the load. Overcurrent protection is to control FET1 to cut off and stop discharging to the load when there is a large current flowing through the load, in order to protect the battery and field-effect transistor. Overcurrent detection uses the conduction resistance of a field-effect transistor as the detection resistance, monitors its voltage drop, and stops discharging when the voltage drop exceeds the set value. In general, a delay circuit is added to the circuit to distinguish between surge current and short-circuit current. The circuit has complete functions and reliable performance, but it is highly professional and dedicated integrated blocks are not easy to purchase, making it difficult for amateur enthusiasts to imitate.

6、 Simple charging circuit:

Many businesses now sell single lithium batteries without charging pads. Its superior performance and low price can be used for the maintenance and replacement of self-made products and lithium battery packs, making it deeply loved by electronic enthusiasts. Interested readers can refer to Figure 2 to make a charging board. The principle is to use a constant voltage to charge the battery, ensuring that it will not overcharge. The input DC voltage should be 3 volts higher than the voltage of the charging pool. R1, Q1, W1, and TL431 form a precision adjustable voltage stabilizing circuit, Q2, W2, and R2 form an adjustable constant current circuit, and Q3, R3, R4, R5, and LED are charging indicator circuits. As the voltage of the charging pool increases, the charging current will gradually decrease. After the battery is fully charged, the voltage drop on R4 will decrease, causing Q3 to cut off and the LED to turn off. To ensure that the battery can be fully charged, please continue charging for 1-2 hours after the indicator light goes off. Please install suitable heat sinks for Q2 and Q3 when using. The advantages of this circuit are: simple production, easy purchase of components, safe charging, intuitive display, and no damage to the battery. By changing W1, multiple series connected lithium batteries can be charged, and changing W2 can adjust the charging current over a wide range. The disadvantage is that there is no over discharge control circuit. Figure 3 is the printed circuit board diagram of the charging board (perspective view from the component surface).

7、 Application examples of single lithium batteries

1. As a replacement for battery pack maintenance

There are many battery packs, such as those used on laptops. After repair, it was found that only a few batteries had problems when this battery pack was damaged. Suitable single lithium batteries can be selected for replacement.

2. Create a bright miniature flashlight

The author once used a single 3.6V1.6AH lithium battery combined with a white ultra-high brightness LED to create a miniature flashlight, which is convenient to use, compact and beautiful. Moreover, due to the large battery capacity, it is used for an average of half an hour every night and has been in use for over two months without the need for charging. The circuit is shown in Figure 4.

3. Replace 3V power supply

Due to the voltage of a single lithium battery being 3.6V. Therefore, only one lithium battery can replace two ordinary batteries to power small household appliances such as radios, walkthroughs, cameras, etc. It is not only lightweight, but also has a long continuous use time.

8、 Storage of lithium batteries:

Lithium batteries need to be fully charged before storage. Lithium batteries can be stored for more than half a year at 20 ℃, indicating that they are suitable for storage at low temperatures. Someone once suggested putting the rechargeable battery in the refrigerator for storage, which is indeed a good idea.

9、 Precautions for use:

Lithium batteries must not be disassembled, drilled, punctured, sawed, pressurized, or heated, otherwise it may cause serious consequences. Lithium batteries without charging protection boards should not be short circuited and should not be used for children to play with. Do not approach flammable or chemical substances. Scrapped lithium batteries should be properly disposed of. 4、 Charging and discharging requirements for lithium batteries;

1. Trickle charging stage. (In the state of battery transition discharge and low voltage)

Below 3.0V. The internal medium of lithium batteries may undergo some physical changes, leading to deterioration of charging characteristics and reduced capacity. At this stage, the lithium battery can only be charged slowly through a trickle, allowing the internal dielectric of the lithium battery to slowly return to its normal state.

2. Constant current charging stage. (The battery has returned to normal state from over discharge state)

An external pin of the IC is connected to an external resistor to determine. The resistance value is calculated based on the formula on the datasheet of the charging management IC.

3. Constant voltage charging stage (already filled with over 85%, gradually replenishing)

When the capacitance of the lithium battery reaches 85% (approximately), it must enter the slow charging stage again. Slowly increase the voltage. Finally, the maximum voltage of the lithium battery reached 4.2V.

The pin output of BAT is connected to the lithium battery end. At the same time, this pin is also a lithium battery voltage detection and measurement pin. The lithium battery charging management IC detects this pin to determine the various states of the battery.

A210 Power Supply Diagram

5V is sent to switch SW2 through D2, and to the lithium battery through charging management ICMCP73831. The voltage at the left point of SW2 is 5V-0.7V=4.3V. Due to the voltage of lithium batteries being 4.3V lower than the voltage at the left point of SW2, whether in a fully charged or non fully charged state. So D1 is the deadline. The charging management IC is charging the lithium battery normally.

If D2 and D1 are directly connected to the BAT pin output of the rear stage LDORT9193, it will cause misjudgment when the charging IC is powered on. A 5V external power supply may be connected, but the lithium battery will not charge, and the LED indicator of the charging management IC is also incorrect. The subsequent load LDO will not receive a normal input voltage (the input voltage is very small). In this case, as long as the voltage input pin of the charging management IC is directly short-circuit connected to the BAT pin, all states are normal again, charging can proceed, and the subsequent load LDO works normally.

At the moment of power on, the IC needs to detect the status of the BAT. Connecting the input pin of the LDO to the branch connecting the BAT and the positive pole of the lithium battery can affect the working status of the BAT pin, causing the charging management IC to enter the trickle charging stage. Short circuit the voltage input of the BAT pin and the charging management IC, forcing the voltage of the BAT pin to increase and causing the charging management IC to determine that the lithium battery has entered the constant current charging stage, thus outputting a large current. Capable of driving downstream loads such as LDO.

D1 and D2 should choose diodes with low voltage drop. Such as germanium diodes, Schottky diodes, MOSFET switching tubes. In designs that require battery switching, a diode with a 10mV forward voltage drop and no reverse leakage current is a luxury for designers. But so far, Schottky diodes are still the best choice, with a forward voltage drop between 300mV and 500mV. But for some battery switching circuits, even if Schottky diodes are selected, they cannot meet the design requirements. For an efficient voltage converter, the saved energy may be completely wasted by the forward voltage drop of the diode. In order to effectively preserve battery energy in low-voltage systems, power MOSFET switches should be chosen instead of diodes. MOSFETs packaged in SOT with a conduction resistance of only a few tens of milliohms can ignore their conduction voltage drop at the current level of portable products.

When switching power using MOSFETs, it is best to compare the diode conduction voltage drop, MOSFET conduction voltage drop, and battery voltage, and consider the ratio of voltage drop to battery voltage as efficiency loss. For example, if a Schottky diode with a forward voltage drop of 350mV is used to switch Li+batteries (nominal value 3.6V), the loss is 9.7%. If it is used to switch two AA batteries (nominal value 2.7V), the loss is 13%. In low-cost design, these losses may still be acceptable. However, when using high-efficiency DC-DC, it is necessary to balance the cost of DC-DC and the cost of efficiency improvement brought by upgrading diodes to MOSFETs.

When a lithium battery consumes 90% of its power at room temperature, the voltage will still remain around 3.5V. Choose a better LDO device. So at 3.5V, the output voltage will still remain stable at 3.3V

According to LDORT9193, when the load resistance is 50 ohms and the load current is 60mA, the relationship between input voltage and output voltage is shown in the table below:

2.8V2.65V

3.4v3.3V

4.0V3.0V

Even when the lithium battery consumes 90% of its power, the output terminal of LDO can still stably output 3.3V. From the analysis of the power supply circuit in Figure A210, with the addition of silicon diode D1, the input voltage of LDO is 3.5-0.7V=2.8V. Therefore, as long as the module burns a program that can work at around 2.4V, the silicon diode can also be used in this circuit.

From the perspective of circuit performance, using germanium diodes or Schottky diodes is the best choice.

The specific circuit design to be adopted also needs to be considered based on the working voltage range and characteristics of other circuits in one's own product, cost, and other aspects.

What is the most suitable current for charging lithium-ion batteries?

The charging of lithium-ion batteries requires constant current charging first, which means the current is constant, and the battery voltage gradually increases with the charging process. When the battery terminal voltage reaches 4.2V (4.1V), constant current charging is changed to constant voltage charging, which means the voltage is constant. The current gradually decreases according to the saturation degree of the battery cell as the charging process continues. When it decreases to 0.01C, it is considered that the charging is terminated. (C is a representation method that compares the nominal capacity of a battery with the current. For example, if the battery has a capacity of 1000mAh, 1C is the charging current of 1000mA. Note that it is mA instead of Ah, and 0.01C is 10mA.) Why is 0.01C considered as the end of charging? This is stipulated in the national standard GB/T18287-2000 and was also discussed. Previously, it was common to end with 20mA, and the industry standard YD/T998-1999 of the Ministry of Posts and Telecommunications also stipulated that regardless of the battery capacity, the stopping current is always 20mA. The 0.01C specified in the national standard helps to charge more fully, which is beneficial for the manufacturer to pass the appraisal. In addition, the national standard stipulates that the charging time should not exceed 8 hours, which means that even if the temperature has not yet reached 0.01C and 8 hours have passed, the charging is considered complete. The optimal charging rate for lithium-ion or lithium-polymer battery packs is 1C, which means that a 1000mAh battery pack needs to be charged quickly with a current of 1000mA. Charging at this rate can achieve the shortest charging time without compromising the performance or shortening the lifespan of the battery pack. For battery packs with increasing capacity, it is inevitable to increase the charging current value in order to achieve this satisfactory charging rate.

What is the most suitable voltage for charging lithium-ion batteries?

The nominal voltage of lithium-ion batteries is 3.7V (3.6V), and the charging cutoff voltage is 4.2V (4.1V, depending on the design of the battery cell brand). How to distinguish whether a battery is 4.1V or 4.2V: Consumers cannot distinguish, this depends on the product specifications of the battery cell manufacturer. Some brands of battery cells are universal for 4.1V and 4.2V, such as A&TB (Toshiba), while domestic manufacturers generally use 4.2V. Charging a 4.1V battery cell to 4.2V will increase the battery capacity, make it feel very useful, increase standby time, but shorten the battery's lifespan. For example, the original 500 times were reduced to 300 times. Similarly, overcharging a 4.2V battery cell can also shorten its lifespan. Lithium ion batteries are very delicate. Since there is a protective board inside the battery, can we rest assured? No, because the cut-off parameter of the protective board is 4.35V (which is still good, with a difference of 4.4 to 4.5V), the protective board is just in case. If it is overcharged every time, the battery will quickly decay.

What is the battery specification for Apple iPhone?

The battery specification of the Apple iPhone is a nominal voltage of 3.7V, a charging cutoff voltage of 4.2V, and a battery capacity of 1400mAh. As we mentioned above, the optimal charging rate is 1C, and a current of 1400mA is required to start charging at a voltage of 3.7V. After the voltage reaches 4.2V, constant voltage charging is started until it reaches 0.01C, which is 14mA and stops charging.

What are the voltage and current of the USB interface and charger, respectively?

The current of the USB interface is 500mA, and the voltage is+5V. If you turn on HWinfo during the charging process, you can also see that the ExternalPower feature is that the 500mA charger is designed for iPhone, and I think everyone understands what the white small square that can be plugged into the power socket is for. Don't you need me to say it again? When using a charger for charging, you can open HWinfo and see that the ExternalPower option is 1000mA. In summary, when you use USB to charge, the voltage is+5V and the current is only 500mA. As we can see from the answers to questions 1 and 2, this method will increase the battery capacity and be enjoyable to use, but it will shorten the battery life. When using a charger to charge, people may ask, didn't you say the best rate is 1C? That's right, the iPhone should have a charging current of 1400mA, that's right, but there is also a regulation in the country that the low rate charging according to the national standard is 0.2C (arbitration charging system). Taking the 1400mAh capacity battery of the iPhone as an example, it is 280mA. In theory, the smaller the charge, the more beneficial it is for the battery. But you can't wait for three days just to charge a battery. (Capacity mAh=Current mA x Time h) So Apple chose 0.7C, and most batteries are between 0.5C-0.8C. You can choose! It is obvious that some people use USB charging, which feels like it takes a long time, but that comes at the cost of sacrificing battery life.

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