Lithium ion battery charging principle and the appropriate charging voltage current

This paper starts from the charging circuit principle of lithium battery, introduces the design of charging circuit of rechargeable battery on the basis of a deep understanding of the principle of lithium battery, and then analyzes in detail how to choose the appropriate charging voltage and charging current, hoping to let everyone have a deep understanding of the basic knowledge of lithium battery that cannot be opened in daily life.

Lithium battery charging circuit principle

I. lithium battery and nickel-cadmium and nickel-metal hydride rechargeable batteries:

The negative polar graphite crystal of a lithium ion battery, and the positive electrode is usually lithium dioxide. During charging, lithium ions move from the positive pole to the negative pole and are embedded in the graphite layer. When discharging, the lithium ions detach from the negative electrode surface of the graphite crystal and move to the positive electrode. Therefore, during the charging and discharging process of the battery, lithium always appears in the form of lithium ion, rather than in the form of lithium metal. So this kind of battery is called lithium ion battery, lithium battery for short.

Lithium battery has the following advantages: small size, large capacity, light weight, pollution-free, high single-section voltage, low self-discharge rate, and more battery cycles, but the price is more expensive. Nickel-cadmium batteries are being phased out because of their low capacity, serious self-discharge and environmental pollution. Nickel-metal hydride battery has a high performance-to-price ratio and does not pollute the environment, but the single voltage is only 1.2v, so it is limited in the range of use.

Ii. Characteristics of lithium battery

1. Higher weight energy ratio and volume energy ratio;

2. High voltage: the voltage of a single lithium battery is 3.6v, which is equal to the series voltage of three nickel-cadmium or nickel-hydride rechargeable batteries;

3. Small self-discharge can be stored for a long time, which is the most prominent advantage of the battery;

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

5, long life. Under normal working conditions, the charge/discharge cycles of lithium battery are much more than 500 times.

6. It can be charged quickly. Lithium batteries can usually be charged by 0.5 ~ 1 times the capacity of the current, so that the charging time is shortened to 1 ~ 2 hours;

7. Can be used in parallel at will;

8. As the battery does not contain cadmium, lead, mercury and other heavy metal elements, it does not pollute the environment and is the most advanced green battery at present;

9. High cost. Lithium batteries are more expensive than other rechargeable batteries.

Iii. Internal structure of lithium battery:

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

The battery is made of a spiral winding structure, which is separated by a very fine and permeable polyethylene film separator between the positive and negative electrodes. The positive electrode includes the lithium ion collector composed of lithium and cobalt dioxide and the current collector composed of aluminum film. The negative electrode is composed of a lithium ion collector composed of a sheet carbon material and a current collector composed of a copper film. The battery is filled with an organic electrolyte solution. There is also a safety valve and PTC element to protect the battery from damage in abnormal conditions and output short circuit.

The voltage of a single lithium battery is 3.6v, and the capacity cannot be infinite. Therefore, a single lithium battery is often processed in series and parallel to meet the requirements of different occasions. String 5

Iv. Charging and discharging requirements of lithium battery;

1. Charging of lithium battery: according to the structural characteristics of lithium battery, the maximum charging termination voltage should be 4.2v, which cannot be overcharged. Otherwise, the battery will be scrapped due to too many lithium ions in the positive electrode. Its charge and discharge requirements are high, can be used for special constant current, constant voltage charger charging. Generally, constant current charging is transferred to constant voltage charging after it reaches 4.2v/node. When the constant voltage and charging current drop to less than 100mA, it should stop charging.

Charging current (mA) =0.1 ~ 1.5 times the capacity of the battery (for example, the charging current of 1350mAh battery can be controlled between 135 ~ 2025mA). The conventional charging current can be about 0.5 times of the battery capacity, and the charging time is about 2-3 hours.

2. Lithium battery discharge: due to the internal structure of lithium battery, lithium ions cannot all move to the positive pole when discharging, and some lithium ions must be kept in the negative pole to ensure that lithium ions can be smoothly embedded into the channel when charging next time. Otherwise, the battery life will be shortened accordingly. In order to ensure that some lithium ions remain in the graphite layer after discharge, it is necessary to strictly limit the minimum voltage of discharge termination, that is, lithium battery cannot be over-discharged. Discharge termination voltage is usually 3.0v/node, and the minimum voltage cannot be lower than 2.5v/node. The length of battery discharge time is related to battery capacity and discharge current. Battery discharge time (h) = battery capacity/discharge current. Lithium battery discharge current (mA) should not exceed 3 times the capacity of the battery. (if 1000mAH battery, discharge current should be strictly controlled within 3A) otherwise, the battery will be damaged.

At present, the lithium battery packs sold on the market are sealed with matching charge-discharge protection board. As long as the external charge and discharge current can be controlled.

V. protection circuit of lithium battery:

The charging and discharging protection circuit of two lithium batteries is shown in figure 1. It is composed of two field-effect tubes and dedicated protection chip s-8232. The over-charge control tube FET2 and over-discharge control tube FET1 are connected to the circuit in series, and the battery voltage is monitored and controlled by the protection IC. When the battery voltage rises to 4.2v, the over-charge protection tube FET1 will stop charging. In order to prevent misoperation, delay capacitance is usually added to the external circuit. When the battery is in the discharge state and the battery voltage drops to 2.55v, the over discharge control tube FET1 is cut off to stop the power supply to the load. Overcurrent protection controls FET1 to stop discharging to the load when there is a large current flowing through the load, in order to protect the battery and fet. The over-current detection USES the on-off resistance of the fet as the detection resistance, monitors its voltage drop, and stops discharging when the voltage drop exceeds the set value. Delay circuit is generally added in the circuit to distinguish inrush current from short circuit current. This circuit function is perfect, the performance is reliable, but the specialty is strong, and the specialized integrated circuit block is not easy to buy, the amateur is not easy to copy.

Vi. Simple charging circuit:

Many businesses now sell single lithium-ion batteries without charging pads. Its superior performance, low price, can be used for homemade products and lithium battery repair and replacement, so by the majority of electronics lovers love. Interested readers can refer to figure 2 to make a charging board. Its principle is: use constant voltage to charge the battery, ensure not overcharge. Input dc voltage is 3 volts higher than the rechargeable battery voltage. R1, Q1, W1, TL431 constitute a precise adjustable voltage regulator circuit, Q2, W2, R2 constitute an adjustable constant current circuit, Q3, R3, R4, R5, LED for charging indicator circuit. As the voltage of the rechargeable battery increases, the charging current will gradually decrease. After the battery is fully charged, the voltage drop on R4 will decrease, so that Q3 will be cut off and the LED will go out. To ensure the battery is sufficient, please continue charging for 1-2 hours after the indicator light goes out. Please install suitable radiator for Q2 and Q3 when using. The advantages of this circuit are: simple production, easy to purchase components, charging safety, intuitive display, and will not damage the battery. By changing W1, multiple serial lithium batteries can be charged, and by changing W2, charging current can be adjusted in a wide range. Disadvantages: no over discharge control circuit. Figure 3 is the printed board diagram of the charging board (perspective view from the component surface).

Application examples of single lithium battery

1. Replace and repair battery packs

There are many battery packs: that kind used in laptops, for example, has been found to be damaged only by repairs to individual batteries. Suitable single lithium battery can be selected for replacement.

2. Make a bright micro flashlight

The author has used a single 3.6v1.6ah lithium battery combined with a white ultra-bright luminescent tube to make a miniature flashlight, which is convenient to use, small and beautiful. Because of the battery capacity, the average use of half an hour a night, has been used for more than two months without charging. The circuit is shown in figure 4.

3. Replace 3V power supply

The voltage of a single lithium battery is 3.6v. Therefore, only one lithium battery can replace two ordinary batteries to power radio, walkman, camera and other small household appliances. It is not only light in weight, but also can be used continuously for a long time.

Viii. Preservation of lithium battery:

Lithium batteries should be fully charged before storage. Visible under 20 ℃ can be stored for more than six months, lithium battery is suitable for preserved in low temperature. It has been suggested that rechargeable batteries should be stored in the refrigerator, which is indeed a good idea.

Ix. Precautions for use:

Lithium battery absolutely cannot be disassembled, drilling, puncturing, sawing, pressure, heating, otherwise may cause serious consequences. Lithium batteries without charging pads should not be short-circuited and should not be used by children. Keep away from inflammable and chemical goods. Scrap lithium batteries should be disposed of properly. Iv. Charging and discharging requirements of lithium battery;

1. Charging of lithium battery: according to the structural characteristics of lithium battery, the maximum charging termination voltage should be 4.2v, which cannot be overcharged. Otherwise, the battery will be scrapped due to too many lithium ions in the positive electrode. Its charge and discharge requirements are high, can be used for special constant current, constant voltage charger charging. Generally, constant current charging is transferred to constant voltage charging after it reaches 4.2v/node. When the constant voltage charging current drops to less than 100mA, the charging should be stopped.

Charging current (mA) =0.1 ~ 1.5 times the capacity of the battery (for example, the charging current of 1350mAh battery can be controlled between 135 ~ 2025mA). The conventional charging current can be about 0.5 times of the battery capacity, and the charging time is about 2-3 hours.

2. Lithium battery discharge: due to the internal structure of lithium battery, lithium ions cannot all move to the positive pole when discharging, and some lithium ions must be kept in the negative pole to ensure that lithium ions can be smoothly embedded into the channel when charging next time. Otherwise, the battery life will be shortened accordingly. In order to ensure that some lithium ions remain in the graphite layer after discharge, it is necessary to strictly limit the minimum voltage of discharge termination, that is, lithium battery cannot be over-discharged. Discharge termination voltage is usually 3.0v/node, and the minimum voltage cannot be lower than 2.5v/node. The length of battery discharge time is related to battery capacity and discharge current. Battery discharge time (h) = battery capacity/discharge current. Lithium battery discharge current (mA) should not exceed 3 times the capacity of the battery. (if 1000mAH battery, discharge current should be strictly controlled within 3A) otherwise, the battery will be damaged.

At present, the lithium battery packs sold on the market are sealed with matching charge-discharge protection board. As long as the external charge and discharge current can be controlled. Lithium battery charging circuit design:

1. Trickling charging stage. (under the condition of excessive discharge of battery and low voltage)

Below 3.0 V. There will be some physical changes in the medium inside the lithium battery, resulting in bad charging characteristics and reduced capacity. At this stage, the lithium battery can only be charged slowly through a trickle of water, and the dielectric inside the lithium battery can slowly return to the normal state.

2. Constant current charging stage. (the battery is restored to normal state from over-discharge state)

A pin external to IC is connected with a resistor to determine. Resistance value is calculated according to the formula on datasheet of charging management IC.

3. Constant voltage charging stage (more than 85% full, slowly replenishing)

When the capacity of lithium battery reaches 85% (about the value), it must enter the slow charging stage again. Slowly increase the voltage. Finally, the maximum voltage of the lithium battery is 4.2v.

The pin output of BAT, which is connected to the lithium battery terminal. At the same time, this pin is also a lithium battery voltage collection pin. Lithium battery charging management IC detects this pin to judge each state of the battery.

A210 power supply diagram

5V is delivered to the lithium battery by D2 to the switch SW2 and by charging management ICMCP73831. The left point voltage of SW2 is 5V- 0.7v = 4.3v. Since the voltage of lithium battery is lower than 4.3v at the left point of SW2 at both full and non-full state. So D1 is cutoff. Charging management IC can charge lithium battery normally.

D2 and D1, LDORT9193 of the later stage are directly connected to BAT pin output, so it will be that when charging IC is energized, there will be misjudgment. There will be an external power supply connected to 5V, but the lithium battery will not be charged, and the LED light indication of charging management IC is also wrong. The LDO of the backstage load will not get the 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 connected to the BAT pin in a short circuit, all the states are normal again, the charging energy can be carried out, and the LDO of the backstage load works normally.

At the moment when IC is connected to power, it is necessary to detect the state of BAT. The input pin of LDO is also connected to the branch connecting BAT and lithium battery anode, which will affect the working state of BAT pin and cause the charging management IC to enter the trickling charging stage. Short circuit the BAT pin and the voltage input of the charging management IC to make the voltage of the BAT pin mandatory increase, which makes the charging management IC judge that the lithium battery has entered the constant current charging stage, so the large current is output. It is capable of driving LDO and so on at the back stage load.

D1 and D2 use diodes with low pressure drop. Such as germanium diode, schottky diode, MOSFET switching tube. In a design that requires battery switching, a diode with a 10mV forward voltage drop and no reverse leakage current is a "luxury" for designers. But schottky diodes are by far the best choice, with a positive voltage drop between 300 and 500mV. However, for some battery switching circuits, even the schottky diode cannot meet the design requirements. For an efficient voltage converter, the energy saved may be completely wasted by the positive voltage drop of the diode. In order to save battery energy effectively in low voltage system, power MOSFET switch should be selected instead of diode. The MOSFET with SOT encapsulation and on resistance of only dozens of milliohms can ignore its on voltage drop at the current level of portable products.

MOSFET to switch power supply, it is best to diode conduction voltage drop, MOSFET conduction voltage drop and battery voltage comparison, the ratio of voltage drop and battery voltage as efficiency loss. For example, if a schottky diode with a positive voltage drop of 350mV is used to switch Li+ batteries (the nominal value 3.6v), the loss is 9.7%; if it is used to switch two AA batteries (the nominal value 2.7v), the loss is 13%. These losses may be acceptable in a low-cost design. However, when using a high-efficiency dc-dc, the cost of dc-dc is weighed against the cost of the efficiency improvement of the diode upgrade to MOSFET.

MOSFET, but also take into account the discharge characteristics of the battery used in the product. The discharge characteristics of lithium battery are as follows:

When the lithium battery consumes 90% of the electricity at room temperature, the voltage will remain around 3.5v. Choose a better LDO device. At 3.5v, the output voltage will remain stable at 3.3v.

From the perspective of LDORT9193, when the load resistance is 50 ohm and the load current is 60mA, the relationship between the input voltage and the output voltage is shown in the following table:

2.8 V2.65 V

V3.3 3.4 V

4.0 V3.0 V

, even when the lithium battery consumes 90% of the electricity, the output end of LDO can still output 3.3v stably. According to the analysis of the power supply circuit of figure 1 A210, after adding silicon diode D1, the input voltage of LDO = 3.5-0.7v = 2.8v. In this way, as long as the module burns down the program that can work around 2.4v, silicon diode can also be used in this circuit.

In terms of circuit performance, germanium diode or schottky diode is the best choice.

What circuit design, but also need to be based on their own products other circuit operating voltage range and characteristics, costs and other aspects of consideration.

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

Lithium ion battery requires constant current charging first, namely must be current, and the battery voltage charging process gradually increases, when the battery voltage of 4.2 V, 4.1 V), constant voltage charging, instead of constant current charging for the voltage must be current depending on the degree of saturation batteries, as the charging process continue to gradually reduce, when reduced to 0.01 C, think charging termination. (C is a way of expressing the nominal capacity of the battery in comparison with the current. For example, the capacity of the battery is 1000mAh, 1C is the charging current 1000mA, note that it is mA instead of Ah, and 0.01C is 10mA.) Why 0.01C is considered as the end of charging: this is stipulated by the national standard GB/ t18287-2000, and it is also discussed. In the past, people generally ended up with 20mA, which is also stipulated in the industry standard YD/ t998-1999 of the ministry of posts and telecommunications. That is, regardless of the battery capacity, the stop current is 20mA. The national standard of 0.01C is conducive to charging more fully, which is favorable for the manufacturer to pass the appraisal. In addition, the national standard stipulates that the charging time shall not exceed 8 hours, that is to say, even if it has not reached 0.01C, 8 hours is considered as the end of charging. (quality no problem of the battery, should be within 8 hours of 0.01 C, battery quality is bad, also meaningless wait) lithium ion or lithium polymer battery pack the best charging rate of 1 C, which means that a 1000 mah battery have to be quick charge current of 1000 ma, charge at this rate can achieve the shortest charging time, and will not degrade the performance of the battery pack and shorten service life. For batteries with increasing capacity, to achieve this satisfactory charging rate, it is inevitable to increase the charging current value.

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

Lithium ion battery nominal voltage 3.7v (3.6v), charging cut-off voltage 4.2v (4.1v, according to the cell brand has different design) how to distinguish the battery is 4.1v or 4.2v: consumers are unable to distinguish, this depends on the cell manufacturer's product specifications. Some brands of cells are 4.1v and 4.2v universal, such as A&TB (Toshiba), domestic manufacturers are basically 4.2v. How about charging the 4.1v cell to 4.2v: it will increase the battery capacity, make it feel easy to use, increase the standby time, but reduce the service life of the battery. Let's say I go from 500 to 300. Similarly, if the 4.2v cell is overcharged, its life will be shortened. Lithium-ion cells are delicate. Since there is a protective plate in the battery, can we rest assured? No, because the cut-off parameter of the protective plate is 4.35v (which is still good, the poor one is 4.4-4.5v), the protective plate should be dealt with in case that the battery will soon decay if it is overcharged every time.

What is the battery specification of apple iPhone?

The battery specification of apple iPhone is the nominal voltage of 3.7v, the cut-off voltage of charging is 4.2v, and the battery capacity is 1400mAh. According to what we have said above, the optimal charging rate is 1C, and the current reaching 1400mA is required to start charging, and the voltage is 3.7v. After the voltage reaches 4.2v, constant voltage charging is started, until reaching 0.01c, that is, 14mA is stopped charging.

4. What is the voltage and current of usb interface and charger respectively?

The current of usb interface is 500mA, and the voltage is +5V. If you open HWinfo in the charging process, you can see ExternalPower. One item is 500mA charger designed for iPhone. In conclusion, when you use usb to charge, the voltage is +5V, while the current is only 500mA. We know that this method will increase the battery capacity by referring to the answer to questions 1 and 2, which is very cool to use but will shorten the battery life. When you use a charger to charge, you may ask, didn't you say the best rate is 1C? Then iPhone should be the charging current of 1400mA, yes, but the country has a regulation, the national standard of low power charging is 0.2c (arbitration charging system), also take the iPhone's 1400mAh capacity battery as an example, is 280mA, theoretically the smaller the battery the better. But you can't wait three days to recharge a battery. (capacity mAh = current mA * time h) so apple chose 0.7c, most batteries are between 0.5c and 0.8c 5 you can choose! It's clear that some people use usb for charging, which feels like a long time, but at the expense of battery life.