Lithium-ion battery charging system differences

At present, the problem of power supply for portable devices poses many challenges to system designers. The use of batteries as the main power source is becoming more and more popular. Therefore, system designers must design highly sophisticated systems to fully tap the full potential of batteries. Each application is different, but one thing is the same: the goal of making the most of a battery's capacity is directly related to how to properly charge a rechargeable battery. In order to design a suitable and reliable battery charging system, it is necessary to have a deep understanding of the battery charging characteristics and the application requirements. Each method has its own advantages and disadvantages, and the specific application and requirements are the key factors to decide which method is the most suitable.

Charging systems often receive little attention in design, especially in cost-sensitive applications. However, the quality of the charging system is critical to battery life and reliability. In this paper, the basic principle of lithium ion battery charging is discussed, and the linear charging solution and the switching solution based on single chip are discussed in detail. The linear charging management controller of Microchip MCP73843 and MCP73861, the single chip PIC16F684 and the MCP1630 pulse width modulator (PWM) are taken as examples for discussion.

Lithium ion charging

The rate of charge or discharge is usually expressed in terms of the capacity of the battery. This velocity is called the C velocity. C rate is equal to the charging or discharging current under specific conditions, which is defined as follows:

I = M x Cn

Among them:

I= charge or discharge current, A

M is equal to a multiple or fraction of C

C= value of rated capacity, Ah

N = hours (for C).

A battery discharging at a rate of 1 C will release its nominal capacity in one hour. For example, if the nominal capacity is 1000mAh, then the discharge rate of 1C corresponds to the discharge current of 1000mA, and the rate of C/10 corresponds to the discharge current of 100mA.

Normally, the manufacturer's standard battery capacity refers to the capacity when n=5, namely 5 hours of discharge. For example, the above battery can provide 5 hours of working time at 200mA constant current. In theory, the battery could provide one hour of working time at 1000mA constant current discharge. However, in fact, the operating time will be less than 1 hour due to the reduced efficiency when the large battery is discharged.

So how do you properly charge a lithium-ion battery? The most suitable charging process of lithium ion battery can be divided into four stages: trickle charging, constant current charging, constant voltage charging and termination of charging.

Charging termination method

There is no doubt that overcharging has always been a major concern for lithium-ion batteries. Accurate charging termination method is very important for a safe and reliable charging system.

Battery temperature monitoring

In general, the range of temperature in the lithium ion battery should be between 0 ℃ and 45 ℃. Charging the battery outside this range can cause the battery to overheat. During the charging cycle, increased pressure inside the battery also causes the battery to swell. Temperature is directly related to pressure. As the temperature rises, so does the pressure, which can lead to mechanical damage or material leaks inside the battery and, in severe cases, explosions. Charging a battery outside this temperature range can also impair its performance or shorten its life expectancy.

Usually, thermistors are used in the lithium-ion battery packs to accurately measure the battery temperature. The charger detects the resistance value of the thermistor. When the resistance value exceeds the specified working range, that is, the temperature exceeds the specified range, charging is prohibited.

Battery discharge current or reverse leakage current

In many applications, the charging system is still connected to the battery even if the input power does not exist. The charging system must ensure that the input power does not exist when the current drawn from the battery is very small. Maximum leakage currents should be less than a few microamps, usually less than one microampere.

Lithium ion charging -- an application example

A suitable charging management system can be developed by fully considering the above considerations.

Linear solution

Linear charging solutions are usually used when there is a well regulated input power supply. The advantages of linear solutions in such applications include ease of use, small size, and low cost. Due to the low efficiency of the linear charging solution, the most important factor affecting the design is the heat dissipation design. Heat dissipation design is the thermal resistance between input voltage, charging current and transfer transistor and ambient cooling air. The worst case scenario is when the device transitions from a trickle charge to a constant charge, in which case the transfer transistor must emit the maximum amount of heat energy and must be weighed against charging current, system size, cost, and cooling requirements.

For example, a single 1000mAh lithium-ion battery needs to be charged at a constant current of 0.5c or 1C using a 5V 5% input power supply. Figure 3 shows how Microchip's MCP73843 can be used to form a low-cost independent solution that requires only a very small number of external components to implement the required charging algorithm. MCP73843 perfectly combines high-precision constant-current charging, constant-voltage voltage regulation and automatic charging termination and other functions.

To further reduce the size, cost, and complexity of the linear solution, many external components can be integrated into the charging management controller. Advanced packaging provides greater integration at the expense of flexibility. Such packaging requires advanced production equipment and in many cases avoids rework. Charging current detection, transfer transistor and reverse discharge protection are usually integrated. In addition, this kind of charging management controller can achieve a certain thermal regulation function. The thermal regulation function can limit the charging current according to the temperature of the device core, so as to optimize the charging cycle time while ensuring the reliability of the device. The thermal regulation function greatly reduces the workload of heat dissipation design.

The total integrated linear solution based on MicrochipMCP73861 is shown in figure 4. MCP73861 contains all the functions of MCP73843, including current detection, transmission transistor, reverse discharge protection and battery temperature monitoring.

The entire charging cycle of MCP73843 at constant current charging rates of 1C and 0.5c is shown in figure 5. Charging at a rate of 0.5c rather than 1C, the charge ends about an hour later. In the process of rapid charging, the charging termination current will be reduced in proportion to the charging current. The result is a 36% increase in charging time with a 2% increase in battery capacity and a reduction in power loss. The charge termination current drops from 0.07C to 0.035C, which increases the final battery capacity from ~98% to ~100%. The system designer must weigh charging time, power loss, and available battery capacity.

Switching charging solution

Switching charging solutions are usually used for applications with wide input voltage fluctuations or large input/output voltage differences. In such applications, the advantages of switching solutions are reflected in the improvement of efficiency, while the disadvantages are complex system, relatively large size and high cost. For example applications need to use the car adapter with 0.5 C or 1 C constant current in a single section 2200 mah lithium ion battery, because the problems such as heat dissipation, linear solution implementation is very difficult, of course, also can be used to support linear solution of thermal regulation, but due to lower charging current charging cycle extended is unacceptable.

The first step in the successful design of a switching charging solution is to select the design configuration: step-down, step-up, liter/step-down, flyback, single-terminal primary inductance (SEPIC) or other form. Depending on input and output requirements and experience, you can quickly narrow the selection to two structures for this application: the step-down or the SEPIC. The advantage of the step-down converter is that only one inductor is required, while the disadvantage is that additional diodes are required for reverse discharge protection, high-end gate drive and current detection, and pulse input current (which can cause EMI). The advantages of the SEPIC topology are low gate drive and current detection, continuous input current, and dc isolation between input and output. The main disadvantages are the need for two inductors and an energy transmission capacitor.

MCP1630 is a high-speed pulse width modulator (PWM) that can be used with a single chip microcomputer. With a single chip microcomputer, MCP1630 can control the duty cycle of the power supply system and provide stable output voltage or current. PIC16F684 MCU can be used for output voltage or current stability, as well as switching frequency and maximum duty ratio adjustment. MCP1630 produces a duty cycle and provides fast overcurrent protection based on different external inputs. External signals include input oscillator, reference voltage, feedback voltage and current detection. The output signal is a square wave pulse. The charger USES the power structure of SEPIC. SCM provides a great deal of design flexibility. In addition, the MCU can communicate with the battery monitor (PS700 of Microchip) in the battery pack, thus greatly reducing the charging cycle time.

The entire charging cycle of the switching charging solution is shown in figure 6. By using a battery monitor in the charging system, the charging cycle can be greatly shortened, and the battery monitor will no longer be required to detect the voltage at both ends of the battery pack protection circuit and the contact resistance of the charging current.

conclusion

In current portable products, the correct implementation of battery charging requires careful design consideration. This paper discusses the linear and switching charging solutions for lithium ion batteries. The guiding principles and design considerations discussed in this paper are actually what all battery charging system designs need to consider.

Introduction to the

At present, the problem of power supply for portable devices poses many challenges to system designers. The use of batteries as the main power source is becoming more and more popular. Therefore, system designers must design highly sophisticated systems to fully tap the full potential of batteries. Each application is different, but one thing is the same: the goal of making the most of a battery's capacity is directly related to how to properly charge a rechargeable battery. In order to design a suitable and reliable battery charging system, it is necessary to have a deep understanding of the battery charging characteristics and the application requirements. Each method has its own advantages and disadvantages, and the specific application and requirements are the key factors to decide which method is the most suitable.

Charging systems often receive little attention in design, especially in cost-sensitive applications. However, the quality of the charging system is critical to battery life and reliability. In this paper, the basic principle of lithium ion battery charging is discussed, and the linear charging solution and the switching solution based on single chip are discussed in detail. The linear charging management controller of Microchip MCP73843 and MCP73861, the single chip PIC16F684 and the MCP1630 pulse width modulator (PWM) are taken as examples for discussion.

Lithium ion charging

The rate of charge or discharge is usually expressed in terms of the capacity of the battery. This velocity is called the C velocity. C rate is equal to the charging or discharging current under specific conditions, which is defined as follows:

I = M x Cn

Among them:

I= charge or discharge current, A

M is equal to a multiple or fraction of C

C= value of rated capacity, Ah

N = hours (for C).

A battery discharging at a rate of 1 C will release its nominal capacity in one hour. For example, if the nominal capacity is 1000mAh, then the discharge rate of 1C corresponds to the discharge current of 1000mA, and the rate of C/10 corresponds to the discharge current of 100mA.

Normally, the manufacturer's standard battery capacity refers to the capacity when n=5, namely 5 hours of discharge. For example, the above battery can provide 5 hours of working time at 200mA constant current. In theory, the battery could provide one hour of working time at 1000mA constant current discharge. However, in fact, the operating time will be less than 1 hour due to the reduced efficiency when the large battery is discharged.

So how do you properly charge a lithium-ion battery? The most suitable charging process of lithium ion battery can be divided into four stages: trickle charging, constant current charging, constant voltage charging and termination of charging.

Stage 1: trickle charge -- trickle charge is used to precharge the fully discharged battery unit (recovery charge). When the battery voltage is lower than about 3V, a constant current of up to 0.1c is used to charge the battery.

Stage 2: constant-current charging -- when the battery voltage rises above the trickle charging threshold, the charging current is increased for constant-current charging. The constant current charging current is between 0.2c and 1.0c. Constant current charging current is not required to be very accurate, quasi-constant current can also be. In linear charger designs, the current often rises with the battery voltage to minimize heat dissipation on the transfer transistor.

Constant current charging greater than 1C does not shorten the whole charging cycle time, so this approach is not advisable. When charged at a higher current, the battery voltage will rise more rapidly due to the overvoltage of the electrode reaction and the voltage rise on the internal impedance of the battery. The constant current charging stage will become shorter, but since the time of the constant voltage charging stage below will increase correspondingly, the total charging cycle time will not be shortened.

Stage 3: constant voltage charging -- when the battery voltage rises to 4.2v, the constant current charging ends and the constant voltage charging stage begins. For optimum performance, the voltage tolerance should be greater than +1%.

Phase 4: charge termination - unlike nickel batteries, continuous trickle charging of lithium-ion batteries is not recommended. Continuous trickle charge will lead to plate plating effect of lithium metal. This makes the battery unstable and can cause a sudden, automatic and rapid disassembly.

There are two typical charging termination methods: using a minimum charging current judgment or using a timer (or a combination of the two). The minimum current method monitors the charging current in the constant voltage charging stage, and terminates the charging when the charging current decreases to the range of 0.02C to 0.07C. The second method is timed from the beginning of the constant voltage charging stage and stops the charging process after two hours of continuous charging.

The above four-stage charging method takes about 2.5 to 3 hours to fully discharge the battery. Advanced chargers also have more security measures in place. For example, if the battery temperature exceeds the specified window (usually a 0 ℃ to 45 ℃), then charge will be suspended.

Lithium ion charging - system considerations

A high performance charging system is needed to complete the charging process quickly and reliably. To achieve a reliable and cost-effective solution, the following system parameters should be considered in the design:

The input source

Many applications use extremely cheap wall adapters as input power. Its output voltage is mainly dependent on the ac input voltage and the load current flowing from the wall adaptor.

The range of ac bus input voltage on standard wall sockets in the United States is generally from 90VRMS to 132VRMS. Suppose the rated input voltage is 120VRMS with a tolerance of +10%,? 25%. The charger must provide appropriate voltage regulation for the battery, so as not to be affected by the input voltage. Charger input voltage is proportional to ac bus voltage and charging current:

VO vin = 2 x (the REQ + RPTC) - a - 1 o 2 x VFDS

REQ is the sum of the resistance of the secondary winding and the reflection resistance of the primary winding (RP/a2). RPTC is the resistance of PTC and VFD is the forward voltage drop of the bridge rectifier. In addition, the transformer core loss will also make the output voltage slightly reduced.

Apps that use car adapters to charge their batteries run into similar problems. The output voltage of a car adapter typically ranges from 9V to 18V.

Constant current charging rate and accuracy

The topological choice for a particular application may be determined by the charging current. Many constant-current charging applications or multi-battery charging applications use switching charging solutions to achieve greater efficiency and avoid generating too much heat. For size and cost reasons, low - and mid-range rapid charging applications tend to use linear solutions, which lose more energy in the form of heat. For linear charging systems, the constant current charge tolerance becomes extremely important. If the voltage tolerance is too large, the transfer transistor and other components need to be larger, increasing size and cost. In addition, if the constant current charging current is too small, the entire charging cycle will be prolonged.

Output voltage stability accuracy

In order to make the most of the battery capacity, the output voltage stabilization accuracy is critical. A small drop in the accuracy of the output voltage can also result in a large reduction in battery capacity. However, for safety and reliability reasons, the output voltage cannot be set too high at will. Figure 2 shows the importance of output voltage stability accuracy.

Charging termination method

There is no doubt that overcharging has always been a major concern for lithium-ion batteries. Accurate charging termination method is very important for a safe and reliable charging system.

Battery temperature monitoring

In general, the range of temperature in the lithium ion battery should be between 0 ℃ and 45 ℃. Charging the battery outside this range can cause the battery to overheat. During the charging cycle, increased pressure inside the battery also causes the battery to swell. Temperature is directly related to pressure. As the temperature rises, so does the pressure, which can lead to mechanical damage or material leaks inside the battery and, in severe cases, explosions. Charging a battery outside this temperature range can also impair its performance or shorten its life expectancy.

Usually, thermistors are used in the lithium-ion battery packs to accurately measure the battery temperature. The charger detects the resistance value of the thermistor. When the resistance value exceeds the specified working range, that is, the temperature exceeds the specified range, charging is prohibited.

Battery discharge current or reverse leakage current

In many applications, the charging system is still connected to the battery even if the input power does not exist. The charging system must ensure that the input power does not exist when the current drawn from the battery is very small. Maximum leakage currents should be less than a few microamps, usually less than one microampere.

Lithium ion charging -- an application example

A suitable charging management system can be developed by fully considering the above considerations.

Linear solution

Linear charging solutions are usually used when there is a well regulated input power supply. The advantages of linear solutions in such applications include ease of use, small size, and low cost. Due to the low efficiency of the linear charging solution, the most important factor affecting the design is the heat dissipation design. Heat dissipation design is the thermal resistance between input voltage, charging current and transfer transistor and ambient cooling air. The worst case scenario is when the device transitions from a trickle charge to a constant charge, in which case the transfer transistor must emit the maximum amount of heat energy and must be weighed against charging current, system size, cost, and cooling requirements.

For example, a single 1000mAh lithium-ion battery needs to be charged at a constant current of 0.5c or 1C using a 5V 5% input power supply. Figure 3 shows how Microchip's MCP73843 can be used to form a low-cost independent solution that requires only a very small number of external components to implement the required charging algorithm. MCP73843 perfectly combines high-precision constant-current charging, constant-voltage voltage regulation and automatic charging termination and other functions.

To further reduce the size, cost, and complexity of the linear solution, many external components can be integrated into the charging management controller. Advanced packaging provides greater integration at the expense of flexibility. Such packaging requires advanced production equipment and in many cases avoids rework. Charging current detection, transfer transistor and reverse discharge protection are usually integrated. In addition, this kind of charging management controller can achieve a certain thermal regulation function. The thermal regulation function can limit the charging current according to the temperature of the device core, so as to optimize the charging cycle time while ensuring the reliability of the device. The thermal regulation function greatly reduces the workload of heat dissipation design.

The total integrated linear solution based on MicrochipMCP73861 is shown in figure 4. MCP73861 contains all the functions of MCP73843, including current detection, transmission transistor, reverse discharge protection and battery temperature monitoring.

Charging cycle waveform

The entire charging cycle of MCP73843 at constant current charging rates of 1C and 0.5c is shown in figure 5. Charging at a rate of 0.5c rather than 1C, the charge ends about an hour later. In the process of rapid charging, the charging termination current will be reduced in proportion to the charging current. The result is a 36% increase in charging time with a 2% increase in battery capacity and a reduction in power loss. The charge termination current drops from 0.07C to 0.035C, which increases the final battery capacity from ~98% to ~100%. The system designer must weigh charging time, power loss, and available battery capacity.

Switching charging solution

Switching charging solutions are usually used for applications with wide input voltage fluctuations or large input/output voltage differences. In such applications, the advantages of switching solutions are reflected in the improvement of efficiency, while the disadvantages are complex system, relatively large size and high cost. For example applications need to use the car adapter with 0.5 C or 1 C constant current in a single section 2200 mah lithium ion battery, because the problems such as heat dissipation, linear solution implementation is very difficult, of course, also can be used to support linear solution of thermal regulation, but due to lower charging current charging cycle extended is unacceptable.

The first step in the successful design of a switching charging solution is to select the design configuration: step-down, step-up, liter/step-down, flyback, single-terminal primary inductance (SEPIC) or other form. Depending on input and output requirements and experience, you can quickly narrow the selection to two structures for this application: the step-down or the SEPIC. The advantage of the step-down converter is that only one inductor is required, while the disadvantage is that additional diodes are required for reverse discharge protection, high-end gate drive and current detection, and pulse input current (which can cause EMI). The advantages of the SEPIC topology are low gate drive and current detection, continuous input current, and dc isolation between input and output. The main disadvantages are the need for two inductors and an energy transmission capacitor.

MCP1630 is a high-speed pulse width modulator (PWM) that can be used with a single chip microcomputer. With a single chip microcomputer, MCP1630 can control the duty cycle of the power supply system and provide stable output voltage or current. PIC16F684 MCU can be used for output voltage or current stability, as well as switching frequency and maximum duty ratio adjustment. MCP1630 produces a duty cycle and provides fast overcurrent protection based on different external inputs. External signals include input oscillator, reference voltage, feedback voltage and current detection. The output signal is a square wave pulse. The charger USES the power structure of SEPIC. SCM provides a great deal of design flexibility. In addition, the MCU can communicate with the battery monitor (PS700 of Microchip) in the battery pack, thus greatly reducing the charging cycle time.

Charging cycle waveform

The entire charging cycle of the switching charging solution is shown in figure 6. By using a battery monitor in the charging system, the charging cycle can be greatly shortened, and the battery monitor will no longer be required to detect the voltage at both ends of the battery pack protection circuit and the contact resistance of the charging current.

conclusion

In current portable products, the correct implementation of battery charging requires careful design consideration. This paper discusses the linear and switching charging solutions for lithium ion batteries. The guiding principles and design considerations discussed in this paper are actually what all battery charging system designs need to consider.

The page contains the contents of the machine translation.