1. Basic knowledge of battery power monitoring
1.1 What is battery power monitoring technology
What it means: Battery fuel gauge is a technology used to predict battery capacity under all system operating and idle conditions.
a.Battery capacity
-percentage
-Time until the battery is exhausted/full
-milliamp hours (mAh)
-Watt hour (Wh)
-Talk time, restricted time, etc.
b. Other data reflecting battery health and safety diagnosis can be obtained
-health status
-Full charging capacity
Battery power monitoring technology is mainly used to report the capacity of the battery. It can also generally provide the health status of the battery and the full charge capacity of the battery.
1.2. Overview
a. Basic knowledge of battery chemical composition
b. Traditional battery power monitoring method
-Based on voltage
-Coulomb counting
c. Impedance tracking technology and its advantages
1.3 Part 1: Basic knowledge of battery chemical composition
First of all, I will introduce to you some knowledge about battery chemical components related to battery power measurement.
[The picture shows: Lithium-ion battery discharge curve: optimal operating time]
The three pictures here are lithium-ion battery discharge curves measured under different conditions. Varying the shutdown voltage with discharge rate, temperature and aging provides the longest possible run time.
From these figures, we can first see that the voltage of the battery will drop quickly after 3.5V under low current conditions at room temperature. Although the lowest voltage that the system can support can be 3.0V or 3.3V, since it reaches 3.5V After that, the voltage will drop quickly. In order to avoid data loss caused by sudden shutdown or sudden interruption of the circuit for loading files, the customer's application system usually tends to set the reference point of the battery's minimum capacity of zero to 3.5V. If it is at low temperature or In the case of large current, or when the battery is very old, if 3.5V is still used as the reference point for zero power, the available capacity of the battery will be greatly reduced. As you can see from these curves, In the case of large current, basically the discharge curve is already close to 3.5V at the beginning, and it is similar in the case of aging or low temperature. Therefore, if 3.5V is fixed as the reference point of zero capacity, then Low temperature or high current conditions, or conditions approaching aging, will cause the reported capacity to decrease. To avoid this, the battery capacity needs to be adjusted based on temperature, discharge rate, and battery aging.
1.4 Battery chemical capacity Qmax
Regarding battery power monitoring technology, there is a relatively important concept, which is the chemical capacity Qmax of the battery. In this picture, the intersection point between the red curve and the abscissa of 3.0V corresponds to Qmax.
This curve is measured when the load current is 0.1C, because to measure Qmax, the load current must be small enough. In theory, Qmax refers to the capacity that can be released when the current approaches zero, but in actual situations In engineering technology, a very small current is used to measure Qmax. Here we use a current of 0.1C. So what is 0.1C?
The concept of C in battery power management refers to the discharge rate of the battery. 1C actually means that if the battery capacity is 2200mAh and the current is 2200mA, it is 1C, so conceptually it is the current required to completely discharge a battery within 1 hour. Therefore, the discharge current corresponding to a 2200mAh battery is 2200mA, and the discharge current corresponding to 0.5C is 1100mA.
The EDV mentioned in this picture refers to the lowest voltage that the system or battery itself can support.
1.5 Available capacityQuse
There is also a corresponding capacity which is available capacity. Because what I just talked about is the chemical capacity of the battery. The chemical capacity of the battery is the capacity measured when the current is very small. It is more determined by the characteristics of the battery itself. In actual use of the battery, not all of this capacity can be released. In actual use, due to a certain discharge current, the discharge curve will be lower than the open circuit voltage curve. You can see this Curve, due to the internal resistance of the battery, the actual discharge curve is the blue curve. The values corresponding to the blue curve and the red curve are obtained by Quse. Quse actually refers to the available capacity of the battery. In this curve We found that because the internal resistance of the battery moves the position of this curve downward, the discharge end voltage will be reached earlier during discharge, that is, the EDV will be reached earlier, so Quse is generally smaller than Qmax.
We can also see from this curve that the greater the current, the smaller Quse will be. In this curve, I*Rbat refers to the drop in battery terminal voltage due to the existence of internal resistance.
1.6 Battery resistance
The internal resistance of the battery has a very important impact on the monitoring of battery voltage. The basic formula can be used to express the impact of the battery's internal resistance on battery power monitoring:
V=Vocv-I*Rbat
In this formula, Vocv refers to the open circuit voltage of the battery, I refers to the charge and discharge current, Rbat refers to the internal resistance of the battery, and V refers to the terminal voltage of the battery.
The impedance of the battery is actually affected by many factors, including ambient temperature, battery capacity, and battery aging. It is a very complex function of these variables. Now it is very difficult to get the specific expression of this function, so in practice, the impedance is often obtained by actual measurement, that is, the impedance is obtained by using a difference meter. Then the internal resistance of this battery will usually double after 100 charges and discharges. This is an empirical value. The deviation between the same batch of batteries can be controlled well at about 10~15%. The deviation in the internal resistance of batteries produced by different battery manufacturers is often greater. Therefore, the battery is a variable whose deviation is difficult to control during production. The internal resistance of the battery is a very difficult variable to control, and it is also a very important variable.
1.7 State of Charge (SOC)
What I just talked about is SOC. SOC actually refers to the capacity percentage, which is the capacity percentage at the corner of the screen when people often use mobile phones or tablets. The meaning of capacity percentage is that the battery is in a certain state until it is empty. How much power is left. The English abbreviation is SOC, which is State of Charge, so it can also be directly translated into state of charge, because Charge refers to charge. So obviously the voltage percentage, or state of charge, of a fully charged battery is equal to 1; the voltage percentage of a completely empty battery is equal to 0. So the formula for voltage percentage.
SOC is equal to Q on this curve (the remaining capacity corresponding to state A) divided by the chemical capacity Qmax of the battery. A concept corresponding to the battery percentage is DOD. DOD refers to the depth of discharge, which is Depth Of Discharge in English. Obviously, when the charging percentage or capacity percentage is 1, the discharge depth should be 0; conversely, when the capacity percentage is 0, the discharge depth should be 1.
We will encounter the concept of DOD in many TI documents. DOD is actually a relative concept to SOC. They actually mean the same thing, which is how much power is left in the battery, or how much power the battery has from full capacity. The charging state indicates how much electricity has been discharged.
1.8 Impedance is related to temperature and DOD
Then the battery's impedance is greatly affected by temperature and capacity percentage, which can also be expressed by the depth of discharge just mentioned, which is DOD. We can see some basic trends from this curve. It can be seen from the figure that the greater the discharge percentage and the greater the discharge depth, the greater the internal resistance of the battery, because the ordinate on this curve refers to the internal resistance of the battery. Resistance, its unit is ohms; the abscissa refers to the discharge percentage, which is DOD. Curves of different colors represent data measured at different temperatures. Obviously, at the same temperature, the greater the discharge percentage, that is, the deeper the discharge, the greater the internal resistance of the battery. Then we can also see in this picture that under the same DOD, that is, the same capacity percentage, the lower the temperature, the greater the internal resistance of the battery. This is a basic concept, and this is a basic understanding that everyone should have about batteries.
1.9 Impedance and capacity change with aging
In addition to the internal resistance of the battery being related to temperature and capacity percentage, another factor that has a greater impact is the service life of the battery, that is, the aging degree of the battery. Generally, the chemical capacity of a battery will be reduced by 3~5% after 100 redischarges. This capacity reduction is not very significant, but its impedance change is more significant. After 100 recharges and discharges, the impedance can increase almost 1 times. . As you can see from these two pictures, the picture on the left is a picture where the 1st and 100th discharge curves are drawn together. From this picture, it can be seen that the increase in the number of discharges does not have an impact on capacity reduction. Very big.
However, the increase in discharge rate has a great impact on the internal resistance. The picture on the right refers to the relationship between the internal resistance of the battery and the increase in the number of discharges. There are many curves in it. The abscissa of this picture is the measurement of the internal resistance of the battery. The frequency used when resisting, the ordinate refers to the internal resistance of the battery. We can see from this picture that when the frequency is very low, the bottom curve is the curve measured for the first time at different frequencies, and the top curve is the 100th time measured at different frequencies. In the obtained battery internal resistance curve, the values at the intersection points of the ordinates of these two curves are basically 1 times different, so after 100 cycles, the battery's internal resistance has doubled. The abscissa of the internal resistance here is frequency, which means that when the frequency is very low, the change in internal resistance is significant as the number of cycles increases, but conversely, as the frequency increases, for example : When the change frequency of the test load increases to 1KHZ, the change in internal resistance is basically negligible. You can see that so many curves basically converge to the same point. So what kind of impedance actually has a big impact on our battery power monitoring?
It is the impedance under relatively low frequency or DC conditions, so we should look at the two intersection points between the curve on the right and the ordinate. From this intersection point, we can see the impact of the number of cycles on the DC internal resistance of the battery.
1.10 Impedance difference of new batteries
This graph shows the difference in impedance of a new battery. The process structure of the battery is stacked layer by layer or rolled up layer by layer, so from the outside, the positive and negative electrodes of the battery can be seen to have characteristics of capacitance, resistance, and inductance. So for the entire battery, if you want to measure its impedance, the impedance can be divided into real cloth and virtual cloth. In this picture, we use an alternating load to measure the internal resistance of the battery. The changing frequency of the battery is the load. The changing frequency of the current changes from 1KHZ to 1mHZ. We are often exposed to the concept of 1KHZ, which means that it changes 1000 times in 1 second; 1mHZ changes once in 1000 seconds. This changing frequency is quite slow, which means that the measurement is actually is a DC impedance.
In these two pictures, you can see that the DC impedance increases monotonically and linearly as the frequency decreases, but the AC impedance has a changing trend. It is small at first, then slowly becomes larger, and then again. It becomes smaller and finally becomes larger again. This is due to the combined influence of factors such as capacitance and inductance inside the battery. However, the DC impedance increases monotonically. As the frequency decreases, the DC impedance becomes larger and larger.
So for battery power monitoring technology, what we care about is the DC impedance at 1mHZ. From this picture we can see that at 1mHZ, the battery impedance deviation is still about 15%. This impedance deviation of about 15% will cause In the case of 1C current discharge, the difference between the terminal voltage and the open circuit voltage of the battery is 40mV at low temperature. If the algorithm you use is to judge the capacity based on the voltage, it will probably cause a capacity error of about 26%.
1.11 Remaining battery capacity (RM)
The following describes the remaining capacity of the battery. The remaining capacity refers to the battery capacity in the current state placed at EDV, which is the end discharge voltage. The current state A in the figure is discharging at a given current. When it is discharged to 3.0V, the corresponding remaining capacity is marked as RM1 in the figure. Then if in state A, a relatively large current is discharged,
At this time, the position of the curve will be a little lower than the open circuit voltage, and also a little lower than the position corresponding to RM1 of the small current discharge just now. Then the remaining capacity obtained at this time is RM2. The difference between the discharge curves corresponding to RM2 and RM1 is that the discharge current used is different. The greater the discharge current, the lower the position of the curve, and the smaller the remaining capacity. Therefore, the remaining capacity of the battery is related to the discharge rate. Different The remaining capacity of the battery is different under the current.
Some users will find that the capacity of the battery changes from less to more when the battery is being discharged during actual use, which they find incomprehensible. In fact, it can be explained here that this situation is caused by changes in the discharge current. When We see that the battery capacity changes from less to more. This is usually caused by the sudden decrease in the discharge current, because the capacity that the battery can discharge is different under different currents. When the discharge current becomes smaller, it can The released capacity can be increased.
1.12 Summary of battery chemical composition
We now briefly review the concepts just introduced.
Qmax refers to the chemical capacity of the battery. The value of this capacity has nothing to do with the load. It refers to the capacity that the battery can discharge under extremely small load current conditions, and its unit is usually expressed in mAh.
Quse refers to the available capacity of the battery. This capacity is related to the load. Under different loads, the available capacity of the battery is different. The greater the load current, the smaller the available capacity of the battery.
Why do Quse and Qmax cause such a difference? This is mainly due to the internal resistance of the battery and the load on the battery.
The electromotive force and terminal voltage directly produce a voltage drop. Another concept is the capacity percentage of the battery, or the state of charge. Its unit is %. This % is actually the remaining capacity of the battery divided by the chemical capacity of the battery. .
The remaining capacity is called RM, and the size of RM also depends on the load. The greater the load, the smaller the remaining capacity is in the same state.
Another concept is that the aging of the battery will affect the impedance and capacity of the battery. The effect of aging on impedance is obvious, and the impedance will double after 100 cycles. The impact of aging on capacity is not as obvious as impedance, but there will be a 3~5% drop after 100 cycles.
2. Traditional battery power monitoring method
2.1 Goal: Make full use of available battery capacity
The main purpose of battery power monitoring is to maximize the utilization of the battery's capacity. Generally speaking, it is difficult for us to utilize 100% of the battery's capacity. Why?
There are 2 factors here
First, when charging, the charging voltage is rarely exactly the full charging voltage of the battery. Usually, in order to prevent the battery from overshooting, the charging voltage error is biased downward. In other words, for a 4.2V battery, the charging voltage It may be 4.18V or 4.15V, so if charging is performed at this low charging voltage, the charged capacity may become smaller;
In addition, due to the inaccuracy of battery power monitoring, users may estimate the battery power more conservatively for safety and to prevent data loss due to sudden critical conditions. In other words, when the actual battery power has not reached 0%, he will report it in advance. 0%, let the system shut down in advance, which can at least avoid the user's data loss. Of course, the user experience feels that the battery capacity has become smaller, which is a shortcoming. The consequence of this is that the battery capacity cannot be fully utilized. The purpose of power monitoring technology is to maximize the monitoring of battery power, allowing users to maximize the use of the current battery capacity. This blue section actually refers to the effective capacity of the battery. Our technology is to The actual effective capacity is expanded upward or downward as much as possible.
2.2 Traditional battery pack side fuel monitor
The traditional battery pack power monitoring technology has such a framework structure. The power monitoring chip is generally placed inside the battery pack. In this picture, the part enclosed by the dotted line is the battery pack. This battery pack generally contains lithium. The battery cell is the battery electrical symbol seen here, and the fuel gauge represented by the red square. This is where the TI device is located.
There is also a protector that controls the MOS tube. This protector switches the MOS to protect the battery core when the battery is overcharged or overdischarged. Generally, a thermistor is placed in the battery pack to monitor the battery pack. Temperature, in addition to the system board of a mobile phone or tablet on the left, the main things related to the fuel gauge on this system board are the power management chip and the host's processor. The host's processor is usually connected through I2C or a single line. The HDQ bus is used to read the power information in the fuel gauge.
With this power information being known, it is decided how long it will take until the battery is completely discharged. When some users want to do certain things, they can be prompted to see whether the power is sufficient. This is a traditional solution, which means that the battery is completely discharged. For the solution of placing the fuel gauge inside the battery pack, TI's main devices in this area include the two main chips BQ27541 and BQ27545; we also subsequently launched the BQ27441, which is a relatively low-cost solution; we also have the BQ27741, which A solution that combines a fuel gauge and a protector; we also have BQ28z560, which is also a solution that combines a fuel gauge and a protector.
2.3 System side impedance tracking fuel meter
With the advancement of technology, or the introduction of TI's impedance tracking technology, there is now an application where the fuel monitor is placed on the main board side of the device, and there is only a protector and MOS tube on the battery side. There is a thermistor, and of course there is a battery inside. What are the benefits of this? The cost of the battery pack has been greatly reduced, and the supplier of the battery pack is easier to find because he moved the fuel gauge from the battery pack to the host side, so such a solution is now feasible. TI also provides this 2 scenarios supported. There is also BQ28z550. This solution is to put the fuel gauge on the system board of the portable device, so that there is no need to discharge the fuel gauge in the battery pack. This can reduce the cost of the battery pack and make it easier to find suppliers. This kind of TI The main fuel gauges include BQ27510 and BQ27520. BQ27441 can also be used in this situation, as well as BQ27425, BQ27421... and other chips.
2.4 What are the functions of the fuel gauge?
a. Communication between battery and user
b. Measurement:
- battery voltage
- Charge or discharge current
- temperature
c. Provide:
-Battery run time and remaining capacity
-Battery health information
- Overall battery power management (working mode)
What are the main functions of the fuel gauge?
The fuel gauge must first complete the communication between the system and the battery. To know how much power the battery has, the system needs to communicate with the bus between the fuel gauges. I just mentioned I2C and single-wire HDQ bus communication to get it. During the communication process , what information can the system mainly obtain?
The first is the measured analog information, such as battery voltage, battery charge and discharge current, and battery temperature. As a fuel gauge, these basic analog information are more important to provide battery capacity information, which is the remaining capacity of the battery, battery running time, and battery health information just mentioned. , and another is that the chip itself must be able to complete the change of working status, that is to say, it must change from normal working mode to low power consumption mode. What purpose does this change achieve? To achieve the purpose of saving power.
2.5 How to implement the power monitoring meter
How to implement power monitoring?
The first method is voltage-based power monitoring. The power percentage or capacity percentage is regarded as a function of the battery voltage. This is a formula obtained from experience. Of course, the expression of the function itself does not necessarily have to be , it only needs to obtain a table corresponding to the open circuit voltage and capacity percentage. The data between the various points of this table can be obtained using the method of difference complementation.
Another method is Coulomb counting. Coulomb counting is an energy obtained by integrating the current charged into or discharged from the battery. We can think of the battery as the fuel tank of our car. How much oil is filled in the tank and how much oil is released can calculate how much oil is left in it. This is also a relatively intuitive algorithm based on life experience.
The latest algorithm now is the impedance tracking algorithm. In fact, this algorithm is based on real-time measurement of the internal resistance of the battery to obtain the battery capacity. Its formula is the formula in the picture, which has been listed just now, that is, the terminal voltage V of the battery. It is equal to the open circuit voltage of the battery minus the current multiplied by the internal resistance of the battery. This current refers to the total current flowing into or out of the battery.
2.6 Voltage-based fuel gauge
Let's first introduce the voltage-based fuel gauge. This picture is the open circuit voltage curve of a battery. The basic idea of this method is that we use different grids to represent the battery capacity for different voltages, for example, at 4.2V It is represented by 4 grids, which is a full battery; when it is 3.8V, I may use 3 grids to represent the battery capacity, and when 3.6V, I use 2 grids; when 3.2V, I may use 1 grid to represent the battery capacity, that is to say, use Different grid numbers correspond to different battery voltages to represent battery capacity. This method has poor accuracy and is usually used in the earliest low-end cellular phones or early digital cameras. What's the problem with this approach?
That is to say, when the current fluctuates, it will jump up and down. For example, when I have a discharge current, or when the current is relatively large, you can see the red arrow during the discharge process. If the current If it suddenly decreases at this point, or I suddenly become 0, the voltage will obviously increase. When the voltage reaches this point, its grid number will become 2 grids, and when it goes down again This change will be more obvious. When you go down, the battery grid number may be close to 0 grid or the red color will be used to indicate the battery grid number. At this time, the jump will change from red color to 2 grids. At this time, it will Jumping back and forth, if the current changes, for example, the call he just made stopped here, and the battery only has 2 bars left. He thought it still had power, and then suddenly another call came, and it suddenly became 0. Therefore, the error in this representation will be relatively large, because as you can see, 4 grids are actually used to represent the battery capacity, because 1 grid corresponds to 25% of the capacity, so skipping one grid will result in a 25% capacity difference. There is a 50% capacity difference in 2 cells, so the error of this method is relatively large. The reason for the relatively large error is that the battery has internal resistance. When the current is relatively large, its grid number will jump more.
2.7 Battery resistance
This is a formula for the open circuit voltage and terminal voltage of the battery. As mentioned just now, the internal resistance of the battery is a function of temperature, state of charge and battery aging; the internal resistance of the battery will double after 100 charges and discharges; the same batch of batteries The impedance deviation may be 10~15%; the internal resistance deviation of different battery manufacturers or manufacturers with poor quality will be greater.
2.8 Impedance is related to temperature and DOD
The information that has the greatest impact on capacity calculation or is the most difficult to obtain is I*Rbat. Of course, I is relatively easy to obtain. All you need to do is measure the current flowing in and out. With current technology, this can be measured with an accuracy of ±1mA. . Then this Rbat is relatively difficult to measure, because it needs to be calculated based on two quantities. This is the relationship between impedance, temperature and capacity percentage. This relationship has been mentioned just now. Basically, impedance increases with the decrease of temperature and increases with the decrease of capacity percentage. This is such a concept.
2.9 Impedance difference of new batteries
This is the deviation of impedance. What kind of concept is this? That is to say, generally speaking, the impedance used has a greater impact on the power measurement. The impedance refers to the impedance in the low frequency state, which is the impedance at 1mHZ. It is actually the DC impedance, rather than what we usually use in the market. The impedance measured by the internal resistance tester is the internal resistance of the battery measured at 1KHZ. Generally speaking, the internal resistance looks relatively small. The above are the three factors that have been introduced on the accuracy of capacity calculation, namely temperature, capacity percentage and aging degree. These will have an impact on the calculation of capacity. This impact refers to the impact of the method of using voltage to monitor power. In addition to the impact of these factors, if the voltage monitoring method is used, there is another impact that cannot be ignored, and this impact is also difficult to deal with. This is a headache for many power management engineers, that is, the battery has transient Response question.
2.10 Battery - Transient Response
You can see from these two pictures that the battery is discharging when it is fully charged. The previous curve represents a discharge process. At this time, the voltage is relatively low, and then the load is removed. At this time, the voltage of the battery It does not immediately return to the time when the current is 0, because everyone thinks that the current will become 0 when the load is removed. Has the voltage at this time returned to the voltage where the current is 0? No. It went back up slowly. It took a long time for it to go back up. You can see this curve. Everyone's daily experience can also prove this. That is to say, after a battery is discharged, and then you remove the load, its voltage is constantly changing. So how long does it take for this voltage change to become stable? Let's see. It takes about 1600 seconds to reach this point. It basically takes 3500 seconds to stabilize. It takes about 2000 seconds to stabilize. This is a discharge with a voltage of about 3.8V to 3.9V, which means that the battery Not yet full. According to what was just introduced, when the battery is relatively full, that is, when the battery capacity percentage ratio is large, the internal resistance of the battery is relatively small at this time. When the internal resistance of the battery is relatively small, it recovers fairly quickly. . In the picture below, you can see that the voltage here is relatively low. It starts to discharge from about 3.3V. After a period of time, the time is also very short, because when the voltage of the lithium battery is relatively low, it will be discharged for a while. The voltage is close to 3.2V, the lowest acceptable voltage for the system. If the discharge is stopped at this time, how long does it take for the voltage to go back up? It basically takes a longer time, such as more than 3000 seconds, to stabilize the voltage. , so during this period of time, its voltage is not stable enough, but there is no load, and the current is always 0. At this time, if you read the voltage, the voltage keeps changing. What is the corresponding capacity percentage? Errors will occur at this time.
2.11 Voltage relaxation and charge state errors
You can see that the difference in voltage between 20 and 3000 seconds can exceed 20mV, so the voltage value of 20mV can already cause a large capacity deviation when calculating capacity, especially in the flat area of voltage discharge. It can cause large capacity deviations, so the transient response of the battery will cause relatively large errors in the measurement method that uses voltage monitoring.
In this curve, this curve inverts the discharge curve of the battery. The ordinate becomes the capacity percentage, and the abscissa is the battery voltage. What does this picture mean? That is to say, at this stage, the battery is actually in the middle stage. If you stretch this platform a little longer, you can see that the voltage change is relatively slow during this period, and the capacity change is relatively large. In other words, during this period you Voltage is used to monitor capacity, so a slight error in this voltage will cause a large error in capacity. The picture on the right refers to the error of the corresponding capacity under different voltages. You can see that at the middle point of the voltage, that is, the voltage of the discharge curve is flat, that is, about 3.7 to 3.8V. The corresponding error during this period is the largest, and the corresponding error during this period can reach 15%. This is the error caused by the voltage method to calculate the capacity.
Therefore, the errors based on voltage monitoring measurement are mainly caused by the following aspects. One is the relaxation error, which is the recovery time of the battery voltage after the load is removed. A typical value here is 20mV relaxation measurement. Error, the actual error will be much larger than this relaxation error. You can take a look after the battery is discharged. When the discharge is completed and the voltage stabilizes, their voltage error is actually very large.
There is also a 15% resistance error between batteries. As mentioned just now, if the same batch of batteries produced by the same supplier has better process control, the internal resistance deviation of these batteries may be 15%. This is still a relatively good situation, but if there are different suppliers, or the supplier's process control is poor, the resistance error between the batteries will be greater. In the picture on the left, we can see that the red one refers to the relaxation error caused by the transient effect of the battery. The light blue curve above is the error caused by the deviation between the individual impedances of the battery. These two Taken together, the total deviation can be about 15%. This is 15% for new batteries, and it is a test result obtained when the current is controlled well.
2.12 SOC error of voltage-based power monitoring
As we all know, another major influencing factor for battery capacity calculation is the battery's service life. In this picture, the errors measured under different service years are shown in red for the 1st or 0th time. The error curve obtained during the period, this curve is about 15% of our icon, and the last mark here is 15%. So after 100 times, we know that the impedance has actually doubled. You can also see the picture just now. After 100 cycles, the internal resistance of the battery has doubled. According to this rule, the error will become larger and larger. The error caused by the impedance error on the capacity will also become larger and larger accordingly, so basically after 300 cycles After the cycle, the error caused when the capacity is relatively low will be very large, 75% or more, so the power calculation technology based on voltage measurement can only be used in those situations where the requirements are not high, and its error is relatively large. Usually the battery in the early digital camera uses this method to calculate its capacity. The biggest impact on this capacity calculation is the internal resistance of the battery. The reason for the large change in the internal resistance of the battery is the manufacturing process of the battery. The process causes deviations in the internal resistance of the battery. Another reason is that the delay in battery use time causes the internal resistance of the battery to change greatly. It is difficult for engineers to know an accurate model for these changes. They can only estimate based on experience. This estimate There will be a relatively large deviation between the results obtained and the actual results.
3. Voltage-based fuel gauge
3.1 Voltage-based fuel gauge
a. Advantages
- Learn without completely discharging
- Self-discharge does not require correction
- Very accurate at small load current conditions
b. Disadvantages
- Poor accuracy due to internal battery impedance
- Impedance is a function of temperature, aging and charge state
To summarize, the disadvantage of voltage-based fuel gauges is poor accuracy due to the internal impedance of the battery. There is a functional relationship between impedance, temperature, aging state and battery capacity percentage. This functional relationship is quite complex. It takes a professional battery person to find a relatively approximate functional relationship. It is difficult to find an accurate functional relationship. Therefore, this model is quite complex. It is difficult for ordinary electronic engineers or software engineers to write very precise relationships. Therefore, the calculation of capacity in software calculations is one of the biggest headaches for engineers. So it also has some The advantage, the advantage is that it can get the current capacity of the battery without completely discharging it.
Because anyone who has done battery or battery pack production, or has used a fuel gauge knows that a fuel gauge generally needs to be fully charged and discharged before leaving the factory. Why do we need to charge and discharge? This is to determine the current battery capacity and the full charge capacity of the battery, especially the full charge capacity of the battery. There are differences in the full charge capacity of different batteries. Of course, you can choose a battery design capacity, but different The deviation between the battery and the designed capacity is still relatively large. To obtain this full charge capacity, a complete charge and discharge is required. Then the specific charge and discharge have higher requirements on the production process. In addition, Lots of direct costs.
In addition, batteries have self-discharge characteristics. If the battery is placed there, even if the load is not working, the battery itself will leak electricity. Over time, the power will become less and less, and the voltage will become lower and lower. Then the voltage monitoring fuel meter only needs to judge the capacity based on the voltage, so now The capacity is reported as much as the voltage, so you don’t have to worry too much about the self-discharge. Therefore, this voltage-based fuel meter can still achieve a certain accuracy if the current is very small. However, currently, various Applications are becoming more and more complex, and current changes are becoming larger and larger, so it is a bit difficult for voltage-based fuel gauges to meet customer requirements.
3.2 Power monitoring based on Coulomb counting
In addition to voltage-based fuel gauges, another type of fuel gauge is Coulomb counting fuel monitoring technology.
The idea of this fuel monitor is to first charge a battery to full. During the charging process, you can know the current capacity of the battery, which is the full charge capacity of the battery. Then during the discharge process, the discharge capacity of the battery is calculated from the existing capacity. By deducting it, you can get how much capacity is left in the battery. The idea is actually to calculate the integral of current versus time to get how much capacity is released, and thus how much capacity is left in the battery.
With this technology, there will be a record of the discharged capacity at the end of each discharge. The recorded capacity is regarded as the full charge capacity of the battery, so Qmax will be updated at the end of each discharge, which is the chemical capacity and maximum capacity of the battery. Capacity will be updated.
3.3 Learning before full discharge
This is theoretically true, but in practice when updating the full charge capacity or chemical capacity of the battery, it is not necessarily necessary to completely drain the battery before updating, because the battery voltage will be very low at this time, and the system may shut down or have a problem. No matter what happens, it is already too late at this time. The usual update is to update when the battery capacity reaches about 7%. The idea of this update is that when the capacity reaches 7%, it means that 93% of the capacity has been released. If you integrate the capacity just released, the mAh number of the capacity will be released. Divide this mAh number by 93% to get the full charge capacity. This also achieves the learning effect, so generally learning will not be set to 0% I usually go to study at 7% of the time. As for learning, what you learn is the full charge capacity of the battery. After you have the full charge capacity, you can calculate the remaining capacity by integrating the discharge current. Therefore, the power of the fully charged battery is also important for the calculation of power. As for the voltage corresponding to 7% and 3%, it depends on the current, temperature and impedance at that time. Generally, when the room temperature current is constant and the impedance difference of the same batch of batteries is not too big, this voltage can also be considered to be basically constant, because it is 3.5V at 7%. At this time, the voltage deviation will not cause a large capacity deviation, so Correction can be made at 7%.
3.4 Compensated end-of-discharge voltage (CEDV)
The 7% point mentioned just now actually means that under a given temperature, current or the same batch of batteries, the voltage at this point is basically fixed, but in fact its current cannot be a fixed current. The current will always change during the process, so the voltage corresponding to 7% is also different, that is, the 7% corresponding to different currents is different.
In this curve, the discharge current is I1, and the voltage corresponding to I1 is 3.5V in this curve, represented by CEDV2. CEDV2 is a function of I1. If the current changes, it is also corrected with 7% of the voltage. , this error is large. It can be seen from the CEDV curve that the voltage corresponding to 7% actually has 30% remaining capacity. If synchronization or learning is performed according to 7% to correct the full charge capacity, there will be 23% The capacity is lost, so a big error is caused at this time, so this algorithm needs to correct the voltage at the 7% point based on the current. The voltage at the 7% point is called CEDV2. Find the voltage at this point As a function of current, different voltages are obtained under different currents. So in the case of current I2, we get CEDV2. In fact, its voltage is a little lower than 3.5V. CEDV2(I2) is actually obtained based on complex calculations. Its formula is roughly like this: CEDV=OCV(T, SOC)-I*R(T,SOC), C of CEDV is compensated, EDV is the termination voltage, that is, the compensated termination voltage is actually equal to the open circuit voltage of the battery minus the voltage drop caused by the internal resistance. The key is that in this formula OCV (T, SOC) can find a function that matches the curve better, and this curve will not change much. However, the latter curve is a function of T and SOC, and it is difficult to find a function for this curve. matching function, so this formula is quite complex.
Due to the difference in internal resistance, the accuracy of this formula is limited during use. It does not mean that this formula is universal during the entire discharge process. Usually when we use this formula to calculate, below 7% It is calculated using this formula, which is a functional relationship such as R (T, SOC). This can be found in our datasheet. The applicable range is only the range after 7% or 12%. This is actually enough, because the correction index only needs to be corrected at about 7%, so the problem of its small applicable range is not a big problem. So before 12%, Coulomb calculation was still used for integration. The error caused by Coulomb calculation integration can be After 12%, compensation is obtained through voltage correction. This is the simple idea of the CEDV algorithm. This formula actually reflects a relationship between impedance, temperature and SOC. This relationship reflects the impedance of the battery. After the general parameters in this formula are determined, the relationship between impedance, temperature and capacity percentage is determined. In fact As the battery's service life increases, the internal resistance will definitely change, but this formula does not actually reflect the difference between the battery's internal resistance and its service life. This formula cannot reflect this difference. Of course, we will add additional Improvements have been made and some linear compensation has been added. This can be done in our chips with the same CEDV, such as TI's BQ3060, the earlier BQ2084, and BQ2085, which are done using the CEDV algorithm.
3.5 Battery management products-battery power monitoring-BQ3060
3.6 Power monitoring based on Coulomb counting
a. Advantages
-Not affected by voltage measurement distortion
-Accuracy is determined by current integrating hardware
-Monitoring error: 3-10% (depends on working conditions and usage)
b. Disadvantages
-Learning cycle is required to update Qmax: battery capacity decreases with aging, Qmax is less: 3-5% (100 charges)
-Without learning, the monitoring error will increase by 1% for every 10 charges, self-discharge must be modeled: not accurate
Main parameters related to aging: Impedance
What are the advantages of power monitoring based on coulomb counting?
Because it mainly calculates electricity based on current integration, the distortion of voltage measurement has a relatively small impact on it. How accurate the current is is determined by the circuit integration hardware. If you control the parameters of the entire CEDV algorithm relatively well, Well, the error can be controlled at 3% or even lower. If the parameters match the actual battery model, it may be larger. The total error is probably around 3~10%, depending on the working conditions and usage.
Its disadvantage is what I just mentioned, because it uses Coulomb counting, that is to say, how much electricity is charged in and how much electricity is released to calculate the capacity. The premise is that it needs to know the full charge capacity of the battery before it can calculate the remaining power in the battery. How much capacity. This full charge capacity generally needs to be updated before leaving the factory, because the deviation between the full charge capacity and the remaining capacity of the battery is relatively large, and the remaining capacity of the battery cannot be directly used as the full charge capacity, so a battery must be updated before leaving the factory. The full charge capacity of the battery is obtained by cycling the charge and discharge cycles. The fuel gauge itself does this by itself, but the cycle requires special tools on the production line, so this is time-consuming.
In addition, the capacity of the battery will also decrease with the increase of its service life. Of course, the decrease is not as significant as the impedance, but there will also be a 3~5% decrease after 100 charges and discharges. This decrease must be managed in a way. Compensation, why? Because in actual use, you will not be able to learn every time you discharge, because when our electrical equipment (mobile phone or laptop) is taken out to discharge, it may not necessarily go from fully charged to empty. , or put it below 93% for you to update the full charge capacity. Generally, it may be discharged to half, or the adapter is plugged in immediately after a slight discharge. In this case, the discharge will be very shallow, and it may not have a chance to discharge. Update the full charge capacity. If there is no update, the monitoring error will increase by 1% every 10 times of charging. If Qmax is not updated, the error will become larger and larger. Therefore, in actual use, if you use an old-fashioned As for the fuel gauge, if you have such experience, you may need to fully charge and discharge the laptop once a month after taking it out, so that it can continuously update the Qmax parameters inside, so that it can be done relatively accurately. Another one is to estimate the self-discharge of the battery. It is inaccurate because the voltage-based power monitoring technology just mentioned determines how much power the battery has and then checks the meter to see how much power is left. As for the battery It doesn't matter how much electricity is discharged internally.
If it is a coulomb meter, it does not judge the capacity mainly based on the voltage. It judges the capacity based on the charge and discharge of the current. The monitoring chip of the coulomb meter cannot detect the charge and discharge inside the battery because the coulomb meter cannot detect this current. The meter is connected to the outside of the battery and can only monitor the current flowing in and out of the battery. It cannot measure the current consumed inside the battery, so it can only use a simple model to estimate how much is discharged during each discharge. , so this result is not very accurate, and the delay in service life will also cause an increase in error, so a relatively large factor here is the aging of the battery. The coulomb meter is more limited in its method of dealing with aging, and aging causes One of the effects is that the capacity will decrease with aging, and the other is that the battery impedance will increase after aging. As mentioned just now, when the impedance of the battery increases, the calculation error of the battery's CEDV will also become larger, because in this formula, the impedance is only related to the temperature and capacity percentage, and the estimation of the capacity is added. This estimation is actually a linear one. , there is still a certain difference from the actual battery, so the error in the contribution of impedance to capacity will become larger and larger as the age of the battery increases. Therefore, the CEDV algorithm takes into account the correction of battery impedance to voltage, but it does not take into account the change of battery impedance over time, or it is relatively simple to consider.
Therefore, the traditional power monitoring method can use voltage monitoring to obtain a more accurate capacity when there is no load, and Coulomb counting can be used to obtain the capacity when there is a load, so these two methods are complementary. In fact, the chips available on the market basically combine these two methods.
3.7 Advantages for typical fuel gauges
3.8 Battery management products-battery power monitoring-BQ3060
3.9 Question test
Therefore, whether it is a voltage-based fuel gauge or a current integral-based fuel gauge, impedance has a greater impact on the calculation of capacity. The impact of this impedance on aging is based on a simple linear model. , or in other words, the early ones did not have this part of the aging effect, so because the model it is based on is relatively simple, the matching success with the battery is actually relatively poor, which means that the error caused will become more and more difficult as time goes by. Obviously, the factor that has the greatest impact on battery power calculation is actually the impedance of the battery. If the impedance of the battery can be obtained anytime and anywhere, then the capacity of our battery can be calculated more accurately.
4. Advantages of impedance tracking technology
Next we will introduce TI's power monitoring technology (we call it impedance tracking technology) and its advantages.
4.1 Current monitoring
-Voltage-based power monitoring: provides accurate monitoring under no-load conditions
-Coulomb counting based power monitoring: provides accurate monitoring under load conditions
-Integrate the advantages of voltage-based and current-based monitoring methods
-Real-time impedance measurement
-Uses open circuit voltage and impedance information to calculate remaining run time given average load conditions
As mentioned just now, the voltage-based power monitoring technology can provide more accurate power monitoring without load. The coulomb counting-based power monitoring can provide accurate power monitoring with load. Our impedance tracking technology actually integrates voltage and current. Advantages of method monitoring, why can it obtain the advantages of the two methods?
Because it measures the impedance of the battery in real time, it does not find a formula for the battery impedance and then compensates for some factors. It finds a method of measuring the impedance in real time. Because it is measured in real time, there is no need to follow the model to compensate for it. When the battery impedance is known, the open circuit voltage and impedance information can be used to estimate how long the system or battery can provide running time or how much capacity the system can provide for system operation at a given current. This formula is a little more detailed here, that is to say, the terminal voltage of the battery is equal to the open circuit voltage of the battery minus the voltage drop above the internal resistance. The voltage drop above the internal resistance is mainly caused by the internal resistance of the battery. The resistance is determined by three factors: temperature, capacity percentage, and age. However, if you want to use a formula to express this internal resistance, it will be quite complicated, and the effect is not ideal. Our approach is actually to measure the impedance in real time.
4.2 Comparison of OCV curves
What is the basic idea of impedance measurement? During the actual use of the battery voltage, the battery terminal voltage will change due to many circumstances. As mentioned just now, the battery terminal voltage may change with the size of the current. Of course, the battery terminal voltage will also change with the current. As the capacity percentage changes, at the same percentage and the same current, the terminal voltage of the battery may also be related to the temperature and the aging degree of the battery, but this is only a superficial phenomenon we see, in fact, it is more essential. In terms of the open circuit voltage curve inside the battery or the electromotive force of the battery, the impact of these external factors on it is not so obvious. Some common things can be found. Batteries produced by different manufacturers perform under given test conditions. For example, at the same temperature, the error of this curve is very small.
This curve is the open-circuit curve measured by combining the batteries made by 5 battery manufacturers. As you can see, these open-circuit curves are basically the same, so they are curves measured at the same temperature, because it is The open circuit voltage is not to mention the current. Of course, its measurement process is also quite cumbersome, because it needs to obtain the open circuit curve in a state where the current is approximately 0. Its testing process is still relatively cumbersome. On this curve we can It can be seen that this curve basically does not change with different manufacturers. There may be relatively large changes in impedance due to different production processes, but this open circuit voltage curve is basically the same for everyone. Most of them The voltage offset is less than 5mV. The SOC prediction error based on this voltage is generally within 1.5%, so once such a curve is found, the same curve can be used to calculate batteries made by different battery cell suppliers. The calculation of this curve can know the open circuit voltage of the battery, and can in turn find the capacity percentage of the battery. It is mainly such a curve. After knowing the battery capacitance percentage, knowing the battery's chemical capacity or full charge capacity, you can know how many mAh of power it has left, then you can calculate how long it will run, and the subsequent capacity percentage can be further calculated.
The picture below is an enlarged picture of the error. This error is a voltage error during the entire discharge process, including the influence of the measurement equipment. This error is between ±15mV. This error is probably between the error of the capacity and the calculation of SOC. The error is within ±1.5%. Why? Because the voltage error here is also related to the measurement accuracy of the instrument, after the measurement accuracy of the instrument is taken into account, the resulting capacity percentage error is within 1.5%.
4.3 How to measure OCV
5. Power monitoring
5.1 Power monitoring
The same goes for this picture. This picture is not of a new battery, but relatively speaking, of an older battery. It is also turned off at 3.5V and turned off when the capacity is 10mAh. In this case, its time is increased by about 58%.
This is the case at low temperature. The effect of time extension is more obvious at low temperature, because the internal resistance at low temperature increases greatly, and the time used here is extended by 121%.
Its load current changes even more under this test condition, in this case it can even be extended to 290%, why? Because if it is in this place at low temperature, it will shut down. This time is quite short. It will shut down not long after it is discharged at the beginning. So if impedance tracking technology is used, it can continue to be left for more than 80 minutes before shutting down, because It is enough to shut down as long as the capacity of 10mAh is retained, so this can extend the time a lot. I do not make a rigid indicator based on the voltage to decide whether to shut down. I decide whether to shut down based on the remaining capacity. In this case, I can only use impedance. Tracking technology can calculate how long it takes to shut down, so using impedance tracking technology can greatly improve the user experience. User experience is a very important factor in expanding sales and bringing competitive advantages among today's portable consumer electronics products.
Therefore, the advantage of impedance tracking technology is that, more specifically, it can be used in some places. What I just introduced are some of the more intuitive advantages that can be thought of. In fact, with the continuous improvement of algorithms in impedance tracking technology, the battery will change during use. The impedance changes caused by the continuous rise and fall of temperature are added to the estimation of the temperature model, thermal simulation is introduced to adjust the heating of the battery, and the changes in the load during use are also studied, that is, a user's usage habits For an electrical device, its current changes have a certain pattern. Then the impedance tracking chip will slowly learn this pattern during use and grasp the voltage drop caused by the load change. These voltage drops are actually Capacity calculation also needs to be considered, and these factors must also be considered. Of course, as mentioned before, impedance tracking calculates the impedance of the battery in real time. It does not need to use a model to estimate the aging battery. It is the measured impedance, so the impact of aging on it is relatively small. Due to its precise computing capacity, it can extend the use time to the maximum extent, as you can clearly see from the pictures just now. With the impedance tracking chip, our host system does not need any algorithm to calculate the battery capacity. It only needs to simply read the specified register to get the capacity. With impedance tracking technology, you can also conduct a thorough analysis of the battery, such as the aging of the battery, the health of the battery... and so on.
5.2 Get the meaning of used battery capacity
What is another advantage? With the use of impedance tracking technology, the battery capacity can be calculated more accurately. In the case of more accurate calculation of the battery capacity, this fuel gauge can actually bring you cost savings.
For batteries, the cost is usually around US$0.15 for a 100mAh capacity. For example, since the discharge termination voltage is reduced to obtain a larger battery capacity, the TV value here is the discharge termination voltage. Reducing it by 500mV can increase the capacity by approximately 5%. For example, if the original 3.5V is reduced by 3V, the capacity of 500mV It can be increased by 5%. For a 1500mAh battery, it actually saves about 5%. For a 75mAh battery, it saves you 0.1 US dollars.
Of course, this is only US$0.1, but as the battery ages, the capacity increased by reducing 500mV is not 5%, but 50%. In this case, the saving is about US$1. Of course, this is for a 1500mAh capacity. The battery capacity is getting bigger and bigger, because today's smart devices consume more and more power, so it will get bigger and bigger, so the money saved will be more and more.
5.3 Losses caused by inaccurate monitoring
Therefore, when designing a system, customers should not only consider the cost of a chip, but also consider how much cost it reduces by choosing a battery for you. Because of the impedance tracking technology, a more accurate power calculation chip, the selected The battery capacity can be more accurate, and there is no need to leave too much margin. In real life, there are many losses caused to customers due to inaccurate monitoring. For example, if the customer charges and discharges every day, 3 months will last about 90 days, and the battery will be charged 90 times, and the internal resistance of the battery will double. At this time, the battery will age 100 times after 100 times. There is no fuel gauge that uses impedance tracking. At that time, the impedance of the battery has doubled, which will cause errors in battery calculation. Because its original fuel meter calculates the battery based on a relatively small internal resistance, the actual internal resistance has already been If it is increased by 1 times, then the calculated internal resistance will inevitably have a relatively large error. What situation will it cause? It will tell you a relatively long running time, but the actual running time is much smaller than this, resulting in a sudden shutdown. This sudden shutdown has a great impact on the system. The sudden shutdown of our laptop may cause the system to crash. Then the user feels that the battery life is greatly shortened, and this shortening may not be caused by the aging of the battery.
The shelf life of the battery may be 1 to 2 years. If this place has only been used for 3 months, the system suddenly crashes. The customer may ask for a return, which will cause economic losses to the company. So this is an example of economic losses caused by incorrect fuel gauges.
5.4 Summary
a. For portable electronic products, accurate monitoring is as important to obtain long running time as reducing the power consumption of the design and using a strong battery
b. There are many types of fuel gauges available, which use different monitoring methods and different compromises.
So when it comes to portable battery products, an accurate meter is as important to achieving long run times as reducing the power consumption of the design and using a robust battery. Because you want a strong battery, you need more mAh, which actually increases the cost. If you use an accurate fuel gauge to calculate the available capacity of the system to the maximum extent, you can use a battery with a relatively low capacity, which can bring about cost savings.
There are many fuel meter solutions available, and they are generally based on a compromise between voltage monitoring or coulomb counting. Pure voltage monitoring or pure coulomb counting are rarely used. Our TI method is basically a power monitoring method that combines the best of both worlds.
Typesetting: Wu Gong, Hardware Engineer's Road to Success, article source: TI Document- SSZB130B
Download link:
https://e2echina.ti.com/support/archived-groups/c8df485b47/m/battery_management/11686?tisearch=e2e-sitesearch&keymatch=TVS#
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