A low-cost solution based on lithium-ion battery MP3 instead of A/D converter

The system designer only needs to set a new voltage detection threshold according to the current battery voltage range that he already knows. When the battery voltage changes to this threshold, the state of the RT9801B output terminal will change, so that the battery Power has reached the new threshold. Level, then change the status of the icon displayed on the screen to let the user know the current battery charge.

The first battery fuel gauges available are based on the Coulomb measurement method. The use of this measurement method has a very solid theoretical basis, because it is a very basic physical common sense to obtain the power by integrating the current. It’s normal, but there is a problem with this operation. You have to know where the zero point of the integral is and where the full point of the integral is, so that you can get an accurate measurement and know the battery energy storage status at each point in time. The proportion of the total capacity is the State of Charge (SoC, State of Charge). Since I know the principle of coulomb measurement, when I started using the first smart phone many years ago, I would often make an action. After the battery was used up, I turned it off and then charged it to full charge. The purpose was to tell it to zero. What is it, what is the full point, otherwise it will often appear that it may suddenly shut down when it shows that there is 10% power.

It suddenly shuts down when the battery is still 10%. The actual situation involved is that the actual battery power can no longer support its external Display voltage. The system automatically shuts down because the battery voltage is too low. The reported power can support it for a long time, so there is a contradiction between the two. Of course, the physical limit itself is the ultimate winner. The data that can be arbitrarily controlled is meaningless at this time. If such a situation occurs in a well-designed system, it will be able to obtain the actual zero position of the battery power from the fact that the power is cut off and shut down, so as to make the most adequate preparations for its next metering cycle, so that the next time The measurement is more accurate, and this is the theoretical basis for me to actively let the mobile phone work until the active shutdown.

There are reasons for the error in the calculation of the electric quantity caused by the coulomb measurement method. One is that the characteristics of the battery itself change with the influence of environment and time, and these changes can be reflected in the change of its internal resistance; the other is that the battery itself has its own characteristics. Discharge, the current measurement on the external circuit of the battery cannot reflect the influence of this part at all; the third is the error of signal sampling, because no digital-to-analog converter can accurately reflect the real-world analog signal itself, just like the National Bureau of Statistics released There is no way for GDP data to reflect the labor remuneration of a part-time worker in my family. The same problem exists in all digital-analog hybrid systems, so the data we can get can only be used as a reference, not as true. Fourth, no digital-to-analog conversion system can collect current information at every moment in a continuous manner. Sampling is only performed at certain points in time, so that it cannot reflect all the actual current information. These problems not only lead to the measurement error of the Coulomb measurement method at every moment, but also because of the integration calculation, they will also be accumulated. The longer the time, the greater the impact will occur. And like me, I will come to clear it from time to time. The zero action is to help it re-calibrate and make it return to the place as close to the origin as possible.

I first became associated with power in my work because I wanted to provide a low-cost solution that can replace A/D converters for MP3 and other devices that use lithium-ion batteries. At that time, I used voltage detection with a detection threshold that can be set by the user. For RT9801B, you only need to change the state of its several input terminals to achieve the purpose. It is very convenient to connect to the GPIO terminal with a lot of surplus digital systems, and ultimately brings the effect of reducing the system cost. The reason why this method can be implemented can be seen from two data. One is the discharge curve of the battery:

A low-cost solution based on lithium-ion battery MP3 instead of A/D converter

The second is the relationship between RT9801B’s voltage detection threshold and its threshold setting terminal:

A low-cost solution based on lithium-ion battery MP3 instead of A/D converter

The system designer only needs to set a new voltage detection threshold according to the current battery voltage range that he already knows. When the battery voltage changes to this threshold, the state of the RT9801B output terminal will change, so that the battery power has reached the new threshold. Level, then change the status of the icon displayed on the screen to let the user know the current battery charge. This method is very inaccurate in the measurement of electricity, but it can already meet the needs of the time, so it is acceptable. In fact, there are still many equipment that do this today. I recently purchased a car washing machine. This is how the battery is displayed, which is enough for me.

The relationship between the apparent voltage of a lithium-ion battery and its actual power is not exactly one-to-one, which has a great relationship with the actual load situation, but if the battery open circuit voltage, the actual voltage and the relationship between the energy storage It is still possible to calculate the state of charge of the battery more accurately due to the lack of current information, and the actual battery power cannot be obtained. What can be given is the relative state of battery energy storage, and the accuracy is still very high. Gao, Li’s first generation of electricity meter products did just that. The following data can tell you its accuracy level: the maximum error can be less than ±3%.

A low-cost solution based on lithium-ion battery MP3 instead of A/D converter

A low-cost solution based on lithium-ion battery MP3 instead of A/D converter

A low-cost solution based on lithium-ion battery MP3 instead of A/D converter

Since the battery state of charge information can be obtained only by measuring the voltage, the application circuit of this fuel gauge is very simple:

A low-cost solution based on lithium-ion battery MP3 instead of A/D converter

This brings it a benefit: as long as the fuel gauge and the battery are connected together, the fuel gauge can quickly get the battery’s state of charge information, without having to go through the process of fully discharging and fully charging the battery, so it can be used. Separated from the battery, the battery becomes a component that can be taken out and accessed at any time, so it can be installed on the system board instead of having to be installed in the battery pack like the coulomb fuel gauge.

The biggest disadvantage of using only the voltage information to calculate the state of charge of the battery is that the specific battery power information cannot be obtained. If the current information and battery capacity information are added on this basis, the two will become supplementary information for the voltage-based fuel gauge, which can not only obtain the actual battery power, but also correct the calculation error of the state of charge. , Improve the measurement accuracy to a whole new level: “±1%, the specific data can be seen in the figure below:

A low-cost solution based on lithium-ion battery MP3 instead of A/D converter

Because to measure the current, it is necessary to add a current detection resistor, so a new application circuit diagram is formed:

A low-cost solution based on lithium-ion battery MP3 instead of A/D converter

In the above figure, the current sense resistor is connected between CSP and CSN of RT9426. In order to improve safety, RT9426 also adds a temperature detection circuit, which can notify the system in time to stop charging and discharging operations when the temperature exceeds the allowable range.

The addition of new features will complicate the circuit, but this effort is valuable. If you explore the brand new fuel gauge product model RT9422A, you will find that it has a more complicated application circuit:

A low-cost solution based on lithium-ion battery MP3 instead of A/D converter

Because it has added a second-level safety control circuit, it can cut off the external fuse in an emergency, so that the connection between the battery and the external circuit is completely disconnected to ensure safety. This function is independent of the protection circuit of the lithium ion battery. . It also adds a key verification mechanism using SHA-1/HMAC algorithm, which can authenticate the battery connected to the system, which is invisible on the circuit diagram, because the key and calculation result are transmitted through SDA The data interface represented by /SCL can be transmitted without other circuits for cooperation.

The needs of the application system are different, and the choice of the electricity meter used is also different. The key is to know what you need, and also understand the characteristics of different electricity meters, so that you can connect the two to choose. No matter what your final choice is, you need to know that the prerequisite for accurate measurement is to have an accurate understanding of battery characteristics. This can be obtained by measuring battery characteristics and inferring battery model parameters from the obtained data. This requires you Heli’s technical support team worked together. If you don’t do this step and use it directly, the results can also be used and have a certain reference value, but the accuracy is always limited.

The Links:   SKKT500-18E MG300Q2YS40