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Power detection scheme for internal lithium battery of mobile power supply

by:dcfpower     2021-03-10
With the advent of large-capacity and high-energy-consuming mobile electronic products, mobile backup power supplies have also evolved. Mobile power, commonly known as power bank, was born to make up for the lack of battery power of mobile electronic products. For example, high-energy-consuming electronic products such as the iPhone. The mobile power supply itself is composed of a large-capacity lithium battery and an internal control circuit. The control circuit is divided into two types, one is a simple boost control chip combined with a single-chip microcomputer, and the other is a special control chip that integrates charging, boosting, overcurrent, overvoltage, and short circuit protection. Comprehensive circuit system, and this kind of scheme is becoming more and more favored by consumers for its advantages such as lower cost, more complete control, and safer system. There is an important control link in the control system of various mobile power dedicated control chips, that is, the power indicator of the internal lithium battery pack of the mobile power supply. This function of indicating the remaining power requires a more accurate power detection method.

1 The current general power display method At present, the general display method is based on the display scheme of four LED lights as the mainstream, so this article also uses the display method of four LEDs to illustrate. The general practice is to divide the total power of the mobile power supply into four parts, 25%, 50%, 75%, and 100%. Use four LED lights to display. The power indicator in the first mobile power supply basically used the equal division of the battery voltage to simply judge the amount of power. For example, the battery voltage is divided into 3.3V, 3.6V, 3.9V, 4.2V, and divided into 25%, 50%, 75%, 100% based on this, that is, when the battery voltage is 4.2V ~ 3.9V, the battery capacity is 100% of the total battery capacity. When the battery voltage is between 3.9V and 3.6V, the battery capacity is 75% of the total capacity. When the battery voltage is between 3.9V and 3.6V, the battery capacity is 50% of the total capacity. When 3.6V~3.3V, the battery capacity is 25% of the total battery capacity. In fact, the above division is not accurate, because the charging and discharging time of the battery and the battery voltage are not simply linear. This will result in uneven power division, which may actually occupy 60% of the battery power in the indicated 50% to 75% part. According to a special test on battery power, this is indeed the case. According to the test results, the voltage segment with the most battery power, that is, the longest power supply time, is between 3.7V and 3.9V, which basically accounts for more than half of the total power. Taking a single cell of 2500mah and outputting 1A current as an example, the discharge time is about 80 minutes when the battery voltage is between 3.7V and 3.9V. In order to solve this problem, engineers adjusted the division of battery voltage. Instead of using the voltage equal division as a reference for the amount of electricity, the voltage corresponding to the time equal division point of battery charging or discharging was used as the reference. For example, when the battery is discharging, it takes a quarter of the total capacity time for the voltage to drop from 4.2V to 3.98V, then 3.98V is used as a reference voltage threshold, and so on. This basically forms the current general mobile power control chip power detection method. There are two problems with the above approach. One is that the battery voltage actually sampled is not the true voltage of the battery due to the internal resistance of the battery, and the battery terminal wire resistance on the system board and the on-resistance of the protection IC switch tube will be superimposed as the internal resistance of the battery. This is A fairly large magnitude, generally close to 100 milliohms. 2 The electric quantity detection scheme with internal resistance compensation A further solution is to collect the battery current synchronously while taking the battery voltage as a reference, and calculate the equivalent battery internal resistance according to the magnitude of the collected current (including wire resistance and Protect the on-resistance of the switch of the IC), and then superimpose the extra voltage drop generated by this internal resistance on the battery capacity reference voltage, so that the detection accuracy of the battery power is better improved, and the detection method is further improved. But after this improvement, the planner also has certain problems. For manufacturers of different battery capacities and mobile power supplies, it is difficult to ensure that this equivalent resistance can be effectively compensated. Therefore, it is necessary to adjust the compensation value for each batch, which means that a special control is required on the control chip. Adjustment pin for resistance compensation. In addition, in the above two schemes, there is still inconsistent battery charging and discharging reference thresholds. In this way, the different reference voltages for charging and boosting judgment will lead to the problem of different states of remaining power when charging and boosting each other. For example: 75% power is displayed during charging, and the result is a phenomenon that it becomes 50% power when converted to boost output. Of course, this is the problem caused by the different charging and discharging curves of the battery. It is currently believed that the better battery power method is the method used in the fuel gauge chip. However, the design of the fuel gauge chip is more complicated. It is not easy to add the circuit design of the fuel gauge to the current production cost of the mobile power supply. It requires MCU, a large amount of digital processing, and complex algorithms. However, this article proposes a method that does not require too complicated digital circuits but can better improve battery power detection, which is quite feasible. 3 Analysis and design of charge metering and detecting power plan The improvement method of this paper is the method of charge metering, which is somewhat similar to the design idea in the fuel meter chip, but the design is simple and easy to implement in the mobile power control chip.

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