High-power battery status detection based on DS2438

Abstract: This paper introduces a high-power battery status detection method. The charging and discharging current value of the battery is measured by the power quality management chip DS2438, and the charging and discharging capacity of the battery is estimated by integrating the time. With the microcontroller STC89C51 as the core, the power quality management chip and peripheral circuits are controlled to achieve accurate measurement of the voltage, current and power of the battery.
Keywords: battery; DS2438; STC89C51; power management

Introduction
The capacity of the battery refers to the level of the current charging status of the battery, which is equivalent to the actual capacity that the battery can discharge, that is, the amount of capacity (remaining capacity) retained by the battery is detected. There are mainly density method (specific gravity method), ampere-hour meter, internal resistance method (conductivity method), etc. This design adopts an ampere-hour meter for battery power detection. It uses a power quality management chip to memorize the battery charging and discharging current, and can accurately calculate the battery charging and discharging capacity, which solves the problem of overcharging and over-discharging of the battery due to inaccurate measurement of power in the traditional power detection method, which affects the battery life. The power quality management chip can simultaneously monitor the voltage, current and temperature of the single battery in real time, thereby managing the charge and discharge of the single battery. This design fully protects the battery, is conducive to extending the number of battery cycles, and has high application value.

1 Hardware Design
This system can measure the terminal voltage, battery temperature, battery discharge current, battery power, etc. of the battery, and the measurement data can be displayed on the LED, stored and uploaded to the PC. In terms of battery power measurement, the system corrects and compensates for the nonlinearity and temperature of the sensor through software. Compared with traditional detection devices, it has the advantages of good stability and high accuracy, and also has an alarm function. The
system structure block diagram is shown in Figure 1. The single-chip microcomputer STC89C51 is the core of the entire system. It can complete the sampling control and data processing of the battery through the intelligent battery monitoring chip DS2438, and is responsible for human-computer dialogue, input control commands, set parameters, output display and output alarm signals. This system communicates with the host computer through the RS-232 communication port. The host computer can use VB language to compile the communication interface and send control commands to the single-chip microcomputer. At the same time, in order to save the power information, the storage chip AT24C02 is used to save the power value and related data after each calculation. The display system uses a common anode digital tube, the alarm system uses a diode, and a relay is used to protect the current sensor.

1.1 Power chip management circuit
DS2438 chip is an intelligent battery monitoring chip launched by Maxim. It is a very small, fast and accurate battery detection chip. It has the advantages of powerful functions, small size and simple hardware wiring, and can easily monitor the operating status of the battery pack. The pin arrangement of DS2438 is shown in Figure 2. GND is grounded, Vsens+ and Vsens- are battery current input terminals, Vad is battery voltage input terminal, Vdd is power input terminal, NC is floating, and DQ is data input and output terminal.

The power management chip can measure temperature, voltage, current and power. The specific parameters are as follows:
① Temperature measurement. DS2438 can measure temperature through the temperature sensor integrated in the chip. The measured temperature range is -55～+125℃ and the resolution is 0.031 25℃.
② Battery voltage measurement. DS2438 has a 10-bit A/D converter integrated in the chip, which can detect the current battery voltage value. The measurement range is 0～10V and the resolution is 10mV.
③ Battery current measurement. DS2438 measures the current flowing through the battery by measuring the voltage on the external resistor Rsens. The voltage value on the reference resistor Rsens is sampled and sent to the current register. The battery current is equal to the value in the current register/(4 096×Rsens), and a compensation register is specially set to eliminate conversion errors.
④ Remaining power tracking. DS2438 can track the remaining power of the battery using the integrated current accumulator (ICA). The total current flowing through the battery is stored in ICA, which can reflect the battery power.
⑤DS2438 is a single bus working mode, data input and output are completed by a data line; and each DS2438 has a unique 64-bit serial number, which can be used to communicate by querying the serial number, so that multiple DS2438s can be connected on one bus.
The detection external circuit of the power quality management chip DS2438 is shown in Figure 3.

The battery voltage is connected to the Vad terminal of DS2438 after being divided by resistors. Because the voltage input range of the power chip is 0-10 V and the voltage of the battery is 24 V, the voltage divider circuit is shown in Figure 4. The voltage range of the current measurement input terminal (Vsens+, Vsens-) of the power chip is -125-+125 mV. This design uses the current sensor LA200-P of LEM Corporation, with a transformation ratio of 2000:1, which can accurately reduce the current in the range of 200A to the range of 100mA, and then input it to the power chip through the 1Ω precision resistor R4, as shown in Figure 3.

The specific calculation, measurement and correction processing will be implemented through the relevant programs of the processor. The current I and the remaining capacity are calculated by equation (1) and equation (2) respectively:

Wherein, Current Register is the value of the current register, and ICA is the value of the current accumulator.
1.2 Current sensor
Since this system measures the power of high-power batteries, and the power quality management chip is generally used to measure batteries with relatively small capacity and discharge current, a suitable current sensor must be selected to ensure the measurement accuracy and measurement range. The battery is used to start the motor to generate a large current of more than 100A. The capacity of the battery is 110Ah. Through the current sensor LA200-P, the high-power battery can be measured using the measurement method of ordinary small-capacity batteries to achieve the purpose of accurate measurement.
LA200-P has become the first choice of frequency conversion manufacturers and uninterruptible power supply manufacturers due to its stable operation, good performance, and long service life. It is widely used in current measurement in various fields. LA200-P has significant advantages in battery management, steering and brake control. As shown in Figure 3, the power supply of LA200-P is ±15V, which can measure positive and negative currents. The output is in the form of current. The current flows through resistor R4 to generate a voltage drop and is transmitted to the power chip.

2 Software Design
The battery status detection system consists of four main parts: power chip test program, display program, memory read-write program, and communication program. The power chip test program realizes the measurement of battery voltage, current, power, and temperature, and is the focus of the software program. The display program mainly realizes the real-time display of measurement results and the output of the alarm system. The memory read-write program completes the access to power and the preservation of important data tables for accurate measurement results. The communication program mainly completes the communication between the microcontroller and the host computer to ensure the realization of the human-machine interface. This article mainly introduces the power test program of the power quality management chip.
DS2438 uses an integrated current accumulator (ICA) to track and measure the remaining power of the battery. ICA is a register that accumulates all the current flowing into and out of the battery after the battery pack is put into use. Therefore, the value of ICA can be expressed as the remaining power of the battery. As mentioned above, the battery current is obtained by measuring the voltage on the external resistor R4 every 27.46ms. According to the positive or negative value of this value, this value is added or subtracted from the value in the ICA register. ICA is an 8-bit binary counter that combines the voltage across the external resistor R4 measured each time. The remaining power can be calculated using formula (2).
The ICA register format is as follows:

The reading of the power register and the conversion function of the power are as follows:



Conclusion
This paper mainly uses the accurate measurement method of the small-capacity battery state to complete the state measurement of the high-power battery, and realizes the measurement of the current, voltage, power and temperature of the high-power battery. The power quality management chip DS2438 fundamentally changes the structure of the battery monitoring system, truly realizes on-site collection, improves the measurement accuracy and anti-interference ability, and makes the monitoring of the remaining battery power accurate and reliable. Real-time monitoring of the battery status will be conducive to the recycling of the battery and effectively improve the maintenance level of the battery.