The Ninth International Conference on Electronic Measurement & Instruments
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High-precision Battery Test System Based on 24-bit ADC Ruijie Shen 1 Rujun Chen 1, 2, 3 Zheyuan Huang 1 1 School of Info-physics and Geomatics Engineering, Central South University, Changsha 410083, China˗ 2 Postdoctoral Station of Computer Science and Technology, Central South University, Changsha 410083, China˗ 3 Postdoctoral Research Station of BGP, CPNC, Zhuozhou, 072751ˈChina E-mail :
[email protected] Abstract – This paper presents an intelligent secondary battery test system based on a 24-bit high-resolution ADC ADS1211. We adopt ultra-high precision voltage reference AD780 and 24-bit ADC ADS1211 to ensure high-precision measurement. The system utilizes a micro-controller to intelligently set the parameters of channel state control circuit, switch charge-discharge mode, switch voltage and current feedback, control voltage and current sampling, test internal resistance of a battery cell, measure the ambient temperature, etc. This system achieves the functions of testing rechargeable battery, including constant current charge, constant voltage charge, constant current discharge and static performance, and the high precision measurement of secondary battery parameters. Furthermore, based on the test result we can analysis the comprehensive performance of the secondary battery and achieve a reasonable battery match to play the best performance. The main characteristics of the system are simple circuit, low cost, multi-function and high precision. Keywords – battery testing; internal resistance testing; 24-bit ADC; ADS1211; AD780.
ĉINTRODUCTION Secondary battery is also called rechargeable battery. There are three main types of conventional storage batteries that are used extensively today: the lead–acid batteries, the nickel-based batteries (mainly NiCd, NiMH and NiZn batteries.) and the lithium-based batteries (mainly lithium-ion and lithium-polymer cells). Their characteristics are described in [1]. Lithium-ion Battery has the advantages of high energy density, no memory effect, good cyclability, long lifetime, low price, etc. Furthermore, it contains no Cadmium, Lead, mercury, etc, beneficial to the environment. As a result, Li-ion battery is widely applied. It becomes the preferred energy storage device for many portable applications including cellular phones, laptop computers, and digital camera, and so on. Furth more, the large-capacity lithium-ion battery has also rapidly developed in the national defense, industrial, telecommunications, aerospace and other fields to reduce the weight and cost [2][3][4]. However, there are some unqualified products which can lead the battery overheating and burning even explosion. The typical example, Dell laptop explodes at Japanese conference in the year 2006, is reported in [5]. Recently, Hewlett-Packard has issued a voluntary recall of about 70,000 Lithium-ion batteries due to fire hazard [6]. In
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addition, the wide using of secondary battery pack has the problem of inconsistency in the performance of single cell battery, resulting in premature battery pack failure. Therefore, the testing of single cell batteries for the assembly must be carried out for sorting. The main testing requirement for secondary battery contains the performance of charge-discharge, capacity, cycle life and so on. Non-intrusive methods of determining in-service capacity are more advantageous, the reason is described in [7]. The impedance and conductance of battery has been suggested as testing parameters. When determining the cycle life of battery, capacity attenuation rate of battery, battery sorting, impedance and conductance testing of battery are very useful. The key to CC-CV charge method is how to smoothly and properly transitioning between the constant current and constant voltage sources [8]. In practice, current and voltage feedback loops are used to regulate the charging process. The transition between two feedback loops is therefore a critical feature for safe and uninterrupted charging sequences. Many transition circuits have been designed about the CC-CV charge circuit. For example, reference [9] uses voltage and current of battery in the circuit to control, respectively. Two PNP transistors are used to achieve the current and voltage loops and the transition between the two loops. The continuous current feedback loop and a switched-sampled voltage feedback loop are operated concurrently during the various phases of the charging process. A much smaller gain-bandwidth than that of the current control loop is used to improve the stability [10]. The multi-mode low dropout (LDO) voltage regulator associated with current sense circuit is adopted to achieve the transition between the two sources [11]. FPGA or micro-controller is used to switch the circuits [12-14], etc. In general, existing battery test equipments use no more than 16-bit ADC for digital-to-analog conversion. According to GB/T18287-2000, while li-ion battery charging at constant voltage, the deadline current is required no more than 0.01C5A. For example, if the battery capacity is 500mAh, 1C is charging current of 500mAh, and then 0.01C5A is 5mA. Therefore, when the charging current is no more than 5mA (or charging time more than 8h in non-normal circumstances) the charge could stop. In the battery test system, when the current signal is converted to voltage signal, the gain is fixed and constant. While the current is small, the measurement error will be increased. High resolution ADC could 1-867
The Ninth International Conference on Electronic Measurement & Instruments enhance the signal resolution and improve its ability to distinguish small-signal [14]. It works together with the ultra-precision voltage reference to improve small-signal accuracy and precision. Concerning the safety of Li-ion battery, in order to play the best performance of single cell batteries used in battery packs and to improve the measurement precision, this paper presents a battery test system based on micro-controller. It adopts ultra-high precision voltage reference AD780 to ensure the precision of high resolution 24-bit ADC and the control accuracy of 16-bit DAC. MOSFET is used to generate constant-current or constant-voltage sources, by controlling the work regions of MOSFET through the current feedback loop and voltage feedback loop. This circuit can control the charge-discharge state and realize the smooth transition between the two loops. It can avoid the side-effect bring
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by the internal resistance changes, and then improves the efficiency of battery charging and does little damage to the battery. Ċ.HARDWARE DESIGH The test system mainly consists of micro-controller circuit, ADC, DAC, voltage reference circuit, the channels work state control circuit, charge-discharge circuit, protection circuit, the ambient temperature test circuit, serial communication circuit, watchdog circuit, etc. And there are 8 completely independent channel set in the system. Each channel is allowed to be set any different mode and does not impact each other. The block diagram is shown in Fig. 1.
Fig. 1. Block diagram of the system
A. Micro-controller control circuit The test system is based on the AT89S52 micro-controller, which is the core of the system. It processes the commands from the operation software running on PC and executes them. It achieves the real-time and efficient controlling of the channel charge-discharge states, data acquisition and others. And it will transmit the sampled data to the computer. The software on the computer analyzes and estimates the performance of secondary battery base on the received data. The AT89S52 is a low-power, high-performance CMOS 8-bit microcontroller. In order to reasonably and 1-868
efficiently use the I/O ports of AT89S52, shift registers are used to implement the transformation from serial to parallel and the pin-outs are time-multiplexing except the interfaces used as decisive control lines, such as chip select lines, latch lines, etc. In this case, it is necessary to consider the drive capability of the I/O ports, so pull-up resistors are connected to P1 and P2 ports, as shown in Fig.2, to increase the load capacity. Crystal with integer value will be in error when loading the initial value, in order to reduce data loss or disorder phenomenon caused by the baud rate error of serial communication process, we select 11.0592MHz crystal.
The Ninth International Conference on Electronic Measurement & Instruments
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RP1 P1.0 P1.1 P1.2 P1.3 P1.4 P1.5 P1.6 P1.7 P2.0 P2.1 P2.2 P2.3 P2.4 P2.5 P2.6 P2.7 P0.7
P1.0 P1.1 P1.2 P1.3 P1.4 P1.5 P1.6 P1.7
P2.1
DIN
P2.2 P2.0
P0.7
VDD
SCLK
DGND
CS
AGND
REF
P2.0 P2.1 P2.2 P2.3 P2.4 P2.5 P2.6 P2.7
OUT
DAOUT
MAX541 OP AMP AIN3N AIN2N AIN1N AIN2 AIN1 AIN3
RP2
AT89S52 VCC5V C1
P2.3 P2.4 P2.5 P2.7
+Vin C2
VCC5V C8
VCC5V
TEMP VOUT C3 GND
AIN2P AIN1P AIN3P
AIN4N AIN4P AGND C7 AVDD
DRDY SDOUT SDIO SCLK REFIN
C6
10 DVDD MODE DGND
C4
VCC5V R1
C5
AD780 ADS1211
Fig. 2. Circuit of the micro-controller, ADC, DAC and voltage reference.
B. ADC circuit
C. DAC circuit
In order to enhance the resolution and the ability to distinguish the small signal, we select 24-bit analog-to-digital converter ADS1211 in this design. The ADS1211 is a high precision, wide dynamic range, self-calibrating, delta-sigma A/D converter capable of achieving very high resolution digital results. Four differential inputs to reduce the common mode rejection are provided, and are ideal for direct connection to transducers or low level voltage signals. The ADS1211 including complete on-chip calibration can correct for internal offset and gain errors or limited external system errors. ADS1211 can work at Master Mode (self-clocked mode) or Slave Mode (externally clocked mode). In the Master Mode, ADS1211 clock frequency needs to be considered. Here slave mode and four-wire interface are used in the design. The interface signals consist of DRDY, SCLK, SDIO and SDOUT, respectively connected to P2.3, P2.7, P2.5 and P2.4 of AT89S52, as shown in Fig.2. In order to ensure ADS1211 to normally operate in high precision, connection between AVDD and DVDD is accomplished via a 10 resistor which, along with the decoupling capacitors, will provide some filtering between DVDD and AVDD. A 10μF capacitor, in parallel with a 0.1μF ceramic capacitor, used to decouple AVDD to AGND, can improve the anti-jamming ability, and then improve the data stability. A 0.1μF ceramic capacitor used to decouple DVDD to DGND could meet the needs.
16-bit digital-to-analog converter is used for the purpose of the control accuracy. The MAX541 are serial-input, voltage-output, 16-bit digital-to-analog converter that operate from a single +5V supply [14]. It provides 16-bit performance (±1LSB INL and DNL) over temperature without any adjustments. The DAC output is unbuffered. Therefore, a buffer circuit, a voltage follower constituted by ultra-low offset current, low-drift operational amplifier LT1012 [14], is used to improve the drive capability. Its connection with AT89S52 is fulfilled by three-wire serial interface, as shown in Fig. 2. In the system, 8 channels share the DAC [14], so as to reduce the system cost and decrease the occupancy of the I/O ports of micro-controller to improve the efficiency of the ports using. Each channel uses a sampling holder to hold the control variable to achieve constant-current and constant-voltage control. D. Voltage reference circuit design The precision and stability of voltage reference directly affects the conversion precision of ADC and DAC. If the reference voltage was instable, the 24-bit ADC would be meaningless. Therefore, ultra-high precision band gap reference voltage AD780 is used. By mean of the precision thin-film resistors and laser trimming techniques, AD780 provides low initial error (f0.04% max) and ultra-low temperature drift (3ppm/°C max). AD780 is the ideal choice to enhance the performance of high resolution 1-869
The Ninth International Conference on Electronic Measurement & Instruments ADCs and DACs. In order to obtain better noise performance, two capacitors, a load capacitor of 100μF between the output and ground and a compensation capacitor of 100nF between the TEMP pin and ground, are added. And then a bypass capacitor of 1 μF (+VIN to GND) should be used due to the load capacitor. E. Charge-discharge circuit design According to GB/T18287-2000, the main test methods of Li-ion battery consist of constant current discharge, constant current charge and constant voltage charge. Utilizing the working characteristics of MOSFET, combined with the theory of negative feedback, a simple charge-discharge circuit is used [14], as shown in Fig. 3. The circuit, by pass of the control of the AT89S52, achieves the stable transition between constant current loop and constant voltage loop, and can testing the performance of secondary battery.
ICEMI’2009
used to control the MOSFET to work at the constant current area; if the current of the charge circuit increased or decreased, the result of the comparison would change to make the MOSFET adjust the current reversely. Thus the current is controlled to a constant value. Similarly, while the circuit is set as constant voltage charge state, the MOSFET will work at the variable resistance area, and the voltage will be regulated by way of adjusting the current. F. Design of internal resistance testing Internal resistance is an important technical indicator to measure the secondary battery performance. In general, the internal resistance of battery is very small, usually using milliohm or micro-ohm as the measurement units. A new rechargeable battery internal resistance is relatively small, but after a long-term using, due to the depletion of the battery electrolyte and the decrease of the internal battery chemical substances, the internal resistance will increase gradually until the internal battery power cannot be released. Internal resistance is an important considerable factor in the battery pack when matching. If a mutation increase of the internal resistance of a battery happened, the power of the entire battery pack will be soon consumed, so that cells are not reasonable and efficient application.
Fig. 3. Diagram of charge-discharge circuit.
The access mode of battery is four-terminal connection. Terminals a and b are the voltage detection output terminals, which are connected to the differential operational amplifier U1. A and B are the connect terminals of the analog current loop. The current of the charge-discharge circuit is converted to voltage signal via a high accuracy and low temperature coefficient constantan wire resistance R0, and then appropriately amplified by operational amplifier U2. In Fig.3, the relay is used to control the switch between charge and discharge, and 2-channel multiplexer is used to set the feedback current feedback or voltage feedback. As the main adjustment the MOSFET is controlled by the output of an open-loop op amp U3, which is used to compare the control variable set by PC software and converted by DAC with the current or voltage feedback variable, to control the gate of MOSFET and then achieve the channel state expected. Due to the share of one DAC, the sampling holder is used to hold the DAC output instead of DAC to reduce the cost. While the circuit is set as constant current charge state, the 7.5V power is switched in by the relay, used to charge the battery. The charge current is set by PC software, transmitted to and processed by the micro-controller, converted by DAC, and then held by the sampling holder. At the same time, 2-channel multiplexer selects the current feedback. The result of the comparison of U3 is 1-870
Fig.4. Block diagram of internal resistance testing circuit.
This system uses AC voltage-drop method to test internal resistance. The block diagram is shown in Fig. 4. An alternating current signal, with a frequency of about 1kHz and a constant current value relatively small, is injected into the battery and the corresponding voltage variation across the battery is measured after the direct current isolated and amplified. According to Ohm's law, U=IR, we can get R=U/I. The magnitude of the battery impedance is then calculated by dividing the AC voltage magnitude by the AC current magnitude. And then the internal resistance is worked out. This method uses an alternating current signal with small voltage amplitude, so internal resistance testing can run either in single mode or run simultaneously with the charge -discharge process, without affecting the charge-discharge process and bringing side effect to the Li-ion battery’s performance. To measure the true RMS of the AC signal, we use an AD736. The AD736 is a low power, high precision, monolithic true rms-to-dc converter. It is laser trimmed to provide a maximum error of ±0.3 mV ±0.3% of reading with sine-wave inputs. Only one external
The Ninth International Conference on Electronic Measurement & Instruments component, an averaging capacitor, is required for the AD736 to perform true RMS measurement. However, additional capacitor will help reduce any output ripple which was not removed by the averaging capacitor. G. Ambient temperature measure circuit
ICEMI’2009
ċ.SYSTEM FIRMWARE DESIGH AT89S52 micro-controller is the core control device of the system, by the way of programming to achieve the efficient control of various functions. SCM C language is used in programming. It’s edited, compiled and debugged by Keil C51 compiler. The main function modules include:
Ambient temperature measure circuit is designed to monitor the temperature in the system chassis. The ambient temperature can be used to calibrate the temperature drift of some components, and monitor the working ambient temperature to prevent the components from overheating which can damage the components. A digital thermometer DS18B20 is used to build a temperature measure system. The circuit is simple and small. Only one port pin is needed to implement communication with the micro-controller. It can measure temperatures between -55 ć and +125 ć. And it has an alarm function with nonvolatile user-programmable upper and lower trigger points.
Prepare for the normal work of the system. First of all, set up the initial values of the state and initialize serial port. And then test whether the channel operation indicator lamps is normal or not. After all the indicator lamps on, initialize DAC to output zero and set sampling holder to sample and hold, set ADC for zero calibration and full-scale calibration. At last, close the channel lights and the initialization is finished, waiting for the control commands.
H. Four-terminal measurement
B. Receiving and processing of the control commands
The contact resistance between the current lines and the battery could not be ignored if the charge-discharge current was relatively large. For example, if the current was 500mA and the contact resistance was 10m, the contact resistance would cause a voltage drop of 5mV. The voltage drop will have a significant impact on voltage measurement, especially in the battery internal resistance test. So the four-terminal measurement method is used. The current connective terminals and voltage detection terminals are separated. As shown in Fig. 5.
As soon as serial interruption happens, the micro-controller begins to receive the control commands. Judge the bytes received, and store them into an array for control commands if meeting the requirements, or else discard them, otherwise the commands are not enough or too long. After receiving the commands, analyze them and then carry out.
Fig. 5. Schematic of four-terminal measurement.
A and B are the analog circuit terminals while a and b are the voltage detection terminals. Terminals a and b are directly connected to the detection circuit with high input impedance. The current across terminals a and b is very small, thereby greatly reducing the detection error caused by the contact resistance, R1 and R2, between the current lines and the battery.
A. Initialization
C. Controlling the working states The system sets eight channels. Each channel operates completely independently. One channel’s state change should not affect other channels. Processing the control commands, the program just reverse the control bit of the channel required to change the operation state while other channels remain unchanged. Because of the sharing of DAC, each channel uses a sampling-holder to hold the control variable. The AT89S52 selects the channels one by one, and set the sampling-holders sample and hold the output of DAC respectively. Due to the various aspects of the leakage and the dielectric absorption effect of capacitor, it is impossible for sampling-holder to maintain the analog control variable for too long time. Therefore, the variable must be refreshed from time to time by the software to correct the retention, so as to reduce the impact of the discharge of the sample-and-hold capacitor and improve the control accuracy. D. Data Acquisition As soon as the AT89S52 receives a sampling, the multiplexer switch voltage, current or the voltage converted from the AC voltage drop into ADS1211, and ADS1211 is controlled to convert the inputs, and then the
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The Ninth International Conference on Electronic Measurement & Instruments converted results are read into the microcontroller and output through the serial port to the PC. E. Measuring Temperature Measure temperature at regular intervals to ensure the system works safely and reliably. Time interval can be adjusted. Č.SYSTEM IMPLEMENTATION AND ANALYSIS OF RESULT
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stable transition from constant current charge to constant voltage charge. It can be applied to the performance test of cellular phone batteries and rechargeable battery for research and further to battery match. ACKNOWLEDGMENT The authors would like to thank the Mittal Steel and professor Li Jie, dean of the Institute of Metallurgy of Central South University, for support. REFERENCES
On the basis of the system circuit designed in this paper, a prototype has been made. The prototype achieves testing the charge-discharge performance of Li-ion battery and measures the ambient temperature. The eight channels can operate independently. The secondary battery test operation system programmed based on visual C++, with good human-machine interface and convenient operation, is used to test the charge-discharge performance of a cell phone Li-ion battery, whose capacity is 520mA. A good charge-discharge curve was gotten and shown in Fig. 6.
[1]
[2]
[3]
[4]
[5] [6] [7]
[8]
Fig. 6. Charge and discharge curve of the cell phone Li-ion battery tested.
Fig. 6 shows two complete charge-discharge cycles. The time of the first cycle is a little longer due to the cut-off current set 5mA in the constant voltage charge stage in accordance with the GB/T18287-2000. The second cycle’s cut-off current is set at 15mA to save test time. From the data sampled, the change of constant current does not exceed 0.05mA and the change of constant voltage is less than 0.5mV. And the measurement of small current signal has reached a better requirement. As seen from Fig. 6, the curve is smooth and stable, without much fluctuation.
[9]
[10]
[11]
[12]
[13]
[14]
č.CONCLUSION The system, based on AT89S52 micro-controller, by way of the adoption of 24-bit ADC, 16-bit DAC and ultra-high-precision voltage reference AD780, achieves testing batteries with high resolution, high accuracy and high precision. The using of MOSFET makes the charge-discharge circuit work stably and achieves the 1-872
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