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  • 1. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 5, Issue 5, May (2014), pp. 28-35 © IAEME 28 AUTOMATED TESTING OF NI-CD BATTERIES USING MICROCONTROLLER BASED CHARGE-DISCHARGE SYSTEM Mahesh Sharma1 , K. Singh2 1 Department of Electronics, Kamla Nehru Mahavidyalaya, Nagpur – 440013, India, 2 Retired Professor, Department of Physics, RTM Nagpur University, Nagpur, India ABSTRACT In this paper we present a microcontroller based low cost system to study charge-discharge characteristic of NiCd. The developed system is able to perform all type of charging/discharging processes specified for the above mentioned batteries. NiCd batteries shows a negative increment on its terminal voltage when the battery has been fully recharged. Negative delta strategy is used in our system to detect state of charge of battery. The system circuit is connected to the personal computer through a USB to serial port. Type of test procedures, magnitude of charge and/or discharge current, end voltages and other quantities are entered in the graphical user interface developed in visual basic. The designed program performs full equipment control and process management. The data is stored in excel format. The GUI also displays the variation of current, voltage and temperature with respect to time. Keywords: Battery Charge-Discharge System, Negative Delta Voltage, Ni-Cd Battery. I. INTRODUCTION Regardless of application or battery type, knowledge of the behaviour of the battery is of great importance. Battery testing is necessary to determining its behavioural characteristics. Observation of current and voltage of a battery over a large period of time is essential for testing the battery for its capacity. Manual testing is labour intensive, time consuming process and inherently is subject to human errors. On the other hand automated system eases the test process and limits the error. They also offer accurate collection and easier storage of accumulated data in system memory, accurate analysis and results. INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING & TECHNOLOGY (IJEET) ISSN 0976 – 6545(Print) ISSN 0976 – 6553(Online) Volume 5, Issue 5, May (2014), pp. 28-35 © IAEME: Journal Impact Factor (2014): 6.8310 (Calculated by GISI) IJEET © I A E M E
  • 2. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 5, Issue 5, May (2014), pp. 28-35 © IAEME 29 The battery finds application at various places such as common electronic gadget like mobile phone; laptops to high power application such as telephone exchange, battery run electric vehicle and HEVs. Three chemistries are widely used for secondary batteries: NiCd, NiMH and lithium-ion (Li- ion) batteries. Each of these types of batteries has their pros and cons. Many portable electronic appliances use NiCd and environmental friendly NiMH battery to cater their power requirement. These batteries have been proved to be most appropriate for its power density, size, cost, re- charge/discharge characteristics and maintenance. Due to the inherent properties, the Ni-Cd battery presents different recharging and discharging characteristics [1]. It has a flat discharge curve that the storage energy can be more efficiently released and an apparent negative increment on its terminal voltage when the battery has been fully re-charged [2]. The negative increment of terminal voltage is mostly known as the negative delta voltage characteristic of the family of NiCd batteries. II. NiCd BATTERY DISCHARGING AND CHARGING METHODS Discharge tests that are performed on the battery are constant current, constant power and constant load discharge. The battery is discharged till the voltage falls to some percentage of the open circuit voltage. Simple method of charging the cell without damaging is tickle charging which is recommended at around 0.1C for NiCd battery[3]. The charger circuit for slow charging does not requires end-of-charge detection circuitry, hence are simple and very cheap. On other hand slow charge takes a long time to recharge the battery. Fast charge typically requires 1 hour recharge time corresponding to a charge rate of about 1C - 5C [2]. Due to low internal resistance and endothermic reaction the Ni-Cd batteries can be charged at high charge rates. Some NiCd cells can tolerate charge rate up to 5C. When the battery reaches full charge, the energy is dissipated as heat within the cell which results in a very sharp increase in both cell temperature and internal pressure along with a fall in terminal voltage of battery. This is called negative delta is indicative of the end of charge [4]. The Time Pulsed Charging (TPC) method is used in some portable electronics in which current is sent in pulses leaving short gaps with zero current being pumped to the battery. This allows the battery cells to relax, thus allowing more charging capacity [4,5]. The battery cell voltage reading is more accurate when charging current is zero hence this method provides a better cell voltage reading. The TPC method also features some more advantages at the cell chemical level [6]. III. HARDWARE SETUP a) Block diagram A generalized block diagram of a battery charge/discharge unit is shown in fig 1 below [7]. The main components are the user interface and controller, data acquisition unit, load, charger and a temperature measurement unit. The user interface and controller allows the operator to specify the test details, control the test and store the test data. The data acquisition unit is responsible for acquiring the relevant data and returning it to the user interface. Temperature measurement is required to know excess battery temperature during charging/discharging so that a suitable action can be taken if the temperature rise is high.
  • 3. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 5, Issue 5, May (2014), pp. 28-35 © IAEME 30 Fig. 1: Block diagram of battery charge unit b) Microcontroller circuit 89S52 is a low power high performance high 8 bit microcontroller consisting of four 8-bit ports, two 16-bit timers, a full duplex UART. Microcontroller control all the function of the test system which consists of ADC, DAC, charging and discharging circuit, the battery temperature measuring circuit and USB to serial interfacing circuit. The microcontroller receives commands from the computer and executes them. These commands are the start and stop commands of the process, open circuit voltage measurement, internal resistance measurement and the type of charging or discharging process to be executed. The microcontroller instructs ADC to measures the battery voltage, the voltage proportional to charging and discharging current and the temperature in a sequence sixteen times. It is then averaged to get the final value. The final data is then sent to computer for further processing. The software in the computer analyzes the performance of test battery based on the received data. All the abovementioned process is embedded in a control word generated in the computer which is sent to microcontroller. The format for the control word is given below: OCV IR - Charge/Discharge - Charge Discharge - 80H => Measure OCV, 40H => Measure Internal Resistance 14H => Charge process 12H => Discharge Process Beside this command the computer also sends a two byte data which is binary equivalent of the DAC voltage which will generate the required charge or discharge current. Fig. 2: Connection of system devices to microcontroller 89S52
  • 4. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 5, Issue 5, May (2014), pp. 28-35 © IAEME 31 Fig. 2 shows the schematic circuit for the test system. DAC is connected to the microcontroller through 4 port pins of Port P1. Since only one of the processes either charging or discharging operates any time hence only one DAC is used here. An analog switch switches the output of DAC to discharging or to charging circuit. Microcontroller is also connected to a 4 channel differential ADC through port P3. These analog channels measure voltage proportional to charging current, discharging current, voltage of the test battery and the temperature sensed by temperature sensor IC LM 35. Microcontroller uses its inbuilt UART to communicate with computer. c) ADC and DAC unit MCP3302 is an 8 multiplexed channel 13 bit SPI SAR ADC from Microchip and has a sampling rate of 100kps. The 8 channels of the ADC can be used either separate 8 single ended channel or 4 differential channel. 12 bits gives the magnitude of the voltage and the 13th bit gives the polarity of the signal voltage during differential mode. Serial ADC requires a smaller area on PCB and just 4 port pins of the microcontroller for its connection at a cost of programming overhead which is slightly higher than the parallel ADC. Measurement of the battery voltage, discharge current or the charging voltage and temperature is taken sequentially 16 times and then averaged to get final value. The reference voltage of 4.092 volts gives a resolution of 1mV. ADC interfaces to microcontroller using port P3.4 – P3.7. MCP4921 is a 12-bit buffered voltage output Digital-to-Analog Converter (DAC). The device operates from a single 2.7V to 5.5V supply with an SPI compatible Serial Peripheral Interface. The device utilizes resistive string architecture, with its inherent advantages of low differential non- linearity (DNL) error and fast settling time. The reference voltage to this IC is set at 4.092 volts which gives the resolution of 1mV. Interfacing to microcontroller is through Port pin P1.0 to P1.3. DAC data is computed in the computer and passed to microcontroller. The microcontroller then serially output the digital data to the DAC. The analog output of DAC is given to both, the charging circuit and the discharging circuit, through the analog switch. d) Temperature measurement During charging or discharging of the test battery if the current is limited then the battery maintains temperature around ambient temperature. Only when the charging is done at much faster rate the battery temperature rises. Higher temperature results in degradation of the battery life time. The main purpose of the temperature measurement unit here is to measure the battery temperature during charging and discharging so that suitable action can be taken if necessary. IC LM35 which has a sensitivity of 10mV/o C is used here to measure the temperature of the battery. The flat surface of LM35 contacts the side surface of the battery in the test assembly. e) USB to serial converter Present day PC seldom have serial port. Industries have done away with serial port in laptop. They have been replaced by USB port. One can find several USB port on a computer and laptop. The disadvantage with USB is that they are not easy to program. While serial port is not only easy to program in microcontroller, there is also a well built library function in all the computer languages. A USB to serial converter is used here to communicate with computer. USB side of the converter is connected to computer while serial side is connected to microcontroller UART. The computer treats USB port as serial port and communicates with the microcontroller.
  • 5. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 5, Issue 5, May (2014), pp. 28-35 © IAEME 32 f) Charging circuit Fig. 3: Schematic diagram of charging circuit The charging circuit consists of op-amp along with a MOSFET in closed loop with unity gain as shown in fig. 3. The voltage at the source of the MOSFET equals the voltage at the control input of op-amp. A resistor R2 is connected between the source of MOSFET and the test battery. The charging current is given by VR2/R2 = (Vcontrol - VB)/R2, where Vcontrol is the voltage from DAC and VB is the battery voltage. g) Discharging Circuit Fig. 4 shows the schematic of the discharging circuit [8]. Operational amplifier U1 drives the MOSFET Q1. The discharge current flows only through resistor R1 and hence the voltage drop across R1 is proportional to it. R1 is a high precision, low temperature coefficient resistance. Large open loop gain and low bias current of op-amp used ensures the voltage drop across the resistor R1 is equal to the voltage applied at the non-inverting terminal input of the op-amp. Battery discharging current remains constant irrespective of the battery terminal voltage if the voltage at the non- inverting terminal of op-amp is maintained at a fixed value and is equal to voltage at non- inverting/R1. Fig. 4: Schematic discharging circuit IV. SOFTWARE User-friendly GUI and the required software to drive the electronic load is developed in Visual Basic. The user enters the type of charge and discharge, the magnitude of the discharging parameter. A timer records the elapsed time of the discharge. Data is stored in excel sheet.
  • 6. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 5, Issue 5, May (2014), pp. 28-35 © IAEME 33 V. RESULTS 1000mAHr capacity AA size NiCd cells were charged and discharged at different current in different ways. Fig. 5a, 5b and 5c shows the discharging of the cell at various constant current, constant power and constant resistance. Pulse discharging was also carried out for various constant current for ON/OFF time ratio of 10:3 minutes is depicted in fig. 5d. Fig. 5: a) Variation of battery voltage at different constant current discharge, b) Variation of battery voltage and discharging current at different constant power c) Variation of battery voltage and discharging current at different constant load d) Pulse discharge of NiCd battery with ON/OFF time of 10/3 minutes Fig. 6a shows the charging of NiCd battery using different constant current and negative delta region was used to determine the fully charged condition of the cell when the charging was stopped. Fig. 6a: Charging of NiCd battery using constant current charge and using negative delta to stop charging
  • 7. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 5, Issue 5, May (2014), pp. 28-35 © IAEME 34 Similarly charging was carried out using pulse charging at 500mA charging current. The variation of the battery voltage is depicted in fig. 6b. Pulse charging may lead to either undercharging or overcharging both of which are detrimental for the battery long life. NiCd cell shows a negative delta region when the battery is fully charged. This can be clearly observed in pulse charging during last pulse. The figure shows a delta voltage of around 100mV. Fig. 6b: Pulse charging of NiCd battery with ON/OFF time of 10/3 minutes A temperature graph shows the variation of the temperature with charging. Since charging of NiCd is endothermic, the temperature falls initially. After the charge is nearly full the temperature shows a steep rise. The variation temperature while charging at 700mA charging is shown in fig 7. It shows a steep rise in temperature during at the end of charging during negative delta region. Fig. 7: The variation temperature while charging at 700mA
  • 8. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 5, Issue 5, May (2014), pp. 28-35 © IAEME 35 VI. CONCLUSION A microcontroller based automated system has been developed which is useful in testing NiCd and NiMH cells. Different types of charging and discharging methods have been included to test so that various types of testing could be carried out on NiCd and NiMH cells. The software is implemented in Visual basic which also graphically shows the variation of current, voltage and temperature with time during the charging process which are also stored in excel format for later reference and analysis. VII. REFERENCES [1] D. Linden, T. B. Reddy, “Handbook of batteries,” McGraw-Hill, NY, 2002, [2] Literature Number: SNVA557, Texas Instruments. [3] Naoki Kat et al. “Establishment of the relationship between capacity and impedance of trickle -charge nickel cadmiurn cells by using electrolyte-deficient model cells”. J. of Power Sources, 62 (1996) 187-192. [4] J.C. Viera, M. Gonzalez, C. Blanco, J.C. Alvarez, “Application range of fast-charging in NiCd batteries”, INTELEC – 2002, pp 259-264. [5] A. Chih-Chiang Hua, B. Zong-Wei Syue, “Charge and Discharge Characteristics of Lead-Acid Battery and LiFePO4 Battery”, IEEE - IPEC 2010 pp 1478-1483. [6] Zoran Vrankovic, Asghar Abedini, and Adel Nasiri, “A Novel Battery Charger for Automotive Applications” IEEE – PESC – 2007 pp 1465-1470. [7] P.E. Pascoe, A.H. Anbuky, Measurement 34 (2003) 325–345. [8] M. Sharma, K Singh, Software Controlled Electronic Load for Testing Battery, J. of Instrument society of India, vol. 43 no. 2, June 2013 pp 104-106. [9] Jadhav Sumedh Damodhar and Phatale Aruna Prashant, “Microcontroller Based Photovoltaic Battery Charging System with Buck Converter”, International Journal of Electrical Engineering & Technology (IJEET), Volume 3, Issue 1, 2012, pp. 123 - 130, ISSN Print: 0976-6545, ISSN Online: 0976-6553. [10] V. P. Labade, N. M. Kulkarni and A. D. Shaligram, “Lead Acid Battery Management System for Electrical Vehicles”, International Journal of Electronics and Communication Engineering & Technology (IJECET), Volume 4, Issue 3, 2013, pp. 97 - 107, ISSN Print: 0976- 6464, ISSN Online: 0976 –6472.