Rechargeable Battery Charge/Discharge
Contents
1. Number 3221 KEITHLEY A Tektronix Company Rechargeable Battery Charge Discharge Application Note Series Introduction Rechargeable or secondary batteries are commonly used in place of disposable batteries in electronic devices such as laptops video game controllers mobile phones digital cameras and remote controls In an effort to improve upon or replace existing battery technologies researchers are studying ways to increase battery life and at the same time decrease the cost of rechargeable batteries Common types of rechargeable batteries include Li ion Lithium Ion Ni MH Nickel Metal Hydride and NiCd Nickel Cadmium The characteristics of a rechargeable battery are commonly tested using discharge and charge cycling Cycle tests provide information about the battery such as its internal chemistry capacity number of usable cycles and lifetime In production testing a discharge charge cycle is often performed to verify battery specifications and to ensure it is not defective A typical battery discharge charge test configuration often includes a programmable power supply an electronic load an electronic switch a voltmeter and an ammeter These systems require extensive software and hardware configurations to synchronize all the instruments into a working test station Another option is to use a potentiostat or galvanostat which are popular instruments used for electrochemical studies A potentiostat is an el2. ectronic instrument designed to control the voltage difference between two electrodes typically a working electrode and a counter electrode A galvanostat is a control and measuring device that has the ability to keep the current through an electrochemical cell constant Controlling the current through a cell like a rechargeable battery is easier than controlling the potential at an electrode Unfortunately there are disadvantages to using a potentiostat or a galvanostat Many of these instruments don t have a display or many control knobs but come with Force HI Figure 1 Battery connections to the Model 2460 SourceMeter SMU Instrument Battery Sense LO Galvanic Cycling Using the Keithley Model 2450 or Model 2460 SourceMeter SMU Instrument designated software to run specific tests on a user supplied PC Users typically don t have control if they want to change a test or re use the equipment for an application not specified in the list of predefined tests Without access to the source code or the programming commands users are unable to perform customized tests or to reuse the equipment for other applications such as basic I V curves in the lab Many potentiostats or galvanostats may also have limited current and voltage source output capacity Galvanostats are often limited to hundreds of milliamps or less in sink discharge or source charge mode High current options may not be able to sink current Batte
3. ed off This will prevent the battery from draining while the output is off To set the Output Off State to High Z press the MENU key and select Source Settings Select High Impedance and then press HOME Model 2450 or 2460 SourceMeter Force HI I Sense HI I Sense LO I Force LO Figure 3 Connecting the Model 2450 or Model 2460 SourceMeter SMU Instrument to the battery Automating the Discharge Cycle Charging and discharging cycles often take several hours so automating the test is important The tests can be executed using any of the supported communication interfaces for the instrument GPIB USB or Ethernet The rear panel connection locations for the remote communication interfaces are shown in Figure 4 Model 2450 rear panel Connect the computer to one of the remote interfaces Figure 4 Model 2450 or Model 2460 SourceMeter SMU Instrument remote interface connections For charging and discharging the SourceMeter should be programmed to perform the following steps 1 Set the measurement to a four wire configuration 2 Set the SourceMeter to measure current This will enable you to monitor the load current 3 Use the High Impedance Output Off State This output off state opens the output relay when the Output is turned off This will prevent the battery from draining when the battery is connected with the Output off 4 Set the SourceMeter to output voltage Even though the unit is set to out
4. names are the property of Keithley Instruments All other trademarks and trade names are the property of their respective companies KEITHLEY A Tektronix Company A Greater Measure of Confidence KEITHLEY INSTRUMENTS 28775 AURORA RD CLEVELAND OH 44139 1891 E 440 248 0400 M Fax 440 248 6168 M 1 888 KEITHLEY www keithley com BENELUX FRANCE ITALY MALAYSIA SINGAPORE 31 40 267 5506 33 01 69 86 83 60 39 049 762 3950 60 4 643 9679 01 800 8255 2835 www keithley nl www keithley fr www keithley it www keithley com www keithley com sg BRAZIL GERMANY JAPAN MEXICO TAIWAN 55 11 4058 0229 49 89 84 93 07 40 81 120 441 046 92 55 5424 7907 886 3 5 72 9077 www keithley com www keithley de www keithley jp www keithley com www keithley com tw CHINA INDIA KOREA RUSSIA UNITED KINGDOM 86 10 8447 5556 080 30792600 82 2 6917 5000 7 495 664 7564 44 1344 39 2450 www keithley com cn www keithley in www keithley co kr www keithley ru www keithley co uk Copyright 2014 Keithley Instruments Printed in the U S A No 3221 10 20 14
5. put voltage it will be operating in constant current mode because it will be in current limit until it reaches the desired voltage For charging the battery Vs gt Vz For discharging the battery Vs lt Vp 5 Turn on the voltage source readback function This will enable the SourceMeter voltmeter to measure the battery voltage as it is either charging or discharging 6 Set the current limit or compliance to the current level at which the battery is to be charged or discharged This is the load current of the test 7 Read back the load current source readback voltage and the relative time stamp 8 Monitor the voltage until the battery voltage reaches the desired voltage level and stop the test Using the Model 2460 SourceMeter SMU Instrument to Discharge a 2500mAh D Cell Battery A 2500mAh D cell 1 2V battery was used to illustrate how to discharge a battery by using the Model 2460 SourceMeter SMU Instrument The instrument was programmed as described in the steps listed in the previous section four wire source voltage measure current etc For this particular test a 2500mAh D cell 2500 mAh D Cell Discharged at 2500 mA 1C Cell Voltage Volts O o o f L2 D gt N ol o o N S Ce 0 2 0 3 0 4 0 5 0 6 0 7 Time Hours Figure 5 Discharge characteristics of 2500mAh D cell battery Model 2460 Front Panel defbuffer1 Measured Load Current gt Measu
6. red Battery Voltage _AUTO_DEL Elapsed Test Time Figure 6 Model 2450 or Model 2460 SourceMeter SMU Instrument displaying the load current battery voltage and elapsed test time while discharging a D cell battery battery was discharged at a rate of 1C by using a 2 5A load current Readings of the battery voltage load current and relative time were taken every ten seconds until the battery voltage reached the specified level 1V The results of measuring the discharge characteristics of the 2500mAh battery are shown in the graph of Figure 5 This TSP code also saves all the current voltage and time readings to a USB drive connected to the front panel USB port of the Model 2460 The code can be easily modified for a specific discharge current and voltage and can be used with either the Model 2450 or 2460 depending upon the specific current and voltage levels In addition to monitoring the readings over the bus the SourceMeter can simultaneously display on its user interface the load current the battery voltage and the elapsed test time while the test is in progress Notice the large easy to read measurements of the D cell battery on the Model 2460 display that is shown in the screen capture in Figure 6 The code used to generate the discharge characteristics is listed in the Appendix Although the SourceMeter s flexibility lets the user choose either SCPI or TSP Test Script Processor commands for p
7. rogramming this code was written with TSP commands using the software tool Test Script Builder This is a free software tool that is provided with the Model 2450 2460 to help users create and modify TSP code and scripts It also has an immediate instrument control console to send commands and receive data from the instrument Before executing this code and connecting the battery to the output terminals of the Model 2460 reset the instrument and then set the Output Off State from Normal to the High Impedance The unit can be reset by pressing MENU gt Manage gt System Reset gt OK Next change the Output Off State by selecting MENU and then pressing the Normal Output Off State button Select High Impedance This output off state opens the output relay when the Output is turned off preventing the battery from draining To execute this code the Model 2460 must be in the TSP command set mode SCPI command is the default To change to the TSP command set press MENU and then Settings Press the Command Set button and select TSP Also make sure you are using the latest version of Test Script Builder which can be downloaded for free from the Keithley website Conclusion The Model 2450 or Model 2460 SourceMeter SMU Instrument is an ideal tool to perform charge and discharge cycle testing on rechargeable batteries because of its accurate four quadrant high power output and the ability to measure both current and voltage accurately U
8. ry testing can be simplified by using an instrument called a Source Measure Unit SMU or SourceMeter instrument A SourceMeter is a type of test instrument that is capable of sourcing and measuring both current and voltage Keithley s Model 2450 and Model 2460 SourceMeter SMU Instruments have the flexibility to source sink current as well as measure voltage and current making them perfect solutions for galvanic cycling of batteries By using the Model 2450 or Model 2460 the user only needs to set up a single unit instead of an entire rack full of equipment As a result the Model 2450 or Model 2460 can charge up the battery by sourcing current discharge the battery by dissipating power and monitor the battery s voltage and load current 100 V 1 05A 210 V 105 mA 21V 1 05 A 20 V 4 A 10 V 5A 7V 7A Output Table 1 Power envelope for Models 2450 and 2460 Battery Charging Discharging Rates for constant current charging and discharging are defined in terms of the battery s capacity which is the amount of electrical charge that the battery can store The capacity is specified in milliampere hours mAh available and should be expressed in terms of a discharge or load current The rate at which the discharge current will discharge the entire battery in one hour is known as the C rate For example a battery rated at 1000mAh will output 1000mA for one hour if discharged at 1C If a 500mAh cell is di
9. scharged at 50mA then it is discharged at one tenth the C rate 0 1C and therefore can source 50mA for ten hours Test Description For both the charging and discharging cycles the Model 2450 or Model 2460 SourceMeter SMU Instrument is configured to source voltage and measure current A simplified circuit diagram of both the charge and discharge cycles is shown in Figure 2 Discharge Cycle Force HI Charge Cycle Force Hl Force LO 2450 or 2460 SourceMeter Force LO 2450 or 2460 SourceMeter SourceMeter in Sink Mode Vs lt Vp SourceMeter functions as Electronic Load Discharge Current i is Negative SourceMeter in Source Mode Vs gt Vg SourceMeter functions as Power Supply Charge Current i is Positive Figure 2 Charge and discharge circuit diagrams A battery is usually charged using a constant current This is accomplished using the SourceMeter as a voltage source set to the voltage rating of the battery with the desired charging current set as the current limit At the start of the test the battery voltage is less than the voltage output setting of the SourceMeter As a result this voltage difference drives a current that is immediately limited to the user defined current limit When in current limit the SourceMeter is acting as a constant current source until it reaches the programmed voltage level As the battery becomes fully charged the current will decrease until it reaches zero or near zero To preven
10. sing a single instrument to perform battery testing simplifies test setup reduces programming time and saves rack space NOTE Please refer to the Model 2450 or 2460 User s Manual for safe operating instructions when charging and discharging batteries Appendix TSP code used to generate discharge characteristics of the 2500mAh battery This code discharges a 2500 mAH 1 2 V battery to 0 9 V with a discharge current of 2 5A 1C Prior to executing code reset instrument from front panel and set the Output Off state of the Model 2460 to the High Z Mode Insert USB drive to save readings clear the buffer defbufferl clear Measurement Settings smu measure func smu FUNC_DC_ CURRENT smu measure range 225 smu measure sense smu SENSE 4WIRE Source Settings smu source func smu FUNC_DC VOLTAGE smu source offmode smu OFFMODE HIGHZ smu source level 0 9 smu measure terminals smu TERMINALS REAR smu source range 0 9 smu source readback smu ON smu source ilimit level 2 5 Set the voltage limit for the battery to stop discharging Set the variable for number of iterations voltLimit 0 9001 iteration 1 Turn on the source output smu source output smu ON Change display to user screen display changescreen display SCREEN USER SWIPE Keep taking readings in the while loop until the measured voltage is equal to the voltage limit while true do Take a reading and get the current
11. t safety hazards or damage to the battery care must be taken not to overcharge the battery When discharging a battery the SourceMeter operates as a sink because it is dissipating power rather than sourcing it The voltage source of the SourceMeter is set to a lower level than the battery voltage The current limit sets the discharge rate When the output is enabled the current from the battery flows into the HI terminal of the SourceMeter As a result the current readings will be negative The discharge current should stay constant until the battery voltage decreases to the voltage source setting of the SourceMeter Making Connections to the Battery To set up the test the Model 2450 or Model 2460 SourceMeter SMU Instrument is connected to the battery as shown in Figure 3 A four wire or remote sense connection is made from the instrument terminals to the battery to eliminate the effects of the lead resistance This enables the battery voltage to be measured as close as possible to its terminals The Force HI and Sense HI output terminals of the SourceMeter are connected to the positive terminal of the battery and the Sense LO and Force LO outputs are connected to the negative terminal of the battery When the output of the SourceMeter SMU Instrument is turned off be sure that it is set to the High Impedance High Z Output Off State With High Z Output Off State selected the output relay opens when the output is turn
12. voltage and relative timestamp curr smu measure read defbuffer1 volt defbufferl sourcevalues iteration time defbufferl relativetimestamps iteration hours time 3600 Compare the measured voltage to the voltage limit Exit the loop if it is if volt lt voltLimit then break end Print the of completed cycles the voltage and the time for the iteration Display information on front panel print Completed Cycles iteration Voltage volt Time time l aO display settext display TEXT1 string format Voltage display settext display TEXT2 string format Current fV volt aa 2fA Time 2fHrs curr hours Il oe Increment the number of iterations and wait 10 seconds iteration iteration 1 delay 10 end Turn the output off when the voltage limit is reached smu source output smu OFF FileNumber file open usbl TestData csv file MODE WRITE file write FileNumber Current Voltage Seconds n Print out the measured values in a 4 column format print nIteration tCurrent tVoltage tTime n for 1 1 defbufferl n do print i defbufferl i defbufferl sourcevalues i defbufferl relativetimestamps i file write FileNumber string format g g g r n defbufferl readings i defbufferl sourcevalues i defbufferl relativetimestamps i end file close FileNumber Specifications are subject to change without notice All Keithley trademarks and trade
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