Testing and evaluating batteries in situ

Transcription

1 Testing and evaluating batteries in situ Tony Schröer, Dieter Brockel Digatron Industrie-Elektronik GmbH, Tempelhofer Str , Aachen, Germany Abstract A new generation of test equipment has been developed based on the latest computer and power electronic technology. The basic construction and the performance characteristics for a new standard Multiple Battery Tester are described. The new modular construction allows a variety of different test circuits to be arranged in a powerful test system. The latest semiconductor technology has been incorporated into the system yielding significant advantages, such as a 60 % reduction in the space required. Each test circuit is equipped with a microprocessor interface for control. An additional signal processor is offered as an option, if required. This allows for an inexpensive version for simple tests, as well as an advanced version for sophisticated technical applications such as pulse charging in the micro second range. A demonstration of the program editor is given using a typical test procedure. The new highly efficient operators for evaluation of the measuring data are discussed. 1. Introduction The continuous development of NiCd, NiMH and lithium batteries as well as of Pb-based batteries, their growing market share and the increasing use of such battery systems made it necessary to develop appropriate test units that guarantee highest possible precision and speed. Apart from complying with these specifications, the manufacturer of such test units has no easy task to fulfill in finding a customized solution at acceptable costs and with reasonable time of delivery. The only way to meet all these requirements is a modular design of the test system. of many types of cells including NiCd, NiMH, Lithium and Pb based cells. The system is rated for individual cells, grouped cells with cell drop out, cell packages or entire batteries. The MBT system is configured to the customer s requirements for voltage range, current range and number of circuits. Each circuit is individually controlled, maximizing the system s flexibility and cost-effectiveness. MBT operates with the proven, widely used Digatron/Firing Circuits BTS-600 software. The flexible, high performance Digatron MBT test system is designed for testing and grading testinginsitu.p65 8/01

2 2. General design of the new MBT test system The MBT is modular in design and is rackmounted in a system cabinet. Modules are used for basic elements, such as power supply, control unit or power circuit which are connected with a few components only, so that a customized solution can easily be achieved. This offers more flexibility for the entire system and allows for combinations of test circuits with different nominal data in one system. Another advantage of the modular construction is the option to exchange modules at a later time for higher or lower currents or voltages without having to vary the system configuration, as the dimensions and pin compatibility can remain unchanged. 3. Customized MBT configuration The following table (fig.1) shows a variety of standard modules that may be combined to an individual test system. Fig.1: MBT configuration

3 4. How to operate lots of circuits The circuits operate with the proven, widely used Digatron/Firing Circuits BTS-600 host computer software. BTS-600 runs under Windows 3.11, 95 and NT, is prepared for network and multi-user connection and can be supplied in future to store data on a relational data base. To make tests realistic, cells or batteries can be placed into a temperature chamber. A task (subprogram running in parallel) in the test program controls the temperature curve via an interface (e.g. RS232 ) The BTS-600 software can also communicate with battery management systems via CAN bus or with Smart Batteries via an I 2 C-interface. Smart battery and CAN bus data can be used to control the test program run or for comparison with other measured values. Cycles within cycles make it easy to run any complex test against a range of current, voltage, power and resistance parameters. The x.328 version of BTS-600 allows for the creation of customized functions and their integration into the overall test program. Digatron offers a detailed programming manual for experienced users to create their own extensions in Turbo Pascal. Less experienced customers, however, should entrust these changes to Digatron software specialists. A number of special features have been included to make the software more user-friendly: The software can be used for almost any application in the battery field. When used in conjunction with appropriate hardware - i.e. test units and other peripherals - testing can be performed as desired, in compliance with standards such as pulse profiles, FUDS cycle, standard cycles and customized specifications with a single test program. Single cells, packs or entire batteries can be tested as required. Due to high speed measuring the voltages of single cells or packs are achieved. Each of those values can interrupt the test program at any time when e.g. a certain voltage, temperature or a gradient thereof is reached. In the same way that external limits can influence the test program run via digital inputs, outputs can be activated by the program to switch on or off fans, valves or load contactors to drop out single cells. Programming of global limits or nominal values for a whole test program or for just one or several test steps Data exchange between different circuits and the respective measuring channels Programming with the battery data or parameters Use of logical channels (i.e. calculated values, such as Ah-CHA, Ah-DCH) in the program Creation of test processes with values from ASCII-tables Prints of customized cycle reports during lifecycle test program run Graphs with depth of discharge vs. cycles, top of charge vs. cycles, capacity vs. cycles depth of discharge vs. top of charge Editor to create FUDS cycle or any other pulse profile In the following the powerful pulse editor (GSM editor) will be explained with a practical program example. The test results are documented by oscilloscope wave forms.

4 5. Pulsed-current test program example using the BTS-600 GSM editor The following example shows the testing of batteries with a pulsed-current test program. As, in particular, cellular phone batteries require the shortest pulse widths, the GSM editor will be presented with the example of a cellular phone battery. It is, of course, also possible to test bigger cells or batteries with higher currents in the same way. In our example a discharge is performed in accordance with the GSM standard (Global Standard for Mobile telecommunication). A standard discharge profile is shown in fig. 4. However, when the test program is executed, the limits of a conventional program editor will soon become evident. To perform the change of step in a program, the microprocessor systems needs a great number of command cycles and subroutines, such as registration of measuring values, refreshing of Ah counters or calculations. The time required for all these activities will soon reach a level that may take the dimension of an entire GSM pulse. If you leave out the time-consuming changes of step, the performance of the CPU will be reduced to the setting and measuring of signals with maximum possible speed. But how can different pulses in one step then be realized? The answer is shown in fig.6 and 7. At first we modify the conventional program (fig.5) by replacing the DCH command in steps 3 and 4 by the PROFILE command. The profile name is user-specific. In our example the profile name is GSM. The execution time of the profile shall be 3 h. Fig.4: GSM profile The following example (fig.6) shows the modified program that simulates the real GSM profile as displayed in fig. 4. At the first sight it may look as if the profile could as well be established with already available test programs and their program editors (fig.5). Fig.6: GSM Test Program using GSM Editor Enter the name of the PROFILE file in the column Nominal Value and you are free to enter limits. Fig.5: GSM Test Program using conventional program editor The registration for the PROFILE operator behaves somewhat different from all other operators. It is possible to perform a registration of a sample pulse at a given time interval.

5 Besides the registration on the sample pulse other important values like Ah and high and low voltage can be registered as well. The registration column should include the sample time and one or two registration formats. If only one format is specified only the values specified in this format will be stored at the specified sample time interval. No sample pulse will be captured. If two formats are specified then a sample pulse will be captured together with all values defined in the first registration format entered. The values specified in the second format entered will be stored only once every sample time period. defined pulse. Our example shows a program with a current pulse of 0.3 A for a duration of 4.4 ms and a current pulse of 1.5 A for a duration of 0.6 ms. The pulse pattern shall be terminated when a voltage of <2.5 V is reached. In contrast to the normal BTS program, the profile is loaded into an additional signal processor. The signal processor handles only the GSM editor, independent from the main CPU. This procedure allows a minimum pulse with of 100 µs. The slew rate is between 4 and 7 µs. Fig.8 shows the oscilloscope wave form of the entire profile. For a better outline of the slew rate and the regulator accuracy the profile is represented in fig.9 with a time scale of 100 µs/ Div. Fig. 7: GSM editor The PROFILE operator is based on a nominal value table (fig.7) with time values and current values for the functions charge or discharge and allows direct editing of the current and time values. You may enter up to 16 individual duty cycles, a voltage limit and/or the maximum number of repeats. The editor also allows a definition of a high and low voltage. One step out of the profile can be defined as the high voltage step and a second one as a low voltage step. The difference between high and low voltage is calculated during the run time of the profile and stored in the variable V_Drop. The high voltage and low voltage points are stored into V_High and V_Low. These points define an envelope of the Fig.8: Scope wave form GSM-profile, 1ms/Div Fig.9: Scope wave form GSM profile, 100µs/Div

6 The registration of the GSM profile in the BTS program is made in accordance with the entered registration conditions. 6. Pulsed-current test results For an overall representation of the test, the measuring values V_Low, V_High and V_Drop are displayed (fig.10). 7. Life cycle test program example using FILE, REG and PROT commands Now some useful commands are presented that - particularly for life cycle test over several months -make an end to the time wasting evaluation with additional auxiliary programs. Already at the beginning measured values and calculated values are defined and directed into different measuring data files. It is possible to define in the test program the conditions when a protocol printout is required in the course of the test. Waiting until the end of a test is a matter only for users of other battery test systems. The operators FILE, REG and PROT are explained with the example of a program in fig. 12. Fig.10: BTS measuring data file, V_Low, V_High, V_Drop V_Low is shown by the lower part of the curve. V_High is shown be the upper part of the curve. V_Drop shows the difference between the upper and lower voltage curves. Depending on the registration condition, a file may contain one or several sample pulses. A sample pulse may be registered, for example, for each cycle, after each n-th cycle or at any random time to be programmed (fig.11). Step Label Procedure Nominal Value Limit Action Registration 1 SET STANDARD 2 DCH 0.5 ACN1 V < CutOff 0.05 V 3 BEG 4 BEG Life 5 FILE Life 6 SET AhCha = 0.0 AhDch = CHA 0.5 ACN5 3 sec & 10 min 2.35 VNC < 0.1 ACN5 0.5 A 8 CHA 0.1 ACN5 0:30 h 0.02 V 10 min 9 SET V_TOC = V 10 DCH 2.0 ACN5 V < CutOff 0.03 V 11 REG AhResult AhResult 12 CYC 50* 13 PROT 14 CYC 10* 15 STO Fig.12: Life cycle test program Fig.12: life cycle test program For a life cycle test it might be useful to save the measuring data registered during this program run in files with different names according to their cycles. This can be done by the entry of the operator FILE and the desired file name into the Nominal Value column. Under the file name Life a number of data files will be generated according to the cycle number. Fig.11: BTS measuring data file, sample pulse It may be of interest to take the TOC voltage after each charge step. This is done with V_TOC = V in step 9.

7 The REG operator can be used to register charge Ah-counter and discharge Ah-counter once after each cycle. Both Ah-counter are saved into the AhResult registration format. The PROT command in Step 13 can activate any other program on the PC which is predefined in BTS-600. Those programs can be statistics or customized report prints. In our case it will print the standard Digatron report after each 50-th cycle 8. Life cycle test results For an overall representation of the test, all cycles are represented in a common graph (fig.13). Fig.15:BTS measuring data file, AhD, AhC, V_TOC vs. cycles The discharged Ah over the charge Ah are represented in fig.16. Fig.13: BTS measuring data file, all cycles Due to the FILE operator it is possible to compare different cycles directly. Our example shows an overlay display of cycles 1, 10 and 20. A sequential display is also possible. Fig.16:BTS measuring data file, AhD vs.ahc 9. Conclusions To allow research and development to test energy accumulators under realistic conditions, the test equipment must be tailored to the actual requirements. The previous introduction of Digatrons new MBT test system outlines a flexible and cost-effective approach for testing NiCd, NiMH, Lithium, Pbbased cells or entire batteries. Fig.14: BTS measuring data file, cycles 1,10, 20 overlaid Due to the AhRES registration format it is possible to have the Ah counter for charge, the Ah counter for discharge and the TOC voltage represented in a common graph over the cycles (fig.15). This lecture does not include an all encompassing list of all features, but a sample of the essential capabilities. The functions described here are particularly those that will save the user time consuming painstaking work in the evaluation of his measuring data files. The high precision of the hardware and the flexibility in the creation of test profiles make the MBT a universal test system that fully meets today s as well as future requirements.