0.0 LABORATORY DEMONSTATION - INTERNAL BATTERY RESISTANCE
0.1 INTRODUCTION This laboratory is used to demonstrate laboratory instruments, experimentation and laboratory reports. A battery is normally considered an ideal voltage source, however it does have an internal resistance which is measurable. The resulting schematic of a realistic battery is shown in Figure 1(a). The test circuit for measuring that circuit is shown in Figure 1(b).
FIGURE 1A TEST FOR INTERNAL BATTERY RESISTANCE
FIGURE 1B TEST FOR WAVEFORMS 0.2 TEST PROCEDURE 0.2.1 MEASURE THE BATTERY VOLTAGE WITH THE SWITCH OPEN(Voc). 0.2.2 MEASURE THE BATTERY VOLTAGE WITH THE SWITCH CLOSED(VL). 0.2.3 COMPUTE THE INTERNAL BATTERY RESISTANCE(Rint). 0.2.4 CONNECT AN OSCILLOSCOPE ACROSS THE BATTERY. 0.2.5 SET THE SCOPE SO THAT ITS SWEEP IS AT 1 CM/SEC 0.2.6 OPEN AND CLOSE THE SWITCH AND OBSERVE THE WAVE FORM ON THE SCOPE 0.3 THINGS TO THINK ABOUT 0.3.1 IS THE BATTERY RESISTANCE SOMETHING TO WORRY ABOUT UNDER CHANGING LOADS? WHY? 0.3.2 WHAT IS THE PERCENTAGE REGULATION DUE TO THE BATTERY RESISTANCE? 0.4 A SAMPLE LABORATORY REPORT IS BELOW NOTE: THIS IS A VERY SIMPLE EXPERIMENT, WHICH RESULTS IN A VERYSIMPLE LAB REPORT. AS THE EXPERIMENTS BECOME MORE COMPLICATED, THELAB REPORTS WILL BECOME MORE EXTENSIVE. REORDER/REWRITE THIS LAB REPORT TO CONFORM TO THE NEW LAB INSTRUCTIONS LABORATORY EXPERIMENT 0 THE MEASUREMENT OF INTERNAL BATTERY RESISTANCE BY DR. H. C. RAWICZ WITH GROUP A CONSISTING OF: 1. DR. H. C. RAWICZ 2._________________ ETC. DATE OF EXPERIMENT ABSTRACT: The internal impedance of an Eveready Energizer 1.5 volt AA size battery was measured to be 0.423 ohms. A 10 ohm resister was used as a test load and the battery voltage was measured with and without the load. It took four measurement sequences before the results were consistent. This was probably because the batteries hadn't been used for a long time. The results were within the expected range of ohmic resistance for a dry cell. INTRODUCTION: Batteries are used in all portable equipment where it is impractical to connect that equipment to a generator. The use of batteries require knowledge of the internal battery characteristics to determine if the battery is appropriate for the application. Some of the battery characteristics are: 1. Maximum current drain 2. Terminal voltage range, fully charged(new) to discharged (end of life) 3. Total power drain 4. Internal impedance The maximum current drain is required to insure that the battery can supply the instantaneous current required for an application. The terminal voltage range is required to insure that the equipment will operate properly over the life of the battery. The terminal voltage range can also be used as a monitor to detect imminent end of battery life(e.g. in smoke detectors). Total power drain is required to determine how long an equipment can operate before the battery has to be changed. And finally, the internal impedance will define how much heat is generated in the battery. This will define how much cooling will be required(if any). In addition, the internal impedance has to be accounted for in some circuit applications(e.g. voltage standards). The battery characteristics can usually be obtained from the battery manufacturer, but the internal impedance changes over the life of the battery. In this experiment the internal impedance of a drycell battery will be measured. EQUIPMENT USED 1. 1.5 VOLT DRY CELL EVEREADY BATTERY COMPANY, ENERGIZER, SIZE AA 2. A 10 OHM RESISTER 0.5 WATT, 5% TOLERANCE 3. A MULTIMETER KEITHLEY MODEL 169 SERIAL NUMBER 298909 2 VOLT SCALE NO CALIBRATION DATE 4. A UTILITY BREADBOARD/WIRING 5. AN OSCILLOSCOPE TEKTRONICS MODEL 2215 SERIAL NUMBER 7 NO CALIBRATION DATE 6. AN OSCILLOSCOPE PROBE TEST PROBES INC. 1:1 TEST SCHEMATIC DIAGRAM
FIGURE 1 TEST FOR INTERNAL BATTERY RESISTANCE
FIGURE 2 TEST FOR WAVEFORMS TEST PROCEDURE 1. CONNECT THE BATTERY AND RESISTOR AS SHOWN IN FIGURE 1 2. CONNECT A WIRE AS SHOWN SO THAT THE RESISTOR CAN BE CONNECTED ACROSS THE BATTERY WHEN DESIRED. 3. CONNECT A MULTIMENTER ACROSS THE BATTERY AS SHOWN IN FIGURE 1 4. SET THE METER TO THE 2 VOLT SCALE AND RECORD THE VOLTAGE. 5. CONNECT THE RESISTOR ACROSS THE BATTERY(CLOSE SWITCH) AND RECORD THE VOLTAGE ACROSS THE BATTERY. 6. DISCONNECT THE RESISTOR FROM ACROSS THE BATTERY(OPEN SWITCH), REPEAT 4 AND 5 UNTIL CONSISTENT RESULTS ARE OBTAINED. 7. USING THE EQUATION IN FIGURE 1 COMPUTE THE INTERNAL RESISTANCE. 8. DISCONNECT THE METER. 9. CONNECT THE OSCILLOSCOPE ACROSS THE BATTERY AS SHOWN IN FIGURE 2 USING A 1:1 PROBE. SETTINGS: A. SET THE HORIZONTAL SCALE TO THE .1 SECONDS/CM B. SET THE VERTICAL SCALE TO 0.5 VOLTS/CM C. TRIGGER INTERNAL D. TRIGGER SOURCE INTERNAL E. HORIZONTAL MODE A F. TRIGGER SLOPE + 12. ADJUST THE HORIZONTAL TRIGGER UNTIL A DOT SLOWLY AND REPEATEDLY GOES ACROSS THE DISPLAY. 13. WHEN THE SWEEP IS ABOUT AT THE MIDDLE OF THE DISPLAY, RECONNECT THE RESISTANCE(CLOSE THE SWITCH). 14. ADJUST THE VERTICAL SCALE AND VERTICAL POSITION UNTIL A VERTICAL CHANGE IS MEASURABLE ON THE SCREEN OF THE OSCILLOSCOPE. 15. DRAW THE RESULTING WAVE FORM. TEST DATA SHEET: TRY #1: OPEN VOLTAGE=1.607V CLOSED VOLTAGE=1.535V TRY #2: OPEN VOLTAGE=1.602V CLOSED VOLTAGE=1.535V TRY #3: OPEN VOLTAGE=1.600V CLOSED VOLTAGE=1.535V TRY #4: OPEN VOLTAGE=1.600V CLOSED VOLTAGE=1.535V OSCILLOSCOPE TRACE
SAMPLE EQUATIONS AND CALCULATIONS: Rint=R((Voc/VL) - 1 ) TRY 1: Rint=10((1.607V/1.535) - 1 ) = 0.469 OHMS TRY 2: = 0.436 OHMS TRY 3: = 0.423 OHMS TRY 4: = 0.423 OHMS RESULTS: The internal impedance of an Eveready Energizer dry cell battery wasmeasured. The impedance measurement ranged from 0.469 ohms to 0.423ohms, constantly decreasing in value, but settling at 0.423 ohms. DISCUSSION OF RESULTS The no load battery voltage decreased as the experiment progressed. The battery was a brand new one and may have taken time to reach a steady state open circuit voltage condition after being quiescent for an extended period of time. The internal impedance of 0.423ohms was within the expected range of 0.05 to 100 ohms. CONCLUSIONS: The Eveready Energizer AA size battery that was tested has an internal impedance of 0.423 ohms after it reaches a steady state condition. The battery is of intermediate quality since it was much better than the 100 ohms, probably expected near end of life, and was not as good as the .05 ohm lower impedance limit.