Current interrupt test

The current interrupt test is particularly easy to perform with single cells and small fuel cell stacks. With larger cells the switching of the higher currents can be problematic. Current interrupts and electrical impedance spectroscopy give us two powerful methods of finding the causes of fuel cell irreversibihties, and both methods are widely used. 

Typical results from three current interrupt tests are shown in Figures 3.10, 3.11, and 3.12. These three examples are shown because of the clear qualitative indication they give of the importance of the different types of voltage drop we have been describing. Because oscilloscopes do not show vertical Unes, the appearance is slightly different... 

Figure 3.10 Current interrupt test for a low-temperature, ambient pressure, hydrogen fuel cell. The ohmic and activation voltage drops are similar. (Time scale 0.2s/div , i = lOOmAcm . )...

Figure 3.11 Current interrupt test for a direct methanol fuel cell. There is a large activation overvoltage at both electrodes. As a result, the activation overvoltage is much greater than the ohmic, which is barely discernible. (Time scale 2s/div. / = 10mAcm. )...

Fuel cell circuit for current interrupt test consisting of load, on/off switch, and a transient recording device such as oscilloscope of data acquisition system (DAS). 

Figure 10. Typical short-circuit behavior of a 18650 lithium-ion cell with shutdown separator and without PTC (positive temperature coefficient) and CID (current interrupt device). This test simulates an external short circuit of a cell.

Repetitive tests gas chromatographic analysis, internal resistance evaluation with current interruption or impedance measurement methods, reference characteristic curves to control aging, and so on. 

Testing section by section can be continued in each direction from the temporary CP location until the changes in the observed currents and potentials as the current interrupter switches on and off are no longer large enough to result in accurate data. The limits of the area that can be maintained above the protected criterion of -0.85 V or better can be established at this same time. 

FIGURE 41.9 ASI vs. DOD for a 3-cm diameter cell with FeSj positive electrode. ASI determined from 30-second current interrupts at various discharge current rates and also from USABC peak power test (From... 

Designing and testing of protective devices, such as PTC (positive temperature coefficient) current limiting devices, CID (current interrupt device) which shuts off power based on internal pressure, and vent system. 

When there is no current flow there cannot be any IR drop. However, when a current is flowing IR drop is included in the measurement. At a certain time, t, the current is interrupted so that 1=0, hence IR is also zero. The test electrode, therefore, shows a potential free fi om IR drop. Hence current interruption provides a good method for measurement of IR free cell potential. Unfortunately, depolarization may also occur when the current is interrupted. If the system is depolarized it is too negative at the anode area and too positive at the cathode. Various types of commercial electronic interrupters are available. The current can be interrupted by as much as 10 milliseconds of each half second. The potential must be read quickly, at the instant of interruption, by a fast response voltmeter such as an oscilloscope. 

It also possible to obtain data that would give information on the current requirement while the coating resistance tests are being made. Such a setup is shown in Fig. 5.42. In the setup a current interrupter is provided (automatic switch). The switch is on position for 30 seconds and in off position for 15 seconds. The data is taken at... 

This test is conducted to verify the suitability of the equipment to withstand a prospective short-circuit current that may develop on a fault. It may also be termed the steady slate symmetrical fault current or the short-time (withstand current) rating of the equipment. When the equipment is an interrupting device, it is referred to as its symmetrical breaking current. 

To isolate the circuit after the lest is over and to also interrupt the test in between, in case the test piece fails. The breaker must possess an instantaneous capacity of more than the test current and the short-circuit MVA of the feeding generator. To achieve the desired voltage it must be suitable to perform the duties of repeated short-circuit tests. 

By the immediate first current zero it is assumed that the contacts have travelled sufficiently apart to achieve the required deionization and have built up adequate dielectric strength to withstand at least 0.95 V, . If the circuit does not interrupt at the immediate current zero at a which is so near to the point of chopping Vt , the interruption will take place only by the next current zero at point h and result in another 260 strikes by then. To study more accurate behaviour of an intenupter, with the number of restrikes and the formation of the actual transient voltage waveforms on current chopping, oscillograms similar to those during a short-circuit test may be obtained (Section 14.3.6). 

---