Electricity: electrical systems and devices – Safety and protection of systems and devices – With specific current responsive fault sensor
Reexamination Certificate
2001-08-31
2004-02-24
Huynh, Kim (Department: 2182)
Electricity: electrical systems and devices
Safety and protection of systems and devices
With specific current responsive fault sensor
C361S093100, C323S909000
Reexamination Certificate
active
06697245
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the field of testing equipment, and, more particularly, to a method and apparatus for applying a variable load to electrochemical energy conversion and storage devices, such as fuel cells and batteries.
BACKGROUND OF THE INVENTION
Known equipment for the testing of electrochemical energy generating and storage devices, such as fuel cells and batteries, suffer from one or more of several drawbacks. For example, known testing systems provide inadequate control of the amount of energy consumed in a load under test coupled to such an energy source. The present invention provides a resistive load used to controllably consume the electrical energy produced or stored by an electrochemical device.
The simplest known system, now rarely used, provides a network of switches and resistors as a test load. Each resistor is in series with a switch, and each resistor-switch pair is in parallel with all of the others. As each switch is closed, the total resistance of the network drops. The resistors may be identical in resistance, or they may provide different resistances to allow finer control of the total resistance of the load. Such a system includes a large number of mechanical contacts brought to the circuit with the switches. Each mechanical contact adds a varied and changing resistance to the network. In a large network, consisting of a dozen or more switches, the network resistance will seldom be exactly the same twice. The unavoidable mechanical wear that occurs during use of such a system only adds to the problem by changing the resistance of different switches differently. Such a system may be improved by the addition of a variable potentiometer in parallel with the switches, but this only replaces one set of mechanical components for another, with all its inherent drawbacks.
The test array just described may be improved by replacing the switches and resistors with field effect transistors (FETs). An FET is a voltage controlled device having a linear region of its operating characteristic and a saturated region at higher input voltages. In the linear region, a FET emulates a resistor having a resistance that can be varied over a wide range through the application of a bias voltage to one terminal of the device. In its simplest form, an FET-based electronic load, which generally consists of a set of FETs mounted in parallel, is controlled by manually adjusting the gate voltage to produce the desired current flow through the system. This system is a distinct improvement on a resistor network. It eliminates the switch contact problems and provides virtually infinite variability in resistance over the available load range. However, such a system requires manual operation to change the output of the system including periodic readjustments to offset variations produced by thermal effects in the circuit or variations in the performance of the source under test.
The circuit can be stabilized by the addition of analog circuitry tuned to measure and compensate for the external changes. This circuit can be sufficiently complex to hold a constant current through changes in the performance of the device under test and to provide for external control of the load via an analog voltage supplied from an external source. A load of this type permits automation of the test system, however it is relatively complex. The large number of components involved increases the difficulty in fabrication and increases the chance of at least one component malfunctioning. The difficulty in fabrication and the number of components make this system expensive to produce and difficult to maintain.
Commercially available electronic loads such as those produced by Hewlett Packard, have been used as loads for fuel cell and battery testing. While they are a distinct improvement on the loads described above, they are still deficient in several respects. They typically have maximum current capabilities on the order of 120 amps. While this is adequate for the uses they were designed for, it is insufficient for a high power density fuel cell. A 50 cm
2
fuel cell operating at 3 A/cm
2
puts out 150 Amps, and some systems are routinely operated at up to 4 A/cm
2
, while many developers use cells with larger areas. These units also have a second deficiency when used to test either single cell fuel cells or batteries. Since they were not designed for fuel cell testing or single cell battery testing, they were designed to operate with a typical minimum potential of 3 Volts. If operated at lower voltages, the maximum current capacity is substantially reduced. Most fuel cells reach their maximum power output at around 0.6 Volts, and at this voltage a 120 Amp load's capacity is reduced to about 40 Amps. This can be overcome by placing a power supply in series with the fuel cell or battery being tested to boost the voltage. While this works, there is some risk involved. For instance, when the device under test is a fuel cell, if the gas supply to a fuel cell is cut off, the cell voltage can go to zero. Since the power supply is still applying a voltage, it is possible for the cell to be forced into reverse and become an electrolyzer. In this mode the cell will generate hydrogen in the compartment that had previously contained oxygen, and oxygen in the one that had contained hydrogen. This can form an explosive mixture in either compartment. This is a situation that is to be avoided.
Still another type of electronic load can be constructed using a high speed chopper to turn the current through a fixed resistor on and off at high frequency (>1000 Hz). When the chopper is on, the current flows at the same level as if the resistor were connected directly to the fuel cell. When the chopper is off the current flow is zero. The duty cycle of the chopper is adjusted so that the total coulomb flux each second is the same as it would be through a fixed resistance larger than that of the actual resistor. One problem with this approach is that a fuel cell responds quite quickly to changes in resistance and tries to track the load. This can bias the results obtained in two ways. One of these is that a fuels cell's performance in a cycling system is not always the same as in a steady state system, even if the two systems have the same time averaged performance. Many types of batteries also show better performance under an intermittent load than a steady one and produce an overly optimistic result when tested under these conditions. The other bias is not in the cell, but in the measurement of the cell voltage under load with a fluctuating load. If the voltmeter being used doesn't average the voltage correctly at the chopper frequency being used the results will be incorrect. For example, a DC voltmeter used to measure a high frequency AC voltage will give a reading greater than the true root mean square (RMS) value, and this will lead to an overly optimistic and inaccurate result.
U.S. Pat. No. 5,512,831 (Cisar) teaches a controlled resistive load consisting of field effect transistors (FETs) operated in the linear region allowing them to function simply as a variable resistor. To increase the power dissipating or current carrying capacity of the overall load, a number of these matched FETs are placed electronically in parallel such that they share the load uniformly. Both the control signal and the applied current are connected in parallel, allowing the multitude of FETs to be controlled in unison by a single external electronic command.
In addition to a conventional FET based load, Cisar further adds the feature that the transistors are placed in a computer controlled feedback loop, in this feedback loop, a command signal is used to place the FETs in a predetermined location in their linear region where they will have a known resistance. The overall current through the electronic load is then measured and this signal digitized and sent back to the controlling computer. The controlling computer then trims or adjusts the FET control signal so that the measured curren
Christian Steven L.
Huynh Kim
Lynntech Inc.
Streets Jeffrey L.
Streets & Steele
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