Electricity: battery or capacitor charging or discharging – One cell or battery charges another
Reexamination Certificate
2001-08-22
2003-05-06
Toatley, Jr., Gregory J. (Department: 2838)
Electricity: battery or capacitor charging or discharging
One cell or battery charges another
C307S009100, C318S139000
Reexamination Certificate
active
06559621
ABSTRACT:
TECHNICAL FIELD
This application relates to a system and method for improving the performance of a hybrid power supply apparatus comprising a power generating device, such as a fuel cell system, and an energy storage device, such as a battery. The purpose of the invention is to provide an equalization charge to the battery from a source other than the power generating device when the battery achieves a predetermined state of charge condition, thereby avoiding the need to operate the power generating device in a low power output mode.
BACKGROUND
Hybrid power supply systems comprising a power generating device and an energy storage device are well known in the prior art. In recent years interest has grown in using fuel cells as power generating devices in such hybrid systems. The fuel cell is used to charge a storage battery which in turn supplies power to a load on an “on-demand” basis. Alternatively, the fuel cell and the battery may jointly supply power to the load depending upon the power requirements.
Many fuel cell systems include fuels processors such as reformers for converting conventional fuels to hydrogen-enriched gas for processing by the fuel cell. In general, the combination of a fuel cell and a reformer makes it difficult for the power generating device to respond quickly to variations in external load since the response time of the reformer is slow. This is particularly the case for loads that fluctuate substantially over time. For example, electric lift vehicles have a pattern of power usage or “duty cycle” which is characterized by loads which fluctuate substantially during the course of a work shift. Hybrid power supply systems offer several advantages in such applications. The addition of a charged energy storage means enables the hybrid system to respond quickly to power demand surges, while maintaining the advantages of a fuel cell system, including extended operating times, low emissions and the flexibility to utilize many readily available fuels. Further, in hybrid systems the size of the power generating device may be minimized to enhance system efficiency and reduce cost.
Hybrid power supply systems are known in the prior art for use in applications subject to sudden load fluctuations. U.S. Pat. No. 4,883,724, Yamamoto, issued Nov. 28, 1989 relates to a control unit for a fuel cell generating system which varies the output of the fuel cell depending upon the state of charge of the battery. In particular, a DC/DC converter is connected between the output of the fuel cell and the battery and is responsive to a control signal produced by a controller. The purpose of the Yamamoto invention is to ensure that the storage battery is charged for recovery within the shortest possible time to reach a target remaining charge capacity under charging conditions that do not cause deterioration of performance of the battery. When the charged quantity of the battery is recovered to the target value, the controller lowers the output of the fuel cell. In the case of no external load, such as during extended interruptions in the operation of the lift truck, the fuel cell is controlled to shut-down after the storage battery is fully charged.
One limitation of the Yamamoto system is that the control algorithm is designed for prolonging the useful life of the storage battery rather than the fuel cell. By varying the fuel cell output to charge the storage battery for recovery within the shortest possible time, the long-term performance of the fuel cell is compromised. For example, frequent changes in fuel cell power output degrade the performance and lifetime of the fuel cell. Further, depending upon the state of charge of the battery, the fuel cell (and hence the fuel processor) may operate for extended periods in a low power output mode which is undesirable.
U.S. Pat. No. 4,839,574, Takabayashi, also discloses a generator system utilizing a fuel cell and reformer. Depending upon the state of charge of the battery the output of the fuel cell may be adjusted in a stepwise fashion. In the Takabayashi system the amount of raw material supplied to the reformer is maintained constant within a range of charged energy to ensure stable operation of the reformer. However, depending upon the state of charge of the battery, the fuel cell and the reformer may once again operate for extended periods in a low output mode.
Typically, using sealed lead acid batteries as an example, a constant voltage charge method is the preferred means for charging the battery cells. Under a constant voltage regime inrush currents are limited by the internal resistance of the battery. Thus, when the battery is in a low state of charge and the internal resistance is low, inrush currents can be very large and energy can be restored to the battery very quickly. As the battery becomes charged, it reaches a transition point where a sudden rise in internal resistance occurs and, under constant voltage, the battery will accept less and less current. This self-regulating effect prevents overcharging of the battery, leading to longer battery lifetimes. Typically such constant voltage charge regimes are conducted at the “float” or “equalization” voltage which is the recommended voltage at which the batteries can be maintained at high states of charge.
Similarly, other advanced batteries such as Nickel Metal Hydride (NiMH) batteries may be charged at higher currents in a constant voltage or constant current regime until a transition point occurs. After this transition a low current equalization charge is required to return the complete capacity of the battery and to ensure that the individual cells within the battery are brought to an approximately equal charge state.
Most of the charging and discharging of the battery is done in the “bulk region” below the transition point. Once the battery reaches the transition point or charge threshold (e.g. at approximately 80% of its total charge capacity), most prior art systems step down the output of the fuel cell and operate the fuel cell in a low power mode until the battery is fully charged (assuming the absence of an external load). In other words, a reduced current equalization charge is provided by the fuel cell to the battery in the “absorption” and “float” regions of the battery above the transition point. Although it is necessary to return the battery to a fully charged state to ensure sufficient capacity to meet future load demands, it is not desirable to operate the fuel cell and fuel processor in low power output modes for extended periods of time to provide such an equalization charge.
A need has therefore arisen for a system and method for providing an equalization charge to a storage battery in a hybrid system from a source other than the fuel cell once the battery achieves a predetermined state of charge condition, such as a threshold state of charge for a predetermined period of time.
SUMMARY OF INVENTION
In accordance with the invention, a hybrid power supply system for supplying electrical power to a load is disclosed. The system includes a power generating device, such as a fuel cell system; a first energy storage device chargeable by the power generating device and electrically connectable to the load; a first detector for measuring the state of charge of the first energy storage device; a second energy storage device chargeable by the power generating device and electrically connectable to the first energy storage device; and a controller for receiving input from the first detector. The controller causes the second energy storage device to provide an equalization current to the first energy storage device when a first predetermined state of charge condition is detected by the controller. For example, the first predetermined state of charge condition may occur when the measured state of charge of the first energy storage device exceeds a first threshold value for a predetermined amount of time.
The power generating device is operable in alternating charging and shut-down or non-charging modes. The controller causes the power generating device to switch
Corless Adrian J.
Leboe David
Cellex Power Products, Inc.
Oyen Wiggs Green & Mutala
Toatley , Jr. Gregory J.
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