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Bidirectional power converter for balancing state of charge among series connected electrical energy storage units

Bidirectional power converter for balancing state of charge among series connected electrical energy storage units description/claims

This application claims the benefit of U.S. Provisional Application No. 60/464,391, filed Apr. 18, 2003 and U.S. Provisional Application No. 60/428,666, filed Nov. 25, 2002. The entire teachings of the above applications are incorporated herein by reference.

Hybrid electric vehicles (HEVs) combine the internal combustion engine of a conventional vehicle with the battery and electric motor of an electric vehicle. This combination offers the driving range and rapid refueling features to which consumers are accustomed with conventional vehicles, while achieving improved fuel economy and lower emissions.

Typical HEV designs such as those already on the market and those which will be introduced shortly, are so-called “parallel” configurations. In the parallel HEV, the battery powered motor is principally used to boost engine torque for hill climbing and high acceleration demands. When boost torque is not required, the engine drives the electric motor as a generator to recharge the battery. The motor is also driven as a generator during braking events, thus, relieving thermal loading of the conventional friction brakes and enabling recovery of vehicle kinetic and potential energy which is returned to the battery.

The battery “pack” of a typical HEV consists of one or more “modules” of series-connected “cells.” Nickel cadmium (NiCad) and nickel metal hydride (NiMH) cells have been successfully employed in recently introduced HEVs while higher performance lithium ion (Li-ion) cells are envisioned for future generation designs.

Desirable attributes of battery cells for HEV applications are high-peak specific power, high specific energy at pulse power, fast charge acceptance to maximize regenerative braking utilization, and long calendar and cycle life. Achieving a favorable HEV battery pack lifetime requires some means to monitor cell state of charge (SOC) and control of cell charging and discharging to assure that all cells in the pack are well “balanced” or “equalized,” for example, at a nominally uniform state of charge. The development of means to achieve affordable and reliable balanced cell operation, especially for newer Li-ion cells, has presented significant technical challenges Lithium ion batteries are now widely used in laptop computer and cell-phone products because of their high specific energy. They also have high specific power, high energy efficiency, good high-temperature performance, and low self-discharge.

Components of lithium ion batteries could also be recycled. These characteristics make lithium ion batteries desirable for HEV applications. However, to make them commercially viable for HEVs, further development is needed to improve calendar and cycle life and cost.

If lithium-ion batteries are to be successfully employed in HEV applications, the state of charge of individual battery cells will need to be continuously balanced to maintain a high cell calendar life and cell capacity. Cells must have their state of charge equalized toward a target state of charge so they uniformly support HEV operation. Furthermore, care must be taken to assure that an individual cell is not charged beyond its safe limit. State of charge may be determined from an open circuit cell voltage measurement, or under load, from a measurement of cell voltage combined with cell impedance and current.

Despite the performance advantages of lithium-ion battery technology, there is a cost tradeoff associated with increased complexity of the controls required to equalize the battery state of charge. The achievement of an affordable solution is particularly challenging in the case of very long high voltage series strings of cells required for an HEV.

According to one aspect of the invention, a bidirectional power converter is provided for balancing state of charge among series connected electrical energy storage units. Cell chemistries other than lithium-ion, such as nickel-cadmium, lead-acid and nickel metal hydride, may also benefit from embodiments of the bi-directional power converter.

According to one embodiment, a system for balancing state of charge among plural series connected electrical energy storage units includes a power converter that selectively couples to an individual storage unit of the a string of electrical energy storage units and transfers energy bidirectionally between the individual storage unit and the string of storage units.

In particular embodiments, the power converter can transfer energy at a controllable rate of transfer. The power converter can monitor voltage and current data of the individual storage unit resulting from the transfer of energy. The power converter can transfer units of energy between the individual storage unit and the string of storage units.

In particular embodiments for transferring energy from the individual storage unit to the string of storage units, the power converter includes (i) a primary inductor; (ii) a first secondary inductor magnetically coupled to the primary inductor; and (iii) a first switch selectively coupling the individual storage unit to the primary inductor. The first secondary inductor further couples to an output capacitor that is coupled in parallel to the string of storage units. When the first switch is on, energy is transferred from the individual storage unit to charge the primary inductor. When the first switch is off, the energy is discharged into the first secondary inductor to charge the output capacitor, which discharges the energy to the string of storage units.

In particular embodiments, the system can include a first pulse generator that provides first enable signals to the first switch. The first switch couples the individual storage unit to the primary inductor in response to the first enable signals, resulting in energy being transferred from the individual storage unit to the string of storage units.

The system can also include a second pulse generator that provides second enable signals to the first pulse generator. The second enable signals control the transfer of energy from the individual storage unit to the string of storage units at a controllable rate with the first pulse generator providing first enable signals in response to the second enable signals.

In particular embodiments, the system can further include a second secondary inductor that is coupled to the individual storage unit with the second secondary inductor having a secondary voltage. A voltage comparator receives the secondary voltage and a reference voltage at its inputs. When the secondary voltage is greater than the reference voltage, the second pulse generator is activated. When the secondary voltage reaches the references voltage, the second pulse generator is deactivated.

In particular embodiments for transferring energy from the string of units to the individual storage unit, the system can include (i) a primary inductor; (ii) a first secondary inductor magnetically coupled to the primary inductor; and (iii) a second switch selectively coupling the first secondary inductor to the string of storage units. When the second switch is on, energy is transferred from the string of storage units to charge the first secondary inductor. When the second switch is off, the energy is discharged into the primary inductor, charging the individual storage unit.

In particular embodiments, the system can include a first pulse generator that provides first enable signals to the second switch. The second switch couples the string of storage units to the first secondary inductor in response to the first enable signals, resulting in energy being transferred from the string of storage units to the individual storage unit.

The system can also include a second pulse generator that provides second enable signals to the first pulse generator. The second enable signals control the transfer of energy from the string of storage units to the individual storage unit at a controllable rate with the first pulse generator providing first enable signals in response to the second enable signals.

• Provisional Patent
• Utility Patent

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