Magnetically coupled autonomous battery equalization circuit

Details Magnetically coupled autonomous battery equalization circuit Magnetically coupled autonomous battery equalization circuit

1999-12-06

2001-04-24

Tso, Edward H. (Department: 2838)

Electricity: battery or capacitor charging or discharging

Serially connected batteries or cells

With individual charging of plural batteries or cells

Reexamination Certificate

active

06222344

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to battery equalization techniques and, more particularly, to magnetically-coupled battery equalization apparatus suitable for autonomous connection to batteries.
2. Related Art
To generate higher voltage than available from a single battery, plural batteries are typically connected in series such that a relatively large total voltage is available to drive a load. As it is desirable to utilize rechargeable batteries, battery charger circuits have been developed which charge all of the batteries in a series string at one time.
Care must be taken to fully charge each battery in the series string without one battery being at a higher state of charge than another battery. If a difference exists between a relatively low charge on one battery with respect to the other batteries in the series, the total effective capacity of the series string of batteries is reduced to the capacity of the battery having the low state of charge.
Battery equalization circuits have been developed to ensure that all batteries in a series string attain substantially the same state of charge. U.S. Pat. No. 5,479,083 to Brainard illustrates a conventional battery equalization circuit which includes a pair of series-coupled transistors connected across a pair of series-coupled batteries. An inductor L is connected between the pair of transistors and the batteries. An oscillator produces gate drive signals to the transistors such that they are alternately biased on and off for substantially equal durations. The inductor operates as a non-dissipative shunt that is alternately switched in parallel with each battery such that excessive charge on one battery is transferred to the other battery. Unfortunately, component tolerances within the Brainard equalization circuit will effect the degree of equalization achieved between the batteries, particularly tolerances which effect the duty cycle of the oscillator and the resultant duty cycle presented by the transistors to the batteries. Therefore, in order to obtain satisfactory equalization, measurements of the charge on each battery must be obtained and fed-back to the oscillator to change the duty cycle as necessary (see FIG. 3 of the Brainard patent).
U.S. Pat. No. 5,710,504 to Pascual discloses a battery equalization circuit which does not require a feedback mechanism from each battery to achieve adequate equalization. However, the circuit of the Pascual patent requires that all switching devices within the circuit be synchronized, no matter how many batteries are in the series combination. When the number of series-coupled batteries is relatively high and results in a high terminal voltage from the uppermost battery to the lowermost battery, the topology of the Pascual circuit may result in undesirable fault conditions.
Turning to FIG. 1 of the Pascual patent, a plurality of series-coupled batteries are shown; all switches 16 are synchronized via control lines 18 and control unit 12. Assuming that the total voltage from the uppermost battery to the lowermost battery is substantially large (e.g., 600 volts), a practical circuit must be designed to withstand a fault from the uppermost battery terminal to the lowermost battery terminal through the wiring of the equalization circuit. Often, the series coupled batteries may deliver many amps (approaching 1000 amps or more) making it difficult to design for surviving a fault and not damaging any of the batteries.
U.S. Pat. No. 5,821,729 to Schmidt discloses a method and apparatus for exchanging charge between a plurality of batteries, where transformer windings are connected in parallel with each battery at predetermined time intervals. Each battery is simultaneously connected to a respective one of the windings in the same winding sense. Unfortunately, the Schmidt apparatus requires precise timing of switching elements which connect the windings to the respective batteries. Indeed, if the timing of the switching elements is not tightly controlled, the common magnetic core of the transformer windings will saturate.
Accordingly, there is a need in the art for a new battery equalization circuit which is capable of autonomous operation (i.e., not requiring synchronization with other equalization circuits servicing the series-coupled batteries) and does not require closed loop compensation or overly complex control circuitry to achieve satisfactory equalization.
SUMMARY OF THE INVENTION
In accordance with the present invention, to overcome the disadvantages of the prior art, a battery equalization circuit is provided for equalizing charge between at least first and second series-connected batteries, with each battery including a positive end and a negative end, where the positive end of the second battery is coupled to the negative end of the first battery at a common node. The battery equalization circuit of the present invention comprises: a switching circuit connectable to (i) the positive end of the first battery at a positive node, and (ii) the negative end of the second battery at a negative node; a transformer having first and second magnetically coupled windings, each winding having a first end defining a polarity of the winding and a second opposing end; and a transformer reset circuit coupled from the windings of the transformer to the positive and negative nodes, the switching circuit being operable to simultaneously couple the first and second windings in parallel with the first and second batteries, respectively, in the same polarity such that a charge is transferred between the first and second batteries as a function of a charge imbalance therebetween, and the transformer reset circuit being operable to couple one of the first and second windings in parallel with one of the first and second batteries in an opposite polarity to direct reset current from the transformer to that battery to decrease the charge imbalance therebetween.
According to another aspect of the invention, the battery equalization circuit includes a first switching transistor connectable at one end to the positive end of the first battery at a positive node; a second switching transistor connectable at one end to the negative end of the second battery at a negative node; a transformer having first and second magnetically coupled windings, each winding having a first end defining a polarity of the winding and a second opposing end such that: (i) the first end of the first winding is coupled to an opposing end of the first switching transistor, (ii) the second end of the second winding is coupled to an opposing end of the second switching transistor, and (iii) the second end of the first winding is coupled to the first end of the second winding; a first diode having an anode coupled to the second end of the second winding and a cathode coupled to the positive node; a second diode having an anode coupled from the negative node and a cathode coupled to the first end of the first winding; and a drive circuit operable to bias the switching transistors ON and OFF substantially simultaneously at ON and OFF times, respectively, and at a duty cycle of less than about 50%.
The invention further provides a method of equalizing charge between the first and second series connected batteries, by using the steps of: simultaneously connecting a different one of first and second magnetically coupled windings of a transformer in parallel with an associated one of the first and second batteries, respectively, in the same polarity such that that one of the first and second batteries having greater charge drives a current into a corresponding one of the first and second windings, to cause an induced current to flow out of the other of the first and second windings and into the other one of the first and second batteries having a lesser charge, such that charge between the first and second batteries tends to equalize; simultaneously disconnecting the first and second windings of the transformer from the first and second batteries; and providing a current path for a reset curre

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