WO2004036666A2

WO2004036666A2 - Rotary voltage equalizer

Info

Publication number: WO2004036666A2
Application number: PCT/US2003/032525
Authority: WO
Inventors: Chaojiong Zhang
Original assignee: Chaojiong Zhang
Priority date: 2002-10-15
Filing date: 2003-10-15
Publication date: 2004-04-29
Also published as: AU2003301366A8; AU2003301366A1; WO2004036666A3

Abstract

A plurality of charge storage elements, each embedded within its own bearing member, are rotated into contact with pairs of contact pads. The contact pads connect to poles of cells 70 of a battery pack. In this way, the cells of the battery pack are equalized.

Description

ROTARY VOLTAGE EQUALIZER

This application claims the benefit of Provisional U.S. Patent Application Ser. No. 60/418,331 filed October 15, 2002.

FIELD OF THE INVENTION

The present invention relates generally to the field of electrical storage devices, and more particularly to a mechanical structure for the equalization of the voltage on the various cells of such a storage device.

BACKGROUND OF THE INVENTION

The present invention is directed to a structure and method of equalizing the charge on the cells of a battery pack during charging, discharging, or conditioning of the battery pack. Equalizing the cells of the battery pack protects the battery from failure or other problems caused by an un-matching of cells. As used herein, the terms "battery" and "battery pack" refer generally to energy storage and conversion devices, such as for example conventional batteries, ultra-capacitors, fuel cells, and the like. The characteristic common to all of these devices is a series connection of at least two voltage sources to develop a predetermined nominal voltage output. It is well known that voltages in series add, and thus a series connection often cells, each having 1.5 volts, develops a voltage source of 15 volts. The present invention provides a mechanical device for equalizing the individual cells of the battery pack.

The capacity of the cells in a battery pack may not match each other perfectly. Also, over time, the cells of the battery pack may develop mismatching during repeated or long charge, discharge, or charge/discharge cycling. This mismatch of the capacities of the cells, appearing as a voltage differential from one cell to another, only gets worse with time and continued use of the battery pack. Unless something is done to alleviate the mismatch, the battery pack may be damaged or severe safety problems may develop for certain types of batteries.

In another application of the present invention, in some high power applications of energy storage devices, it is important that the cells of the devices share the load relatively equally.

However, even within manufacturing tolerances, some cells will be weaker than others. For this reason, there remains a need for a device which will equalize the cells of the device during discharge. This is particularly true of certain high, but intermittent demand battery powered systems, such as electric vehicles. The need for equalization of the cells of a battery pack is useful in the formation, charging, and discharging of the cells of these types of systems. A number of known techniques have been used in the past to attempt to accomplish this equalization process. Most such known methods of performing the function of equalizing the cells of a voltage source include the use of such things as electrical equalizers with electrical switches along with passive circuit elements such as capacitors, inductors and/or resistors. Such methods are by their very nature limited in capacity and do not equalize the cells effectively. Some equalizers transfer energy using capacitors or inductors. Most such equalizers use single or double throw switches and have electronics or a micro-controller for control. Some equalizers simply use bypass resistors to discharge weak cells during charging or discharge healthier cells during discharging. This process dissipates the extra energy as heat and needs precise control from an on-board micro processor, making such system unnecessarily complex and inefficient.

One known technique for equalizing the cells of a battery pack uses capacitors and double pole - double throw relays to equalize adjacent pairs of cells by passive equalizing. There aren-1 capacitors for n cells. However, this technique has a number of problems. First, electronic switches are usually high resistance and expensive while mechanical relays have very limited cycle life although it has lower resistance and cost is relatively low. Second, equalization is limited to adjacent cells. Equalization of an entire battery pack depends on the propagation of such equalization throughout the pack and can take too long time to reach an acceptable balance. In theory, it may happen that the difference of highest cell voltage and the lowest cell voltage is much higher than the tolerable difference although voltage differences of adjacent cells are all very low. This is especially true when number of cells of the battery pack is large.

Thus, there remains a need for a simple, robust, effective system for the equalization of the cells of the battery or battery pack, and the present invention is directed to solving this need in the art. SUMMARY OF THE INVENTION

The present invention addresses this need by mechanically moving a plurality of charge storage elements between the various cells. The mechanical movement of the charge storage elements, preferably capacitors, is accomplished with a rotary movement by a stepper motor. This rotary battery equalizer prevents the development of mismatching by equalizing the voltage of the cells, which is actually transferring energy from cells of higher voltage to the cells of lower voltage. During charging, extra energy is transferred from weak cells to healthier cells. Conversely, during discharging, the system transfers energy from healthier cells to weaker cells.

In addition to the application of the present invention to battery packs used in electric vehicles, The present invention is useful in improving the formation procedure of batteries, ultra- capacitors, and others. F or example, c ells c an b e c onnected in s eries for formation. T he equalizer of this invention makes it possible to provide cell voltage control while cells are connected in series without bypassing switches. This technique enables one charge/discharge circuit to carry out the formation of multiple cells, in contrast to the typical method in which one charge/discharge circuit is applied to one cell or bypass circuits have to be employed.

In an alternative, most preferred embodiment, this invention creates a way to realize faster and complete battery equalization with a simple, passive structure. The structure uses robust rotary switches and high capacity energy storage devices such as ultra-capacitors or batteries, which may be referred to herein as CAP. The technique as taught by this invention equalizes all cell voltages simultaneously to the average voltage directly from beginning of the process, instead of equalizing between adjacent cells and incrementally and asymptotically approaching the average voltage of the cells in the battery pack.

These and other features and advantages of this invention will be readily apparent to those skilled in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features, advantages and objects of the present invention are attained and can be understood in detail, more particular description of the invention, briefly summarized above, maybe had by reference to embodiments thereof which are illustrated in the appended drawings.

Figure 1 A is a top view of a rotary equalizer of this invention. Figure IB is a side of such an equalizer, and Figure 1 C is a detail view of a charge storage device finding application in this invention.

Figure 2 A is a side view of a rotary equalizer, similar to the view of Figure IB. Figures 2B and 2C are circuit schematic diagram showing the connection of the equalizer to a battery pack.

Figure 3 is a side elevation view of a rotary equalizer including a stepper motor for rotary motion.

Figure 4 is a circuit schematic diagram showing the connection of the equalizer to a battery pack.

Figure 5 is a circuit schematic diagram showing the equalization function over time.

Figure 6 is a circuit schematic diagram showing the equalization function over time.

Figure 7 is a side elevation view of a rotary equalizer including a stepper motor for rotary motion and having a plurality of equalization elements.

Figure 8A is a top view of an alternative embodiment of the rotary equalizer of this invention. Figure 8B is a side of such an equalizer, and Figure 8C is a detail view of a charge storage device finding application in this invention.

Figure 9 is a circuit schematic diagram of an alternative embodiment in which every other pad of the positive terminal and every other pad of the negative terminal are coupled together, rather than connecting to a battery.

Figure 10 is top view of the most preferred embodiment of a rotary equalizer of this invention.

Figure 11 is a circuit schematic diagram of the equalizer of Figure 10. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Figures 1A, IB, and 1C illustrate a presently preferred embodiment of a rotary equalizer

10 of this invention. The equalizer comprises a wheel 12mounted for rotational drive about an axis 14. The axis 14 corresponds to a drive hole 16 through the wheel 12 which receives a drive shaft of a stepper motor, shown and described below in regard to Figure 3. The stepper motor drives the wheel 12 in a rotational direction shown by an arrow 18.

The periphery of the wheel 12 is encircled by a pair of relatively soft rubber rings 20.

Surrounding the wheel and the rings 20 and spaced apart from them is a support frame 22. The support frame 22 is spaced away from the rubber rings, thereby defining an annulus 23 having a width adequate to accommodate aplurality of cylindrical bearing members 24. Thus, from one point of view, the annulus defines a race, like that of a bearing.

One such cylindrical bearing member 24 is illustrated in Figure lC. The cylindrical bearing member is made of an insulative material, such as for example an acrylic, and includes a charge storage member 26 such as a capacitor embedded or otherwise mounted therein. The ends 28 of the cylindrical bearing member 24 are covered with a conductive material, such as for example copper, to define terminal contacts 28. The charge storage member is described as a capacitor, although an ultra-capacitor, as well as electrolytic or other capacitors, can be used for this element. An ultra-capacitor typically has much higher capacity and higher energy density than a regular capacitor with much lower cost, although regular capacitors have a longer cycle life than an ultra-capacitor.

Alternatively, a battery may be used for the charge storage device, and it functions in identically the same way for purposes of the present invention. A battery usually has a much higher capacity and higher energy density than a capacitor with much lower cost. However, a capacitor has longer cycle life than a battery. When using battery as the charge storage element, the battery usually should have same chemistry as the battery pack. Embedded into the support frame 22 are a plurality of contact pads 30, flush with the inner surface of the support frame. The contact pads must be flush with the inner surface of the support frame to accommodate the rolling of the bearing members along this inner surface. The contact pads are positioned opposite the rubber rings, and the rubber rings press the terminal contacts 28 of the cylindrical bearing members intermittently against the contact pads as the wheel rotates.

A lead or conductor 32 is electrically coupled to each contact pad 30 and the lead 32 extends through the support frame 22. Each complementarypair of leads 32 provides a coupling to one cell 34 of a battery pack to be equalized by this invention. Furthermore, each cell in a battery pack will connect to one of such pair of leads and therefore one pair of contact pads 30.

In operation, the wheel 12 turns, rolling the bearing members across the contact pads. Each bearing member includes a charge storage device having a particular voltage. When the terminal contacts encounter a pair of pads 30, if the voltage on the capacitor 26 is higher than the voltage on the cell 34, then charge is imparted to the cell. If the voltage is lower on the capacitor

26, then charge is transferred to the capacitor. As the wheel continues to turn, each capacitor 26 is brought into contact with the next cell 34. Charge is again transferred in one direction or the other, until all cells in the battery pack have been brought to the same voltage.

This feature of the invention is illustrated conceptually in Figures 2A, 2B, and 2C. Figure 2A shows the wheel 12 rotated to a position where the bearing members 24 are in contact with contact pads, thereby equalizing the voltage on the respective cells 34. As the wheel turns, each bearing member comes in contact with each pair of contact pads 30, each in turn, exchanging charge as illustrated in Figure 2C, sometimes into the bearing member charge storage device 26 and sometimes into the cell of the battery pack to be equalized. Note that the pads connect to leads, to which the cells of the battery pack are to be electrically coupled; the pads are not connected to each other. Furthermore, the contact pads 30 are paired, providing electrical coupling on positive and negative terminals of one cell of the battery pack. All pairs of pads are equally spaced around the support frame so that the connecting and disconnecting of the capacitors 26 with the pads 30 can be synchronized.

As previously described, cells in a battery pack are connected in series to develop a predetermined, nominal voltage output, hi the operation of this invention, each cell in a battery pack connects to one pair of pads. Polarity of all cells must be the same from each to the next and to the polarity of the capacitors 26. h some battery packs or some applications, cell groups, instead of single cells, may be equalized. In such applications, perhaps the cell connections require equalization in cell groups. In other applications, equalization of cells in groups may be performed to minimize the cost of the operation. The present invention functions equally well in such applications. The nominal voltage of each cell group should be the same in order to use the rotary equalizer as just described.

As previously described, the wheel 12 is preferably rotated by a motor, preferably a steppermotor, although other types of prime movers ormotors maybe used within the scope and spirit of the invention. This feature of the invention is illustrated in Figure 3. A motor 40 is mounted on a pedestal 42 for support, with an axle 46 extending vertically up through the drive hole 16, previously described in regard to Figure 1A. The support frame 22 is supported by a plurality of vertical posts 44. The motor rotates the wheel 12, thereby driving the bearing members with their embedded capacitors 26 around for electrical contact with the cells 34. Each capacitor 26 connects to and equalizes each cell in turn. The polarity of the capacitors does not change and is always same as that of the cells, unless the cell's polarity is reversed. It is also important to recognize that, at any given moment throughout rotation of the wheel, only one capacitor 26 can contact apair of pads 30 (Figure IB). In other words, a capacitor cannot contact pads from different pairs at the same time. Preferably, all of the capacitors 26 should be contacting a set of contact pads simultaneously. The motor 40 then steps to a new position wherein the capacitors are in contact with another set of contact pads and pauses briefly for the capacitors to equalize with the new corresponding cells. The time for equalization is a function of the RC time constant for the circuit which includes the capacitor 26. The motor then rotates forward quickly to the next pause position to contact with the next pair of contact pads for the next step of equalization. With rapid movement between equalization steps, a large ratio of working time to travel time is attained. This also reduces wearing on the bearing members, thereby extending the expected working lifetime of the rotary equalizer of this invention.

It should also be noted that it is not necessary that every pair of pads 30 be connected to a cell. Thus, a surplus of pads and leads is provided to accommodate a wide range of numbers of cells in a battery pack. However, every cell must be connected to a pair of pads, h this way, one bearing m ember 24 may connect to a large number of cells, and the capacity of each capacitor may be made relatively large. This means fewer but more effective moving and wearing components, longer time periods for charging and discharging cycles, and hence a slower rotating rate for the mechanism. On the other hand, one cell may be connected to more than one pair of pads, if desired.

When a particular battery pack includes more cells than can be accommodated by one set of bearing members as illustrated in Figures 1 A and 3, then two or more rotary equalizer sets can be ganged together, as illustrated in Figures 4 and 7. Each equalizing assembly may share one or more cells with another equalizing assembly. Further, capacitors from more than one equalizing assembly preferably will connect to the same cell, simultaneously or not, and hence connect to each other directly or indirectly, h this way, the battery pack will be equalized as a whole to the same voltage overall, instead of sections of the battery pack having different voltages. This is shown in Figure 4, wherein bridge cells 50 couple between sections of the battery pack. Because voltage differences between cells are very small, the value of resistance of the entire electrical circuit from the charge storage element to the battery pack cell is critical for the speed of equalization. Mechanical switches, used in prior art equalization systems, have a shorter cycle life than in the present invention, especially under high voltage, which may cause a spark. hi the rotary equalizer of this invention, however, the voltage applied for equalization is the voltage difference of cells, which is very small. By using a stepper motor and a charge storage element with a high capacity, rotation speed of the wheel can be very small, which means less switching time.

A comparison between Figures 5 and 6 illustrates an operational situation in which, in the first case of Figure 5, there is a one to one correspondence between the number of charge storage elements 26 and the number of battery pack cells 34and, in the second case of Figure 6, there are fewer charge storage elements than cells, hi step N of Figure 5, a charge storage element labeled "CAPN" equalizes voltage with a first cell in the battery pack. In step N+l, CAPN has been incremented by one unit, now equalizing charge with the next cell in line. Further, in step N+2, CAPN has been incremented a further element to continue to equalize charge. Note that each cell receives charge at each increment. However, in Figure 6, in step N, CAPN is positioned to equalize a first cell, and then in step N+l, the CAPN with equalized charge in the previous step are now equalizing the next cell which was not equalized in the step N.

Finally, Figures 8A, 8B, and 8C illustrate another presently preferred embodiment of a rotary equalizer 50 of this invention. A wheel 52 is oriented about an axis of rotation 54 for rotary movement, as before. Around the periphery of the wheel 52 is a pliant grommet 56 adapted to press a plurality of bearing members 58 against a rail 60. The rail 60 includes a plurality of contact pads 62 distributed radially about the rail, hi this embodiment, the charge storage elements 58 are dumbbell shaped, meaning that each element has ends which are of a larger diameter than the inner portion which rides between the rail and the grommet. This embodiment provides the advantage of a centrally positioned alignment shape for the charge storage elements, hi all other aspects of operation, the embodiment of Figures 8 A through 8C operate in the manner previously described.

Figure 9 illustrates an alternative embodiment to that previously described, h the circuit of Figure 9, every other pad of positive terminal is connected together, rather than connecting to a battery and the pads of corresponding negative terminal are also connected together. After a group of capacitors have been equalized with cells, they are connected together to equalize to their average voltage, which should be close to the average of cells of the group they just equalized to. Then, this group of capacitors is incremented to the next step and equalized with the same group of cells to bring those cells voltage closer to the average voltage. With this configuration, faster equalization is accomplished due to the sharing of charge between charge storage elements.

Figures 10 and 11 depict the most preferred embodiment of the invention. The basic wheel structure remain the same as previously described. As before, all pairs of pads are equally spaced around the large, outer support frame. Contact and disconnection of all CAP cylinders with pads is synchronized. Each pad pair has one positive pad or terminal and one negative pad or terminal. All positive pads are on one side of the bearing and negative pads are on the other side.

In this embodiment, however, alternating pairs of pads are electrically coupled together with leads 70. The number of pad pairs is even and the pads are divided to two groups in such a way that every pad pair of one group is adjacent to two pad pairs of the other group. All positive pads of Group A are connected together and all negative pads of group A are connected together.

As previously explained, cells in a battery pack are connected in series. Each cell of a battery is connected to a pair of pads of group B with a certain polarity arrangement. When charge storage elements are connected to pads of group B, each cap will be charged/discharged by (or equalized with) the connected cell and the voltage of the charge storage element and the cell will be equal or very nearly so. During next step or increment, all charge storage elements are connected to all pads of Group A, their voltage will be equalized to their average voltage, which should be very close to average of the cell voltage. The motor drives charge storage elements to rotate and move forward. When the charge storage elements in the bearing advance, charge storage elements are cycling between these two processes: equalizing with each cell and equalizing to each other, as shown in Figure 11.

It should be borne in mind that the number of charge storage elements may equal to the number of pairs of pads and be grouped into two alternating groups. As the wheel rotates, the charge storage devices of one group are equalized to the average voltage of the group, while the other group of charge storage devices is equalized to cells to which they are coupled. The wheel then increments, and the charge storage devices of the second group equalize to the average voltage of the group and the first group equalizes to the cells of the battery pack. When the wheel increments, the first step is then repeated. This method of equalizing is much faster than that shown and described in respect of Figure 1, and brings the voltages of the cells in the battery pack quickly into equilibrium.

It should also be recalled that multiple wheels can be ganged together in order to accommodate a larger number of cells.

The principles, preferred embodiment, and mode of operation of the present invention have been described in the foregoing specification. This invention is not to be construed as limited to the particular forms disclosed, since these are regarded as illustrative rather than restrictive. Moreover, variations and changes may be made by those skilled in the art without departing from the spirit of the invention.

Claims

I claim:

1. A voltage equalizer for equalizing a plurality of cells of a battery pack, the equalizer comprising:

a. a wheel defining a periphery;

b. a support frame around the periphery of the wheel and spaced therefrom;

c. a plurality of charge storage elements in the space between the wheel and the support frame;

d. a plurality of contact pads in the support frame positioned for contact with the plurality of charge storage elements; and

e. a lead extending from each of the plurality of contact pads.

2. The voltage equalizer of claim 1 , wherein each of the plurality of charge storage elements comprises a cylindrical bearing member.

3. The voltage equalizer of claim 2, wherein the cylindrical bearing member comprises an insulative material with a capacitor embedded therein and electrical contacts on respective ends of the bearing member.

The voltage equalizer of claim 3, wherein the capacitor is polarized.

5. The voltage equalizer of claim 1 , further comprising a pair of spaced apart soft rubber rings around the periphery of the wheel and between the wheel and the plurality of charge storage elements.

6. The voltage equalizer of claim 1, wherein the contact pads are aligned along the support frame in pairs.

7. The voltage equalizer of claim 1, further comprising a prime mover to rotate the wheel.

The voltage equalizer of claim 7, wherein the prime mover comprises a stepper motor.

9. The voltage equalizer of claim 1, wherein rotation of the wheel results in the charge storage elements contacting each of the contact pads in turn.

10. The voltage equalizer of claim 1, further comprising electrical conductors connecting selected leads into groups.

11. The v oltage e qualizer o f c laim 1 , further c omprising a first p lurality o f c onductors connecting every other lead and pad together to define a first group of pads, and a second plurality of conductors connecting the remaining leads and pads to cells of the battery pack to define a second group of pads.

12. The voltage equalizer of claim 1 , wherein each of the plurality of charge storage elements comprises a dumbbell-shaped bearing member.

13. The voltage equalizer of claim 12, wherem the cylindrical bearing member comprises an insulative material with a capacitor embedded therein and electrical contacts on respective ends of the bearing member.

4. The voltage equalizer of claim 13, wherein the capacitor is polarized.