GB2293059A

GB2293059A - Equalization of charge on series connected cells or batteries

Info

Publication number: GB2293059A
Application number: GB9518261A
Authority: GB
Inventors: David R Pacholok; David T Fouchard; Walter B Ebner
Original assignee: Rayovac Corp; Ray O Vac Corp
Current assignee: Spectrum Brands Inc
Priority date: 1994-09-09
Filing date: 1995-09-08
Publication date: 1996-03-13
Also published as: GB9518261D0; FR2724501A1; CA2157814A1; DE19533542A1

Description

2293059 EQUALIZATION OF CE1ARGE ON SERIES CONNECTED CMLS OR BATTERIES

FIELD OF THE INVENTION

This invention relates generally to the field of battery charging and particularly to the charging of multiple cells or batteries connected in series.

BACKGROUND OF TRE INVENTION

Each type of electro-chemical cell has a characteristic "full" charge voltage. A lower cell voltage indicates a state of charge less than "full". To obtain a higher voltage than can be provided from a single cell, cells are connected in series, often with internal or integral connections between the cells, to form a battery which has the desired level of output voltage. Certain types of electro-chemical cells, such as rechargeable alkaline manganese, lithium, and lithium ion cells have no internal charge control mechanism. Consequently, if charging of such cells is not carefully controlled, overcharge can result which will cause irreversible changes in cell chemistry, performance loss, and, in extreme cases, cell venting. The charging of series connected cells of these types is difficult. Since individual cell voltages is and capacities may not be equal, charging of the entire series of connected cells may result in some cells being overcharged.

To address this problem, equalizers have been developed which monitor the voltage across each cell and connect resistors or current sinks across the cell or cells having an excessive voltage to partially discharge the cell and thereby accomplish charge equalization among the cells. However, equalizing the charge on cells in this manner wastes power and also causes undesirable heating of the battery pack since the equalization circuitry is typically physically located in the battery housing. In addition, the rate at which the cells can be equalized, and thus the rate at which the cells can be recharged, is limited to the amount of power dissipation that can be tolerated.

SUMNARY OF THE INVENTION Equalization of charge an multiple series connected cells (or batteries) is accomplished in accordance with the present invention rapidly and substantially without unnecessary dissipation of power. Equalization is accomplished automatically without requiring comparison of voltages across individual cells or batteries (cell units) and can be and preferably is carried out during charging of the cell units. Further, the present invention provides peak current flow to a cell unit in proportion to the difference in the voltages between cell units. Energy is transferred in this manner from the most highly charged cell unit to the cell unit or units having lesser charge.

The equalizer apparatus of the present invention includes a capacitor which can be selectively and sequentially connected in parallel with each cell unit. Pairs of controllable switching devices are connected to each cell unit and to the capacitor, with one switching device in each pair connected in series between the positive terminal of each cell unit and one terminal of the capacitor, and the other switching device connected between the negative terminal of each cell unit and the other terminal of the capacitor. A controller is connected to the switching devices to provide control signals to the pairs of switching devices associated with the cell units. The control signals switch each pair of switching devices on and off in sequence so that at any time only one cell unit is connected across the capacitor. A separate battery charger may simultaneously supply charging current to the series connected cell units during the equalization operation.

The equalizer of the present invention may be used to equalize the voltage on two or more cells connected in series. If only two cells are to be equalized the controller may be implemented as an oscillator having two complementary outputs connected to the two pairs of switching devices. During onehalf of the oscillator cycle the most highly charged cell unit will thus be connected across the capacitor and the capacitor will be charged to the voltage level of that cell unit. During the other half of each oscillator cycle the less charged cell unit will be connected across the capacitor, and during this half cycle the charged capacitor will discharge into the less charged cell unit. In this manner, energy is transferred from the more highly charged cell unit to the lesser charged cell unit. The equalization effect is achieved for more then two cell units by use of a controller which switches each cell unit in sequence across the capacitor.

Because control of power flow is carried out by switching elements which are either ON or OFF, very little power loss occurs in the equalization circuit and very little heating of the components occurs. Consequently, equalization can be carried out at a very rapid rate compared to conventional equalization circuits, and the equalization is carried out with high energy efficiency.

is Because the transfer of energy from the most highly charged cell unit to a lesser charged cell unit is carried out automatically by the inherent operation of the circuit, no complex voltage comparator circuits are required, minimizing the complexity and expense of the circuit. In addition, because no voltage measurements need be made, the present invention operates without regard to temperature and no compensation is required for temperature changes which may result in changes in cell unit voltages.

Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

Fig. 1 is an illustrative block diagram of the equalizer of the present invention connected to provide equalization of two series connected cell units being charged.

Fig. 2 is a schematic circuit diagram of an equalizer in accordance with the present invention for providing equalization of the charge to two cell units.

Fig. 3 is a schematic circuit diagram of a controller circuit for providing control signals to the equalizer switching devices.

DETAILED DESCRIPTION OF THE INVENTION

With reference to the drawings, a block diagram of a charging system is shown in Fig. 1 which includes the equalizer 10 of the present invention connected to two series connected electro-chemical cell units 11 and 12. The cell units 11 and 12 may comprise various types of electro-chemical cells, such as lithium-ion, rechargeable lithium, and rechargeable alkaline manganese batteries of the type commonly used in, e.g., cellular phones, video tape recorders and players, cameras, cordless tools, portable communications equipment, electrical vehicles, and so forth. The cell units 11 and 12 also may comprise batteries of internally or externally connected cells where a requirement exists to properly equalize the charge across the two or more separable cell units. As used herein, the term "cell uniC is intended to refer both to single cells and to internally or externally connected batteries of cells. The illustrative charging system of Fig. 1 includes a charger 14 which supplies a charging current I, which passes in series through the cell units 11 and 12. The charger 14 may be any type of conventional charger including constant current, float and trickle chargers, which are well known in the art. The output voltage of the charger 14, V,, is applied across the series connected cell units 11 and 12. A connecting line 17 extends from the equalizer 10 to the positive terminal of the cell unit 11 and a connecting line 18 extends from the equalizer to the negative terminal of the cell unit 11 and to the positive terminal of the cell unit 12. Another connecting line 19 extends from the equalizer to connection to the negative terminal of the cell unit 12. Although the equalizer 10 is thus effectively connected in parallel with the cell units 11 and 12 across the charger 14, the equalizer 10 draws substantially no power from the charger. It is understood that the equalizer 10 inay operate whether the charger 14 is charging the cell units or not. If desired, the equalizer 10 may be automatically actuated when the charger 14 supplies current to the cell units 11 and 12.

A schematic circuit diagram of an embodiment of the equalizer 10 for equalizing the two cell units 11 and 12 is shown in Fig. 2. The equalizer 10 includes a capacitor 20, two pairs of controllable switching devices, 21 and 23, and 22 and 24, and a controller for the switching devices implemented as an oscillator 25. For a typical application with cell units having nominal voltage is levels of 3 volts each, a 100 microfarad (gF) capacitor rated at 10 volts may be used. The controllable switching devices 21-24 must be able to conduct current in both directions through the device, and they must be able to block voltage in the reverse direction when the devices are switched off. Therefore, conventional power MOSFETs, which have their body-source diodes internally connected, cannot be used unless the MOSFETs are modified to disconnect the body diodes. Other types of switches, mechanical as well as solid state, may be used where appropriate, e.g., reed switches, symmetrical junction FETs or other bidirectional semiconductor devices, as well as appropriate combinations of devices such as insulated gate bipolar transistors, gate turn-off thyristors, power bipolar transistors, etc. The switching devices are controlled to switch on and off by a control signal applied to a "gate" input of each device. The switching device pairs selectively connect each cell unit 11 and 12 in parallel with the capacitor 20. The switching device 21 is connected between the positive terminal of the cell unit 11 and a first terminal (e.g., the "positive" terminal) of the capacitor 20. The switching device 23 is connected between the second terminal of the capacitor 20 and the negative terminal of the cell unit 11. Similarly, the switching device 22 is connected between the positive terminal of the cell unit 12 (which is also connected to the negative terminal of the cell unit 11) and the first terminal of the capacitor 20. The switching device 24 is connected between the second terminal of the capacitor 20 and the negative terminal of the cell unit 12 (which in this case is connected to the circuit ground). The oscillator controller 25 provides two control signals, 5 and 9 (which is the complement of 5). The control signals are preferably square wave functions such that S is HIGH when is LOW, and S is LOW when 9 is HIGH, with equal length half cycles. The S control signal is provided on lines 27 to the gates of the switching devices 21 and 23 associated with the cell unit 11. The control signal is provided on lines 28 to the gates of the switching devices 22 and 24 associated with the cell unit 12.

The charge equalizer 10 operates in the following manner. During the first half of an oscillator cycle the control signal S is HIGH and the control signal is LOW. During this half cycle the switching devices 22 and 24 are turned off and the cell unit 12 is disconnected from the capacitor 20. Simultaneously, the switching devices 21 and 23 are ON and the cell unit 11 is connected across the capacitor 20. The capacitor 20 is thus charged (or discharged) to the voltage level of the cell unit 11. During the next half cycle the control signal S is LOW and the control signal 9 is HIGH. Thus, during this half cycle, the switching devices 21 and 23 are OFF, disconnecting the cell unit 11 from the capacitor 20. Simultaneously, the switching devices 23 and 24 are ON, connecting the cell unit 12 across the capacitor 20. If the cell unit 11 is more highly charged than the cell unit 12, the capacitor 20, having been charged to the voltage level of the cell unit 11 in the previous half cycle, will have a higher voltage than the cell unit 12 and will discharge into the cell unit 12 and thereby charge that cell unit. If the cell unit 12 is at a higher voltage level than the cell unit 11, the capacitor 20 will have a lower voltage then the cell unit 12 and will charge to the voltage level of cell unit 12 and, during the next oscillator half cycle, will discharge into the cell unit 11 thereby charging that cell unit 11. Consequently, by switching each of the cell units 11 and 12 alternately across the capacitor 20, energy will be transferred from the more highly charged cell unit to the less highly charged cell unit. As the voltages of the two cell units approach one another the level of current flow into and out of the capacitor during each oscillator half cycle will decrease and become substantially zero when the two cell units are equally charged.

The oscillator controller 25 may be implemented in various ways using either integrated circuits or discrete coinponents. An exemplary implementation for the oscillator controller 25 for driving the gates of MOSFETS used as the switching devices is shown in Fig. 3. An inverter Schmitt trigger 30 is biased with a resistor 31 and a capacitor 32 to act as an oscillator, putting out a square wave signal at a selected frequency, e.g., 500 Hz. The switching frequency and the value of the capacitor 20 are selected based on the internal resistance of each cell unit and the ON resistance of the switching devices so that the effective RC time constant allows essentially full charge or discharge of the capacitor during each half cycle. The square wave signal from the Schmitt trigger 30 provides the control signal used to control the switching devices 22 and 24. This same signal is also provided to a second inverter 33, the output of which is the control signal S used to control the switching devices 21 and 23. Since the source voltage for the switching device 21, where MOSFETs are used as the switching devices, is based on the positive terminal of the cell unit 12 plus the voltage across the capacitor 20, a voltage doubler is used to provide a higher gate drive voltage for the switching devices 21 and 23. This voltage doubling may be implemented by a conventional voltage doubler circuit composed of an inverter 35 switching between V, (the voltage across the cells 11 and 12) and ground, two capacitors 36 and 37, and two diodes 38 and 39.

The equalizer 10, as described above, may be easily extended to equalize the charge on more than two cell units connected in series. An additional pair of switching devices is required for each cell unit to be equalized. one switching device of each pair is connected between the positive terminal of the cell unit and the first terminal of the capacitor 20 and the other switching device is connected between the negative terminal of the cell unit and the second terminal of the capacitor 20. A controller is used to sequentially switch on and off each pair of switching devices for each cell unit such that each cell unit is connected, in turn, across the capacitor. Such a controller may be implemented in various conventional ways, e.g., an oscillator driving a ring counter (or Johnson counter) having outputs driving the gates of each pair of switching devices.

If desired, the switching frequency of the output of the oscillator controller 25 can be varied in response to the need for equalization. For example, the switching frequency may be either of two levels: a high switching frequency to provide high equalization current during charging and a low switching frequency to provide a low equalizing current when the cells are inactive to compensate for differences in the self- discharge characteristics of the individual cells.

It is understood that the invention is not confined to the particularly embodiments set forth herein as illustrative, but embraces all such forms thereof as come within the scope of the following claims.

is

Claims

CLAIMS 1 2 3 4 6 7 a 9 11 1 2 3 what is claimed is:

1. An equalizer for equalizing the charge on two or more cell units having positive and negative terminals connected in series comprising:

a) a capacitor having first and second terminals; b) a pair of controllable switching devices for each cell unit wherein one switching device of the pair is connected between a positive terminal of each such cell unit and the first terminal of the capacitor and the other switching device of the pair is connected between a negative terminal of such cell unit and the second terminal of the capacitor; and C) control means connected to the switching devices for providing a control signal to each switching device to turn each pair of switching devices on and off in turn in progression at a selected switching frequency, such that one pair of switching devices is conducting while each other pair of switching devices are not conducting, whereby the capacitor is sequentially charged by the cell unit having the highest voltage and discharged into a cell unit having a lower voltage such that energy is transferred from a higher charged cell unit to a lower charged cell unit.

2. The equalizer of Claim I wherein the switching devices are power MOSFETs which predominantly conduct current in only one direction when turned on, and having gates, and wherein the control means is connected to supply control signals to the gates to turn each pair of MOSFETs on and off sequentially.

3. The equalizer of Claim 2 wherein there are two cell units and the control means includes an oscillator which provides complementary square wave output signals to the gates of each of two pairs of MOSPETs.

4. The equalizer of Claim 3 including means for providing the control signal to one pair of the MOSFETs with a peak voltage approximately double that of the control signal provided to the other pair of MOSFETs.

5. The equalizer of Claim 1 in combination with a battery charger connected to supply a charging current to the series combination of cell units.

6. A method of equalizing a charge on two or more cell units connected in series, comprising the steps of:

a) providing a capacitor; b) sequentially connecting each cell unit across the capacitor such that the capacitor is charged to the voltage of the most highly charged cell unit and discharged into a lesser charged cell unit, and repeating the step of sequentially connecting each cell unit across the capacitor thereby to equalize the charge on the cell units.

1

7. The method of Claim 6 including the additional step of providing a charging current through the cell units connected in series.