WO1993010589A1 - Apparatus for and a method of balancing the state of charge of sub-units of a battery - Google Patents

Apparatus for and a method of balancing the state of charge of sub-units of a battery Download PDF

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Publication number
WO1993010589A1
WO1993010589A1 PCT/GB1992/002147 GB9202147W WO9310589A1 WO 1993010589 A1 WO1993010589 A1 WO 1993010589A1 GB 9202147 W GB9202147 W GB 9202147W WO 9310589 A1 WO9310589 A1 WO 9310589A1
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WIPO (PCT)
Prior art keywords
sub
charge
units
unit
state
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Application number
PCT/GB1992/002147
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French (fr)
Inventor
Michael Francis Mangan
John Molyneux
Original Assignee
Silent Power Gmbh Für Energiespeichertechnik
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Silent Power Gmbh Für Energiespeichertechnik filed Critical Silent Power Gmbh Für Energiespeichertechnik
Publication of WO1993010589A1 publication Critical patent/WO1993010589A1/en

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Classifications

    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J7/00—Circuit arrangements for charging or depolarising batteries or for supplying loads from batteries
    • H02J7/0013—Circuit arrangements for charging or depolarising batteries or for supplying loads from batteries acting upon several batteries simultaneously or sequentially
    • H02J7/0014—Circuits for equalisation of charge between batteries
    • H02J7/0016—Circuits for equalisation of charge between batteries using shunting, discharge or bypass circuits

Definitions

  • the present invention provides apparatus for and a method of balancing the state of charge of a plurality of serially connected sub-units of a battery.
  • Batteries providing a large voltage are necessarily formed of serially connected sub-units. For instance for a sodium sulphur battery in order to provide 24 volts it is necessary to serially connect together twelve cells. However, in order to allow for cell failure these cells can be arranged in blocks of parallel connected serial strings.
  • the battery During the normal operation of a battery formed of a plurality of serially connected sub-units (which may be cells or blocks of cells), the battery will be discharged and charged many times. During the discharging and charging cycles, the charge held by the sub-units may become out of step. This is manifest as a variation in the state of charge of the sub-units when the first sub-unit reaches full charge. The remaining sub-units may not have reached a full charge state and since the charge capacity of a battery is limited to the charge capacity of the sub-unit containing the least charge, the capacity of the battery can become limited.
  • the present invention provides a method of balancing the state of charge of a plurality of serially connected sub-units of a battery, comprising the steps of monitoring the state of charge reached by each said sub-unit at the end of a charge cycle; and discharging at least one sub-uni r which reaches a higher state of charge than another sub-unit, to reduce the inequality in the state of charge between said sub-units.
  • the method includes the steps of monitoring the voltage across each said sub-unit during said charge cycle; for at least one sub-unit, determining a time at which the voltage across a sub-unit exceeds a predetermined threshold voltage; and determining the charge supplied to a sub-unit during the time the threshold voltage is exceeded until said charge cycle is terminated.
  • the charge supplied to a sub-unit is determined by measuring the current supplied to said sub-unit and integrating said current over the time the threshold voltage is exceeded until said charge cycle is terminated.
  • the method includes the steps of calculating the difference between the charge supplied to each of said sub-units since said predetermined threshold voltage was exceeded by voltages across respective said sub-units; and discharging at least one of said sub-units, to equate the state of charge of said sub-units with the state of charge of one of said sub-units at a lower state of charge.
  • the present invention also provides apparatus for balancing the state of charge of a plurality of serially connected sub-units of a battery, comprising state of charge monitoring means operative to monitor the state of charge reached by each said sub-unit at the end of a charge cycle; and discharging means operative to discharge at least one sub-unit, which reaches a higher state of charge than another sub-unit, to reduce the inequality in the state of charge between said sub-units.
  • the present invention is thus able to balance the state of charge of each sub-unit of the battery by discharging the sub-unit or sub-units at a higher charge state than the lowest charge state sub-unit.
  • the discharge operation can preferably take place by discharging the sub-units through a resistor such that the discharge current is quite low compared with the normal maximum load current.
  • the discharge operation is thus preferably a low current operation performed over a long period of time.
  • the discharge operation according to one aspect of the present invention can take place during the operation of the battery under a discharging load.
  • the discharging of the sub-units is under the control of a processor which is opto-isolated from the discharge circuit and which monitors the voltage across each sub-unit and the charge supplied to each sub-unit in order to determine the difference in the charge stored by each sub-unit.
  • a processor which is opto-isolated from the discharge circuit and which monitors the voltage across each sub-unit and the charge supplied to each sub-unit in order to determine the difference in the charge stored by each sub-unit.
  • Figure 1 illustrates the cell voltage during a constant current recharging cycle for a sodium sulphur cell
  • Figure 2 illustrates the voltage across monoblocs of a battery during a charging operation
  • Figure 3 illustrates a circuit diagram of a monobloc balancing circuit according to one embodiment of the present invention.
  • FIG. 1 of the drawings illustrates such a curve for the sodium sulphur cell.
  • the voltage across the cell increases steadily initially.
  • the voltage is then constant until charging is nearly complete whereupon the voltage increases rapidly due to an increase in internal resistance of the cell.
  • this curve can then be used to monitor the charge state of the cell at or near full charge.
  • Figure 2 illustrates the voltage curve during recharging of a sodium sulphur battery formed of four sub-units, termed monoblocs, wherein each monobloc is formed of at least one string of four serially connected cells.
  • Figure 2 illustrates a case wherein the four monoblocs are out of step.
  • a first monobloc becomes fully charged resulting in the termination of the charge cycle when the voltage detected across this monobloc reaches a maximum voltage of about 9.1 volts.
  • the remaining three monoblocs are not fully charged. It is therefore desirable to bring the state of charge of the monoblocs into phase in order that during the next recharging cycle, the available capacity of the battery can be increased.
  • the state of charge reached by the monoblocs can be balanced by slightly discharging the monoblocs having reached the highest state of charge.
  • One or more monoblocs at a time can be discharged in this manner.
  • the amount of discharge required from each monobloc can be measured by monitoring the voltage across each monobloc during the charge cycle and logging the time at which the voltage for each monobloc exceeds a predetermined threshold voltage.
  • This threshold voltage could typically be 8.7 volts for a monobloc formed of four sodium sulphur cells in series.
  • the current then supplied to each monobloc from this time is measured until the charge cycle is terminated.
  • the charge supplied to each monobloc can then be calculated by multiplying the current by the time for which this current was supplied to the monobloc.
  • the monobloc having the highest state of charge which will be the monobloc which terminated the charge cycle, will therefore be discharged by an amount equal to or slightly greater than the difference between the charge supplied to this monobloc since the logged time and the charge supplied to the monoblocs having the lowest charge state.
  • the threshold voltage was 8.7 volts, then only the second monobloc would trigger a logged time, and hence only the first monobloc would be discharged to reduce the inequality in the state of charge reached between the first and second monoblocs.
  • the inequality between the second, third and fourth monoblocs can be reduced at the next recharging cycle.
  • this monobloc can be discharged by the amount of charge supplied since the threshold voltage was reached. This would only be necessary if the state of charge of at least one of the monoblocs was very much advanced compared to the others.
  • the discharging operation of the monoblocs is preferably at a low discharge current of typically 1 amp over a long time period of typically two hours or may be as low as 100mA for a time period of ten to twenty hours. It is not necessary to terminate the balancing discharge before the battery next undergoes discharge through a load, since it is possible, under processor control, to carry out the balancing discharge during normal discharge of the battery through a load.
  • Figure 3 illustrates a circuit arrangement capable of balancing the charge in a battery formed of serially connected sub-units or monoblocs 1.
  • Figure 3 illustrates a string of 1 to n monoblocs.
  • each monobloc might comprise a number of parallel connected serial connected strings of four cells. The voltage of such a monobloc would be 8 volts.
  • the voltage across each monobloc V Q/ V., Vo-- " ⁇ is measured by providing connections to a processor (not shown) .
  • Each monobloc is provided with a discharge circuit comprising two resistors R in series with two fuses F limiting the discharge current to 1 amp.
  • the discharge circuit is switchably controlled by a photo-transistor controlled by a photo-diode D .
  • the photo-transistor T and the photo-diode D form an opto-isolator 1.
  • the p oto-diode is provided with a 5 volt supply and earth and is connected to respective terminals T 1 through to T .
  • the operation of the discharge circuit can be controlled by signals input to terminals T. to T . These control signals are provided by a processor (not shown) which can control the discharge current to bring the stored charge of each monobloc into balance, in response to the measured voltages V Q through to V in accordance with the method described hereinabove.
  • the processor is also able to monitor the voltage V Q through to V after the discharge operation in order to ascertain that the balancing operation has been undertaken.
  • the resistors R can be provided integrally with the monoblocs. Thus any temperature rise caused by the discharge current through the resistors R is input into the monoblocs helping to maintain battery temperature.
  • the present invention is thus able to balance the state of charge reached by sub-units of a battery by discharging the sub-units which reach the highest state of charge. This helps to keep the maximum capacity of the battery available for use.

Abstract

Apparatus for balancing the state of charge of a plurality of serially connected sub-units (1) of a battery comprises state of charge monitoring means operative to monitor the state of charge reached by each sub-unit (1). Discharging means (R) operate in response to the state of charge monitoring means to discharge sub-unit which store more charge than another sub-unit. The discharge is performed for a period of time in order to reduce the inequality in the state of charge between said sub-units (1). The state of charge monitoring means preferably includes a processor which monitors the voltage (V0, V2, V3... Vn) across the sub-units and applies control signals to terminals (T1, T2...Tn) in order to control the balancing discharge of the sub-units (1).

Description

APPARATUS FOR AND A METHOD OF BALANCING THE STATE OF CHARGE OF SUB-UNITS OF A BATTERY
The present invention provides apparatus for and a method of balancing the state of charge of a plurality of serially connected sub-units of a battery.
Batteries providing a large voltage are necessarily formed of serially connected sub-units. For instance for a sodium sulphur battery in order to provide 24 volts it is necessary to serially connect together twelve cells. However, in order to allow for cell failure these cells can be arranged in blocks of parallel connected serial strings.
During the normal operation of a battery formed of a plurality of serially connected sub-units (which may be cells or blocks of cells), the battery will be discharged and charged many times. During the discharging and charging cycles, the charge held by the sub-units may become out of step. This is manifest as a variation in the state of charge of the sub-units when the first sub-unit reaches full charge. The remaining sub-units may not have reached a full charge state and since the charge capacity of a battery is limited to the charge capacity of the sub-unit containing the least charge, the capacity of the battery can become limited.
It is an object of the present invention to provide apparatus for and a method of balancing the state of charge of sub-units of such a battery.
The present invention provides a method of balancing the state of charge of a plurality of serially connected sub-units of a battery, comprising the steps of monitoring the state of charge reached by each said sub-unit at the end of a charge cycle; and discharging at least one sub-uni r which reaches a higher state of charge than another sub-unit, to reduce the inequality in the state of charge between said sub-units.
According to one aspect of the present invention the method includes the steps of monitoring the voltage across each said sub-unit during said charge cycle; for at least one sub-unit, determining a time at which the voltage across a sub-unit exceeds a predetermined threshold voltage; and determining the charge supplied to a sub-unit during the time the threshold voltage is exceeded until said charge cycle is terminated.
Preferably the charge supplied to a sub-unit is determined by measuring the current supplied to said sub-unit and integrating said current over the time the threshold voltage is exceeded until said charge cycle is terminated.
If only one sub-unit exceeds said predetermined threshold voltage during a charge cycle then preferably an amount of charge is discharged from said sub-unit substantially equal to the charge supplied to said sub-unit since the threshold voltage was exceeded. If the voltage across a plurality of sub-units exceeds said predetermined threshold during said charge cycle, the method according to one embodiment of the present invention includes the steps of calculating the difference between the charge supplied to each of said sub-units since said predetermined threshold voltage was exceeded by voltages across respective said sub-units; and discharging at least one of said sub-units, to equate the state of charge of said sub-units with the state of charge of one of said sub-units at a lower state of charge.
The present invention also provides apparatus for balancing the state of charge of a plurality of serially connected sub-units of a battery, comprising state of charge monitoring means operative to monitor the state of charge reached by each said sub-unit at the end of a charge cycle; and discharging means operative to discharge at least one sub-unit, which reaches a higher state of charge than another sub-unit, to reduce the inequality in the state of charge between said sub-units.
The present invention is thus able to balance the state of charge of each sub-unit of the battery by discharging the sub-unit or sub-units at a higher charge state than the lowest charge state sub-unit.
The discharge operation can preferably take place by discharging the sub-units through a resistor such that the discharge current is quite low compared with the normal maximum load current. The discharge operation is thus preferably a low current operation performed over a long period of time. The discharge operation according to one aspect of the present invention can take place during the operation of the battery under a discharging load.
Preferably the discharging of the sub-units is under the control of a processor which is opto-isolated from the discharge circuit and which monitors the voltage across each sub-unit and the charge supplied to each sub-unit in order to determine the difference in the charge stored by each sub-unit.
Examples of the present invention will now be described with reference to the drawings, in which:-
Figure 1 illustrates the cell voltage during a constant current recharging cycle for a sodium sulphur cell;
Figure 2 illustrates the voltage across monoblocs of a battery during a charging operation;
Figure 3 illustrates a circuit diagram of a monobloc balancing circuit according to one embodiment of the present invention.
The voltage of any cell forming a battery, during the recharging of the battery can be well characterised. Figure 1 of the drawings illustrates such a curve for the sodium sulphur cell. When the cell is deeply discharged and recharging commences, the voltage across the cell increases steadily initially. For most of the charge cycle the voltage is then constant until charging is nearly complete whereupon the voltage increases rapidly due to an increase in internal resistance of the cell. For the sodium sulphur cell this curve can then be used to monitor the charge state of the cell at or near full charge. Figure 2 illustrates the voltage curve during recharging of a sodium sulphur battery formed of four sub-units, termed monoblocs, wherein each monobloc is formed of at least one string of four serially connected cells.
Figure 2 illustrates a case wherein the four monoblocs are out of step. A first monobloc becomes fully charged resulting in the termination of the charge cycle when the voltage detected across this monobloc reaches a maximum voltage of about 9.1 volts. The remaining three monoblocs are not fully charged. It is therefore desirable to bring the state of charge of the monoblocs into phase in order that during the next recharging cycle, the available capacity of the battery can be increased.
The state of charge reached by the monoblocs can be balanced by slightly discharging the monoblocs having reached the highest state of charge. One or more monoblocs at a time can be discharged in this manner. The amount of discharge required from each monobloc can be measured by monitoring the voltage across each monobloc during the charge cycle and logging the time at which the voltage for each monobloc exceeds a predetermined threshold voltage. This threshold voltage could typically be 8.7 volts for a monobloc formed of four sodium sulphur cells in series. The current then supplied to each monobloc from this time is measured until the charge cycle is terminated. The charge supplied to each monobloc can then be calculated by multiplying the current by the time for which this current was supplied to the monobloc. The monobloc having the highest state of charge, which will be the monobloc which terminated the charge cycle, will therefore be discharged by an amount equal to or slightly greater than the difference between the charge supplied to this monobloc since the logged time and the charge supplied to the monoblocs having the lowest charge state. In Figure 2, if the threshold voltage was 8.7 volts, then only the second monobloc would trigger a logged time, and hence only the first monobloc would be discharged to reduce the inequality in the state of charge reached between the first and second monoblocs. The inequality between the second, third and fourth monoblocs can be reduced at the next recharging cycle. If only the first monobloc reached the threshold voltage during the charge cycle then this monobloc can be discharged by the amount of charge supplied since the threshold voltage was reached. This would only be necessary if the state of charge of at least one of the monoblocs was very much advanced compared to the others.
The discharging operation of the monoblocs is preferably at a low discharge current of typically 1 amp over a long time period of typically two hours or may be as low as 100mA for a time period of ten to twenty hours. It is not necessary to terminate the balancing discharge before the battery next undergoes discharge through a load, since it is possible, under processor control, to carry out the balancing discharge during normal discharge of the battery through a load. Figure 3 illustrates a circuit arrangement capable of balancing the charge in a battery formed of serially connected sub-units or monoblocs 1. Figure 3 illustrates a string of 1 to n monoblocs. For a sodium sulphur cell each monobloc might comprise a number of parallel connected serial connected strings of four cells. The voltage of such a monobloc would be 8 volts. The voltage across each monobloc VQ/ V., Vo--"^ is measured by providing connections to a processor (not shown) .
Each monobloc is provided with a discharge circuit comprising two resistors R in series with two fuses F limiting the discharge current to 1 amp. The discharge circuit is switchably controlled by a photo-transistor controlled by a photo-diode D . The photo-transistor T and the photo-diode D form an opto-isolator 1. The p oto-diode is provided with a 5 volt supply and earth and is connected to respective terminals T1 through to T . The operation of the discharge circuit can be controlled by signals input to terminals T. to T . These control signals are provided by a processor (not shown) which can control the discharge current to bring the stored charge of each monobloc into balance, in response to the measured voltages VQ through to V in accordance with the method described hereinabove.
The processor is also able to monitor the voltage VQ through to V after the discharge operation in order to ascertain that the balancing operation has been undertaken.
The resistors R can be provided integrally with the monoblocs. Thus any temperature rise caused by the discharge current through the resistors R is input into the monoblocs helping to maintain battery temperature. The present invention is thus able to balance the state of charge reached by sub-units of a battery by discharging the sub-units which reach the highest state of charge. This helps to keep the maximum capacity of the battery available for use.

Claims

CfrAIMg
1. A method of balancing the state of charge of a plurality of serially connected sub-units of a battery, comprising the steps of monitoring the state of charge reached by each said sub-unit at the end of a charge cycle; and discharging at least one sub-unit, which reaches a higher state of charge than another sub-unit to reduce the inequality in the state of charge between said sub-units.
2. A method as claimed in Claim 1, including the steps of monitoring the voltage across each said sub-unit during said charge cycle; for at least one sub-unit, determining a time at which the voltage across a sub-unit exceeds a predetermined threshold voltage; and determining the charge supplied to a sub-unit during the time the threshold voltage is exceeded until said charge cycle is terminated.
3. A method as claimed in Claim 2, wherein the charge supplied to a sub-unit is determined by measuring the current supplied to said sub-unit and integrating said current over the time the threshold voltage is exceeded until said charge cycle is terminated.
4. A method as claimed in Claim 2 or Claim 3, wherein the voltage across only one sub-unit exceeds said predetermined threshold voltage during said charge cycle; and an amount of charge is discharged from said sub-unit substantially equal to the charge supplied to said sub-unit since the threshold voltage was exceeded.
5. A method as claimed in Claim 2 or Claim 3, wherein the voltage across a plurality of sub-units exceeds said predetermined threshold voltage during said charge cycle; said method including the step of calculating the difference between the charge supplied to each of said sub-units since said predetermined threshold voltage was exceeded by voltages across respective said sub-units; and discharging at least one of said sub-units, to equate the state of charge of said sub-units with the state of charge of one of said sub-units at a lower state of charge.
6. A method as claimed in any preceding claim, wherein the charging operation is terminated when the voltage measured across a sub-unit reaches a predetermined maximum voltage.
7. A method as claimed in any preceding claim wherein said sub-units are discharged at a low current, relative to a normal maximum load current of said battery.
8. A method as claimed in any preceding claim wherein said sub-units are discharged to reduce the inequality in the charge stored by each sub-unit, during a normal load discharge of said battery.
-9. A method as claimed in any preceding claim wherein said sub-units are discharged through respective loads integral with respective said sub-units.
10. Apparatus for balancing the state of charge of a plurality of serially connected sub-units of a battery, comprising state of charge monitoring means operative to monitor the state of charge reached by each said sub-unit at the end of a charge cycle; discharging means operative to discharge at least one sub-unit, which reaches a higher state of charge than another sub-unit, to reduce the inequality in the state of charge between said sub-units. - lO -
ll. Apparatus as claimed in Claim 10, wherein said state of charge monitoring means comprises voltage monitoring means to monitor the voltage across each said sub-unit during said charge cycle; voltage threshold means operative for at least one sub-unit, to determine a time at which the voltage across a sub-unit exceeds a predetermined threshold voltage? charge determining means to determine the charge supplied to a sub-unit during the time the threshold voltage is exceeded, until said charge cycle is terminated.
12. Apparatus as claimed in Claim 9, wherein said charge determining means comprises current measuring means to measure the current supplied to said sub-unit; and integration means to integrate said current over the time the threshold voltage is exceeded until said charge cycle is terminated.
13. Apparatus as claimed in Claim 8 or Claim 9, wherein said voltage measuring means detects the voltage across only one sub-unit exceeding said predetermined threshold voltage during said charge cycle; said discharging means being operative to discharge an amount of charge from said sub-unit which is substantially equal to the charge supplied to said sub-unit since the threshold voltage was exceeded.
14. Apparatus as claimed in Claim 8 or Claim 9 , wherein said voltage monitoring means detects the voltage across a plurality of sub-units exceeding said predetermined threshold voltage during said charge cycle; said comparison means being operative to calculate the difference between the charge supplied to each of said sub-units, since said predetermined threshold voltage was exceeded by voltages across respective said sub-units; said discharging means being operative to discharge at least one of said sub-units, to equate the state of charge of said sub-units with the state of charge of one of said sub-units at a lower state of charge.
15. Apparatus as claimed in any of Claims 10 to 14, including processing means operative to control said discharging means in order to reduce the inequality in the charge stored by each sub-unit.
16. Apparatus as claimed in any of Claims 10 to 15, wherein said discharging means comprises respective resistors connected across respective said sub-units of said battery.
17. Apparatus as claimed in Claim 16, wherein said resistors have a resistance value such that said sub-units are discharged at a low current relative to a normal maximum load current.
18. Apparatus as claimed in either Claim 16 or Claim 17, wherein respective resistors are provided integrally with respective sub-units of said battery.
19. Apparatus as claimed in any of Claims 15 to 18, wherein said discharging means includes isolation means to provide electrical isolation between said discharging means and said processing means.
20. Apparatus as claimed in Claim 19, wherein said ftr isolation means comprises an opto-isolator.
21. Apparatus as hereinbefore described with reference to the drawings.
22. A method as hereinbefore described with reference to the drawings.
PCT/GB1992/002147 1991-11-20 1992-11-20 Apparatus for and a method of balancing the state of charge of sub-units of a battery WO1993010589A1 (en)

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GB9124672.8 1991-11-20
GB919124672A GB9124672D0 (en) 1991-11-20 1991-11-20 Apparatus for and a method of balancing the state of charge of sub-units of a battery

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