Maximizing life of capacitors in series modules
View Patent ↗A device ( 1 ) monitors and/or balances an ultracapacitor ( 3 ) and/or a module ( 4 ) comprising a plurality of ultracapacitors ( 3 ) connected in series, the module ( 4 ) being connectable in series or in parallel with other modules ( 4 ). The device comprises an electronic board ( 2 ) comprising digital control and/or command means, such as a microcontroller ( 5 ), executing a program for monitoring and balancing the ultracapacitor ( 3 ) and/or the module ( 4 ). The relative capacitances of the capacitors are measured, and this information is employed to determine when to carry out a controlled discharge of particular capacitors. Temperature information is also employed to determine when to carry out a controlled discharge of particular capacitors. In this way the lifetime of any particular capacitor is, desirably, extended to be no shorter than the lifetime of other longer-lived capacitors in the module.
1. A method for use in a series module of capacitors, each capacitor having a respective relative capacitance, each capacitor at a particular moment having a respective charge, each capacitor at a particular moment having a respective voltage, the method comprising:
arriving at an estimate of the respective relative capacitance of each capacitor by measuring voltages at each of the capacitors;
identifying a first capacitor in the series module having an estimated relative capacitance smaller than the estimated relative capacitance of a second capacitor in the series module; and
causing the first capacitor to be charged to a respective level of charge lower than the respective level of charge of the second capacitor.
2. The method of claim 1 wherein of causing the first capacitor to be charged to a respective level of charge lower than the respective level of charge of the second capacitor comprises connecting a resistor to the first capacitor for some duration, thereby reducing the charge of the first capacitor.
3. The method of claim 1 wherein the capacitors are ultracapacitors.
4. The method of claim 1 wherein the series module at a particular moment has a respective current passing therethrough, the respective current at various times defining positive, negative, and zero values, and arriving at an estimate of the respective relative capacitance of each capacitor by measuring voltages at each of the capacitors comprises carrying out a multiplicity of relative capacity measurements and averaging the relative capacity measurements, each relative capacity measurement comprises:
at a first time when the current passing through the series module is at a value smaller than a first predetermined threshold, measuring the respective voltage at each of the capacitors, thereby defining a sum of the voltages representing a voltage across the series module;
at a second time, after the current passing through the series module has exceeded the first predetermined threshold and has later dropped to a value smaller than the first predetermined threshold, again measuring the respective voltage at each of the capacitors, thereby again defining a sum of the voltages representing a voltage across the series module;
for each capacitor, a difference between the respective voltage at the first time and the respective voltage at the second time defining a respective voltage change;
for the series module, a difference between voltage across the series module at the first time and the voltage across the series module at the second time defining a respective voltage change for the series module;
for each capacitor, a ratio of its respective voltage change to the voltage change in the series module defining the each relative capacity measurement.
5. The method of claim 1 wherein causing the first capacitor to be charged to a respective level of charge lower than the respective level of charge of the second capacitor is not performed at any of the times when measurements of voltages take place.
6. The method of claim 1 wherein the measurements of voltages take place only at times when the first capacitor is not caused to be charged to a respective level of charge lower than the respective level of charge of the second capacitor.
7. An apparatus for use with a series module of capacitors, each capacitor having a respective relative capacitance, each capacitor at a particular moment having a respective charge, each capacitor at a particular moment having a respective voltage, the apparatus comprising:
means for arriving at an estimate of the respective relative capacitance of each capacitor by measuring voltages at each of the capacitors;
means for identifying a first capacitor in the series module having an estimated relative capacitance smaller than the estimated relative capacitance of a second capacitor in the series module; and
means for causing the first capacitor to be charged to a respective level of charge lower than the respective level of charge of the second capacitor.
8. The apparatus of claim 7 wherein the means for causing the first capacitor to be charged to a respective level of charge lower than the respective level of charge of the second capacitor comprises a resistor selectively connectable to the first capacitor, thereby reducing the charge of the first capacitor.
9. The apparatus of claim 7 wherein the capacitors are ultracapacitors.
10. The apparatus of claim 7 wherein the series module at a particular moment has a respective current passing therethrough, the respective current at various times defining positive, negative, and zero values, the means for arriving at an estimate of the respective relative capacity of each capacitor by measuring voltages at each of the capacitors comprises means for carrying out a multiplicity of relative capacity measurements and averaging the relative capacity measurements, each relative capacity measurement comprises:
at a first time when the current passing through the series module is at a value smaller than a first predetermined threshold, measuring the respective voltage at each of the capacitors, thereby defining a sum of the voltages representing a voltage across the series module;
at a second time, after the current passing through the series module has exceeded the first predetermined threshold and has later dropped to a value smaller than the first predetermined threshold, again measuring the respective voltage at each of the capacitors, thereby again defining a sum of the voltages representing a voltage across the series module;
for each capacitor, a difference between the respective voltage at the first time and the respective voltage at the second time defining a respective voltage change;
for the series module, a difference between voltage across the series module at the first time and the voltage across the series module at the second time defining a respective voltage change for the series module;
for each capacitor, a ratio of its respective voltage change to the voltage change in the series module defining the each relative capacity measurement.
11. The apparatus of claim 7 wherein the means for causing does not cause the first capacitor to be charged to a respective level of charge lower than the respective level of charge of the second capacitor at any time when the measuring voltages take place.
12. The apparatus of claim 7 wherein the means for arriving measures the voltages at times when the means for causing is not causing the first capacitor to be charged to a respective level of charge lower than the respective level of charge of the second capacitor.