Capacitor measurement

A system for measuring a voltage drop of a reserve capacitor in a supplemental restraint system (SRS), the reserve capacitor having a capacitance value and an equivalent series resistor value, the system comprising:

• a resistor having a first terminal and a second terminal, the first terminal of the resistor coupled to a terminal of the reserve capacitor;
• a discharge circuit coupled to the second terminal of the resistor, the discharge circuit configured to supply a discharge path for the reserve capacitor during a discharge routine;
• and
• circuitry configured to:
• measure a first change in voltage at the first terminal between a first time and a second time during a discharge routine of the reserve capacitor, the discharge routine comprising sinking a current through a discharge circuit coupled to the resistor from the first time to the second time,
• measure an integration of a first change in voltage at the terminals of the resistor during the discharge routine between the first time and the second time,
• measure a second change in voltage at the first terminal at the first time,
• measure a first change in voltage across terminals of the resistor at the first time,
• measure a second change in voltage across the terminals of the resistor at the second time,
• measure a third change in voltage at the first terminal at the second time,
• compute the capacitance value based on the measured first change in voltage at the first terminal, the measured integration, and an inverse of a resistance value of the resistor, and
• determine a health of the reserve capacitor based on a difference between the computed capacitance value and a threshold value.

A circuit for measuring a voltage drop of a reserve capacitor having a capacitance value and an equivalent series resistor value, the system comprising:

• a discharge circuit couplable to a first terminal of a resistor, the discharge circuit configured to supply a discharge path for the reserve capacitor during a discharge routine, a second terminal of the resistor couplable to a terminal of the reserve capacitor;
• and
• circuitry configured to:
• measure a difference in voltage at the terminal between a first time and a second time during a discharge routine of the reserve capacitor, the discharge routine comprising sinking a current through the discharge circuit from the first time to the second time;
• measure an integration of a difference in voltage at terminals of the resistor during the discharge routine between the first time and the second time;
• compute the capacitance value based on the measured difference in voltage, the measured integration, and an inverse of a resistance value of the resistor;
• and
• determine a health of the reserve capacitor based on a difference between the computed capacitance value and a threshold value.