A detection system (59) for determining spark voltage of a spark plug (54) in a capacitive discharge ignition system (52) is disclosed. The detection system may have a sensor circuit (60) configured to detect occurrence of a spark by measuring a voltage on a low side (78) of a primary coil (68). The detection system may also have an electronic control unit (58) connected to the sensor circuit. The electronic control unit may be configured to receive a signal from the sensor circuit indicating that the voltage exceeded a threshold. The electronic control unit may be further configured to determine a spark time based on the signal, and determine a spark voltage based on the spark time.