The ferroresonant charger is a simple & robust device, basically consisting of: a transformer, a capacitor, one or more diodes (rectifiers), and some wires and terminals.
The charginbg voltage of the ferroresonant charger is essentially constant throughout the charge period.. The output current
, however, is limited by the battery voltage. At the beginning of the charge, the battery voltage is considerably lower than the charging
voltage, and the maximum charging current
flows. As the battery is recharged, its voltage increases, gradually reducing the charging current to the finish rate.
If the fault is in the transformer, the unit is not likely worth repairing.
If the problem was a failed diode, you may be able to find a reasonably-priced replacement.
However, unless the charger was very seriously overloaded, the problem is most likely in the capacitor - a $10 fix. The capacitor must be designated as a “motor-run” capacitor, not a motor-starting capacitor.
Without the capacitor, a basic ferroresonant charger changes its output (charge) voltage with variations in input voltage. The capacitor causes a current to flow, that induces a strong magnetic field in the core
. The resulting "saturated field” condition then stabilizes the voltage on the winding that provides current to the rectifier diode(s). The result is a constant output voltage, even when the input voltage varies over a reasonably wide range.
Testing the Capacitor
Connect a DC voltmeter to the output, and apply AC power to the unit.
Note the DC voltage.
Disconnect the AC power and remove one wire from the capacitor. Apply AC power and check the DC voltage.
A lack of change in measured DC voltage is a pretty good indication that the capacitor is not working.
Disconnect the AC power and substitute the new capacitor for the old one (only two wires are involved). Reapply the AC power and connect the output to both a voltmeter and a 12-volt lead-acid battery (the battery must be connected for the voltage to reach the correct value).
If the problem was the capacitor, the DC voltage at the battery will be very close to 13.5 Volts (13.2 - 13.9V), if the battery is reasonably well charged.