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Battery charger repair

Feb 20, 2006

If your boat has a shore-power connection, it likely has a 120-volt AC-powered battery charger. If the boat is more than about 15 years old, the odds are the charger is a ferroresonant unit, a Raritan Crown converter, a Cruisair Sentry, a product of Charles Marine or Professional Mariner. You can determine if your charger is a ferroresonant unit by checking the nameplate or owner's manual, or by the fact that it hums when operating (or hummed when it was working) and that it is quite large and heavy. Other clues are that it has no switches to select the type of battery to be charged and no provision for equalization charging. Its most telling feature is its plainness.

Although the ferroresonant charger is just about the most reliable piece of electrical equipment on your boat, it can fail. The charger on my boat, a Professional Mariner Pro 40-60 recently stopped charging after 25 years and about 200,000 hours of virtually constant use. I bought a new charger immediately to continue testing some new 12-volt powered equipment. However, knowing the elegant simplicity of the ferroresonant design, I decided to try to repair the veteran unit. (I can't have too many sources of well-regulated 12-volt power.) In the event you face the untimely demise of your charger, you may find the following test and repair suggestions of some use.

First, how it works. The ferroresonant charger is an elegantly simple device. The component list is short: a transformer, a capacitor, one or more diodes (rectifiers), and some wires and terminals. A marine battery charger has to be safe first of all. The design must ensure that there is no direct electrical connection between the incoming AC power and the DC power used to charge the batteries. The second requirement is that the voltage supplied to the batteries is constant, regardless of substantial variation in the AC supply voltage. Dockside voltage varies much more than what we encounter in our homes. Since the difference between undercharging and overcharging a battery is a matter of a few tenths of a volt, the charger needs to compensate for line voltage changes automatically.

Automatic control of output voltage from a ferroresonant charger is accomplished by controlling the strength of the magnetic field in the iron core of the transformer. The capacitor connected to a special winding on the transformer causes a current to flow that induces a strong magnetic field in the core. The "saturated�VbCrLf field condition then stabilizes the voltage on the winding that provides current to the rectifier diode(s). The result, constant output voltage even when the input voltage varies over a reasonably wide range.

If the failure of your charger is not accompanied by smoke and a burned smell the odds are the only part that failed is the capacitor. A slightly audible hum usually will confirm that power is reaching the charger. Its output voltage, with the battery disconnected may be about 8 to 10 volts. The easiest way to confirm the likely failure of the capacitor is to substitute a known good capacitor for the suspect one. The capacity and rated voltage of the capacitor usually will be marked on the capacitor's can, although depending on the unit's age, it may be a bit difficult to read the numbers. Six microfarads at 470 volts is a typical value for a 40-amp charger. The capacitor must be designated as a motor-run capacitor, not a motor-starting capacitor.

Repair begins with opening the charger's case. You may have to drill out some pop rivets. These devices were so reliable, the manufacturers did not intend them to be field serviced. With the unit opened up, 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 13.2 volts, if the battery is reasonably well charged.

If the fault was in the transformer, the unit is not likely worth repairing. If the problem was a failed diode, you may be able to find a replacement. However, unless the charger was very seriously overloaded, the problem is most likely in the capacitor. If that is the case, you have found and fixed the fault in the charger, likely at a cost of less than $7 for the new capacitor. Run the charger for a few days to be sure there are no other faults, then button it up and prepare to enjoy another 20+ years of service.

Contributing Editor Chuck Husick is an electrical engineer, pilot and sailor - among many other passions and pursuits. He lives in Tierra Verde in Florida, where he rides his bike every day.

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