Zena Solves the Alternator Problem

The Shearwater II is a power hungry sailboat.  She has two 12 volt refrigeration units and a 12 volt water maker plus a number of other smaller devices that produce a base load of about 20 amps.  The house bank is nearly 900 amp hours.  If I restrict usage to the upper 50% of the bank’s capacity to extend the life of the Absorbed Glass Mat (AGM) batteries, she must be recharged about every 20 hours if everything runs all the time.

We are satisfied to charge twice each day for about two hours so we must operate two alternators that each deliver at least 100 amps to the house bank beyond what it takes to operate the house load.  That way when we charge or motor-sail on one engine, we can still charge at optimal rate.

When we commissioned the Shearwater II, we installed an additional alternators on each engine, both auxiliary alternators were Balmar 95-210s on special brackets.  These never worked completely correctly, producing below their rated power, requiring three rebuilds in three years, and being a constant source of concern.  Their 3-year cost was well over $2,000 each including rebuilds and the initial cost.

It was time for a change.

We asked friends, examined the web sites and requested help from Maxpower and Ample Power in choosing the correct alternators for our installation.

Previous success with Maxpower regulators almost made up our mind to order a Maxpower for a test.  They have proven to be very easy people to work with and the regulators are well designed yet simple to operate.  Their bolt-to-bolt compatible unit that replaces our Balmar 95-210 was about $400 less than Balmar – a strong vote in their favor.

Ample Power declined to work with us and was generally critical of our attempt to generate lots of power.  Careful reading of their adds gave me the feeling they intend their units primarily for battery charging, not for continuous delivery of high power (base load operation).  We must have caught them on a bad day (for either them or me).

One unique device made by Zena, caught our attention.  It is advertised as a “high frequency DC welding generator” and is sold in sizes between 150 amp and 800 amp.  At $495, Zena’s 200 amp Marine Alternator the least expensive 200 amp dc source we could find - by a wide margin.  The unit has a 3” mounting foot that was easily adapted to fit the 4” saddle mount employed by Balmar. The pulleys and belt offset are nearly identical-no shimming was necessary.

Best of all, Zena alternators come with a three year warranty – even in marine use.  Additionally, they come with Tony Blazina, a patient and knowledgeable man who explained why Zena regulators seldom need warranty work.  They are designed for continuous service at 160 degrees and have a front bearing the size of a wheel bearing on a good size car.  While I hope the new alternator (and the second one I expect to install on the other engine) always works as well as it does today, I’m confident Tony will be there to help me get it working again if it should fail.

According to the Zena website, DC high frequency welders are ideal for marine battery charging.  The output consists of a number of spikes of direct electrical current all in the same direction.  DC welders come in two basic types (relative to our marine needs).  One type has the positive field voltage switched by the voltage regulator and the other type supplies a constant positive voltage while the negative voltage is switched on and off as needed.  Most marine regulator installations are positive switched.  Our Balmar and Powermax regulators are both positive switched as are all other marine regulators I examined.

Zena buys automotive cases and adapts them to the Marine alternator.  The two control leads are labeled “fld” and “sta”  just as they are on normal marine alternators – the “sta” lead is not connected to the stator, but rather the other end of the field winding from the “fld” lead.  In a positive switched installation such as ours, a wire is connected between the “sta” and the negative-ground lead, effectively converting the unit to a “normal” marine alternator.  No output is presented for a tachometer in the Zena configuration.  They provide a “simple” circuit on their web if you require a tach sender from this alternator.  Owners with Yanmar engines do not normally require a tech sender from their alternator.

The red or blue wire from your voltage regulator (Zena sells one, unevaluated, for $50), connects to the “fld” lead.  Be sure to use 10 or 12 gauge wire (depending on the length) and that your regulator is capable of supplying 8 amps continuous.  Our Balmar regulators accept a switch signal to turn the regulator on and off.  We supply that signal from the “fld” lead on the built-in regulator (it draws little current), effectively providing a way to turn the high power regulator off from the helm station by turning off the key. 

Our engine will not provide the needed horse power at idle to operate the Zena alternator when the battery is low.  Thus, we must charge at a higher RPM and turn off the key when maneuvering at idle speeds with a low battery.  This is not a problem after an hour or so of charging.

Installation is straightforward.  Since we are dealing with currents approaching 200 amps, the red and black wires in the photo on the left are “00” gauge and are limited to 6’ in length in our installation.  If your lead lengths are longer, you may need heavier wire.  Each alternator is connected directly to its respective ½ of the battery bank through a 200 amp fuse.  The two halves of the house bank are selected via an "a", "b" switch that is normally operated in the "both" position.

The black wire in the lower part of the diagram is the “sta” wire described above while the blue wire comes from our Balmar 612 alternator (not shown) about 3 feet away.  The small red wire is the sense wire for the regulator so it can determine the charging voltage, while the wire above it is the temperature sensor to limit the output should the new Zena begin to heat beyond its 160 degree limit.

Except for the temperature sensor, the unit worked correctly as installed and is quietly delivering 96 amps (plus house load of 36 amps into a partially charged 13.35 volt house bank.  If we use the microwave when the batteries are under charge with one engine, the alternator delivers just over 160 amps at 2500 engine rpm - and continues to charge the batteries at about 15 amps while powering the microwave.