In response to increasing DC electrical load on most boats, some years ago one or two companies began to take a close look at what would constitute an “ideal” battery charger for marine applications. Today, we have the fruits of that investigation: several companies market high-tech marine battery chargers, all of which appear to be improvements over previous designs.
For input power, battery chargers use AC, which cycles from positive to negative (forming a sine wave) 60 times a second in the U.S. (60 hertz) and 50 times a second in Europe (50 hertz). The line voltage may be a nominal 120 volts (U.S.) or 240 volts (Europe). A transformer is used to step this voltage down to appropriate battery-charging voltages.
On the output side of a transformer we still have AC. To be of any use in charging a battery, it must be rectified to DC, a process whereby the positive and negative pulses are separated and alternately fed to the positive and negative poles of the battery being charged. Rectification is done via a diode bridge of one kind or another.
The two basic components of a battery chargera transformer and some method of rectificationcan be put together in numerous different ways. The most common in the marine field is the ferro-resonant charger. These will have transformers with the appropriate primary and secondary windings such that, given the rated input voltage and frequency (e.g., 120 VAC at 60 Hz), the desired output voltage (e.g., 13.8 VAC) results. All that remains is to rectify the output to DC, something that can be readily accomplished with diodes. Transformer construction is critical, but if it’s done right, the transformer itself will hold the desired voltage without the need for any further regulation or control.
Other types of chargers use transformers that are wound to produce a higher finishing voltage than a ferro-resonant transformer. Their output is then regulated with electronic circuitry using either silicon-controlled rectifiers (SCRs), triacs (also called a bidirectional triode thyristor), or metal oxide semiconductor field effect transistors (MOSFETs)all devices for switching circuits on and off in response to a control signal. SCRs and triacs are found in conventional battery chargers, with SCRs operating on the secondary (battery) side of the transformer, while triacs operate on the primary (incoming or line voltage) side of the transformer.
MOSFETs are used in the latest wave of chargers (called high-frequency switchers) that are an outgrowth of power supplies used in the computer market. High-frequency units operate by pumping up the incoming AC line frequency from 50 or 60 Hz to 50,000 or even 100,000. (The higher the frequency, the greater the efficiency of the transformer.) Then the high-frequency transformer output is rectified with MOSFETs, which also regulate the charger’s output by using more or less of the available pulse (pulsewidth modulation). The end result is a charger that is smaller, lighter, more sophisticated and more powerful than anything else on the market. However, it is also considerably more complex.
A yardstick for comparison
Given the market dominance of ferro-resonant chargers over the past few decades, a good way to open up a discussion on the subject of battery chargers is to look at the pros and cons of this type.
The most obvious advantage is the extreme simplicity of the design. It is relatively economical to produce and has proven incredibly reliable over the years. But price and reliability are one thing; performance is another. The simplicity of design that results in this excellent track record is also the ferro-resonant’s chief stumbling block. Since there are no regulating circuits, the only way to avoid overcharging a battery with a ferro-resonant charger is to wind the transformer in such a way as to keep its finishing voltage below 14.0 volts (in practice, most manufacturers set a target of between 13.6 and 13.8 volts). Once a battery’s voltage has been driven up to this level, the transformer’s construction must ensure that its output is down to zero amps (or at least, no more than milliamps).
The nature of the transformer is such that the current tapers off as the finishing voltage is approached. As a result, once a battery is just 50% charged many ferro-resonant chargers are already below 50% of rated output. By the time the battery is 75% charged, the output of even a large charger will be down to a trickle of amps. To bring a deeply-discharged battery back to a state of full charge is likely to take the better part of 24 hours.
If a boat is only used for weekends and then left in a slip with shore power for the following week, the necessary extended charging times will not be a problem. But in any situation where there is pressure on charging times, the batteries will not be properly charged and sooner or later will succumb to progressive sulphation and permanent failure.
In spite of this weak charging performance, once the batteries are fully charged, the continuing trickle charge from many a ferro-resonant charger is enough to slowly boil away the water, or dry out the gel, in any unused battery that is left permanently connected. (This is precisely the operating condition of many batteries when a boat is left in its slip connected to shore power.) Boat yards, marinas, and battery dealers have numerous horror stories relating to ferro-resonant chargers. (Marine Development Corporation has a modified ferro-resonant chargerthe Sentrywhich has a slightly higher finishing voltage than most, with an additional automatic shutdown circuit to prevent overcharging during extended charging. This does prevent overcharging but at the price of another circuit and added complexity.) There is another problem. Since the output voltage and current of a ferro-resonant charger are determined by the input voltage and frequency, fluctuations in the line voltage or frequency (which are quite common with inadequate dockside wiring and on-board generators) will cause uncontrolled fluctuations in the output, resulting in under- or over-charging. The frequency, in particular, must be accurate to within two Hz.
When taken overall, this is not a very rosy picture. The bottom line is that if high performance is desired, a different design is needed.
To achieve fast charge rates, a transformer must be wound to give a finishing voltage well above the voltage on the DC system. The charger will then have the ability to maintain a high voltage differential over the batteries it is charging. The higher the differential, the more current that can potentially be pumped in, depending on the overall capability of the charger and the battery’s ability to absorb the charging current.
However, the other side of this picture is that any charger with the capability to pump in high current at high battery voltages also has the capability to destroy the battery. Some kind of a regulation circuit is essential to control the output of the charger as the battery comes up to full charge.
A basic SCR or triac charger is likely to have a constant voltage regulation circuit similar to that found on an automotive alternator. This will taper down the charger’s output as the regulator’s voltage set-point is approached. The charging curve is not too dissimilar to that of a ferro-resonant charger, with the significant exception that the rate of charge is considerably higher during the important latter stages of charging. Once the battery reaches the regulator’s voltage set-point, the charger either switches off or trips to a lower float setting to avoid overcharging. In addition to a better output curve, the higher potential finishing voltage on the transformer, which is then regulated down, provides some leeway to compensate for changes in line voltage and frequency. The net result is a significant gain in performance, but with a commensurate increase in complexity.
This has pretty much been the state of the art until the recent introduction of “smart” (multi-step) chargers. These have a transformer that is wound to give continuous output at high charging voltages. The regulator, which may be of the SCR, triac, or MOSFET type, is then controlled by a microprocessor. The sophistication of the charging regimen is now limited solely by the ingenuity of the programmer. In theory, a customized program can be tailor-made for any battery in any application. In practice, most smart chargers have a standard set of programs, including bulk charge, absorption charge, float charge, and maybe an equalization cycle. A few units even have something called a sulphation recovery program.
The bulk charge is a near-constant current charge, at or above the charger’s rated output, which is maintained with little or no tapering effect right up to the voltage regulator’s set- point (generally between 14.2 and 14.4 voltsconsiderably higher than a traditional charger). By the time the voltage trip point is reached, a battery will be 80% or more charged. A powerful, smart charger will bring a discharged set of batteries up to this level of charge several times faster than any traditional charger.
An absorption charge (found on some chargers but not on others) is a constant voltage charge maintained at the level of the termination voltage of the bulk charge (in other words, the regulator tapers off the charger output, keeping it to a level which will just sustain the regulated voltage). The absorption charge is continued either for a specific time, or until the current the battery will accept at this voltage has tapered to a pre-set level (the specific mechanism for determining when the charger switches from absorption to float constitutes one of the subtle differences between one high-tech charger and another). The absorption charge is particularly important on a wet-type battery that is cycledit holds the battery at the gassing level long enough to knock sulphates off the plates and mix up the electrolyte, preventing damaging stratification. At the end of an absorption cycle a battery will be 90 to 100% charged.
A float charge is a low-level charge (between 13.2 and 13.6 volts depending on battery type and application) which is designed to compensate for internal losses in a battery but without overcharging the battery. It is initiated at the termination of either the bulk charge cycle or the absorption cycle. If any DC load comes on line during the float cycle, the charger prevents battery cycling by increasing its output to meet the load while maintaining a constant float voltage.
An equalization capability is, once again, found on some chargers but not on others. When present, the charger maintains a low constant-current charge for several hours, driving already-charged batteries to as high as 16.2 volts. Since the high voltages involved can damage sensitive electronic equipment (e.g., a depth-sounder), equalization is always a process that must be initiated by the user and which can only be continued for a strictly limited time. It is generally done after isolating the battery in question, or disconnecting it from all equipment. An equalization capability is an especially valuable function on a boat with wet-type, deep-cycle batteries that are cycled on a regular basis. The high voltages will restore partially sulphated plate areas to active duty, thus maintaining capacity and extending life.
A sulphation recovery program can, in certain circumstances, bring completely flat and badly sulphated batteries back to life. Resurrection is achieved by hitting the battery with very high voltages (20 volts or more on a 12-volt system) at very low amperages until the battery begins to accept a current, and then backing off the voltage levels as the battery state of charge comes up.
While some multi-step charger programs are almost entirely factory pre-set, others allow the user to determine the various voltage trip points, current- and time-limiting parameters, and the equalization program. A charger may have a single output, but most have at least dual outputs.
Line frequency v. high frequency
Such multi-step programs can be employed with both line frequency transformers (whose output is regulated by SCRs and triacs) and also with high-frequency transformers (regulated with MOSFETs). There is an ongoing debate about the merits of the two approaches. On the one hand, we have the “conservatives” who see high-frequency switchers as notoriously unreliablethe electronics are complex and difficult to regulate. On the other hand, there are those who point out that in the computer world, manufacturers changed over from ferro-resonant to high-frequency switchers about five or six years ago, and that problems have long since been solved.
There is no question that the high-frequency switchers have significant advantages over conventional battery chargers, and not just in terms of size and weight. The very nature of the high-frequency process makes these chargers both insensitive to input frequency (whether it be the fluctuating frequency of a poorly governed AC generator or the difference between European and American frequencies) and also tolerant of high variations in input voltage (many nominal 120-volt chargers will still produce full output with input voltages as low as 80v or as high as 150v). The output of a high-frequency charger is also pure DC, in contrast to many conventional chargers which have a superimposed AC ripple, which can be destructive to batteries, particularly gel-cells.
On the other side of the coin there is the question of complexity and its potential impact on reliability, and the fact that high-frequency technology, in its “raw” state, is electrically very dirtythe rapid on/off switching of high currents has a tendency to generate large amounts of radio frequency interference (RFI). Manufacturers of high-frequency switchers have invested, and are continuing to invest, with varying levels of success, considerable sums of money to filter out RFI.
Wave of the future?
The new generation of microprocessor-controlled chargers represents not simply a linear development of ferro-resonant chargers but a radical advance. The computer age has caught up with this segment of the marine marketplace.
The charger is no longer seen simply as a crude device for pushing current into a battery, but is regarded as an essential element in ensuring a battery’s good health. Not only can batteries be kept more fully charged in less time than was previously possible, but their life expectancy can also be extended, sometimes considerably. The larger and more expensive the batteries, the greater the potential benefit: a good-sized bank of deep-cycle batteries may well repay the cost of a high-tech charger out of extended battery life alone.
But, as always, there are drawbacks to any system; the likelihood of charger failure increases with its complexity. For this reason, ferro-resonant chargers will remain attractive to the sailor whose boat spends much of its time in a slip with shore power, and who is primarily concerned with cost and reliability, rather than performance.
In contrast, there are many boat owners, particularly blue water sailors, who will benefit from a sophisticated high-tech charger, and who will find the new breed of chargers to be an invaluable tool. That is, just so long as these chargers prove themselves to be reliable in rugged marine use; and only time will reveal if that is the case.
Contributing editor Nigel Calder is the author of several books, including Boatowner’s Mechanical and Electrical Manual, published by International Marine.