Batteries are generally misunderstood and almost universally ignored until they fail. In this series of articles I will summarize battery theory application and maintenance and delve into the various methods of recharging commonly utilized on boats. Although there are differences between the systems used on power and sail craft the basics are similar and the principal differences are the length of time between recharge and applications of appropriate equipment. The following charts illustrate specific gravity and voltage readings for batteries at various states of charge.

While these articles are primarily focused on recharging equipment some discussion of battery types is needed as applications and recharge requirements vary between the design and application differences. Three types of batteries are commonly used in marine applications, flooded lead acid, gel and absorbed glass mat (AGM). All of these depend upon lead acid chemistry, the difference being primarily the media in which the acid is contained and the composition of the grid material either lead antimony or lead calcium. A further breakdown is by application either engine starting or house batteries. Although many boats use the same batteries for both, the tasks are very different and batteries specifically designed for the application should be used

Engine starting is a relatively easy task for a battery and is usually best served by a battery constructed with many thin plates which allow large amounts of current (amps) to flow readily on demand from the starting motor, and conversely are quickly recharged by an engine driven alternator with a simple automotive type regulator.

House batteries, that is, the batteries which supply power to your electronics, lighting, refrigeration, pumps and all the other electrically driven appliances on your boat are by necessity a different matter entirely. These batteries are required to supply power over periods of time when there may or may not be a charging source available. For these applications thick plate “deep-cycle” batteries are in order. These batteries are designed to allow deep discharge (up to 50% of rated capacity) and recharge many times. Although batteries claiming to be “deep-cycles” vary widely in their ability to actually do so, a general rule is that top quality flooded types such as Rolls will give longest cycle life by a substantial margin followed by AGMs and gels in descending order, and as you have probably already guessed require different treatment when being recharged.

This installment will deal primarily with battery chargers powered by alternating current, (AC) either from a shore-side source or an onboard generator. In subsequent articles we will discuss other charging systems, primarily engine driven alternators, solar, wind and water powered charging sources and the means to monitor them.

To further complicate the matter, how you use your boat changes charging requirements too. If you only use your boat on weekends and it sits in a slip hooked to shore-power most of the time one set of rules apply.

If you are cruising and deeply cycling your batteries every day then another one does. If you live aboard a third set of circumstances dictates and while there is crossover between these circumstances the purpose of this article is to help define and choose which charging system or systems is best suited to your application.

AC battery chargers vary widely in design and cost. The least effective and naturally the least expensive is the simple single stage charger you can purchase at your local discount or automotive outlet. These chargers are not appropriate for charging deep-cycle battery banks as they charge slowly and cannot fully recharge to rated battery capacity in any reasonable period of time. There are no automatic current or voltage adjustments during the recharge cycle and with out constant monitoring, the single stage charger will excessively gas the battery causing electrolyte loss and may overheat and warp the plates causing premature failure.

Fig.1 Courtesy Analytic Systems

The graph in Fig.1 illustrates a charge cycle from a single stage charger. As you can see charge current drops quickly necessitating very long charge time and if you are not there to monitor the process your batteries will pay the price. Additionally these chargers usually have steel cases not well suited to the marine environment, are not ignition protected and utilize inexpensive auto transformers to drop the voltage from 120 to 12 volts rather than isolation transformers utilized in quality chargers designed for marine applications with the subsequent potential hazard of electric shock, severe injury or death, neither are they ignition protected, especially important in gasoline engine powered boats. A final but not unimportant drawback is that these types of chargers will allow substantial amounts of alternating current known as AC ripple to pass through into the batteries, a condition which is very destructive to battery plates. These chargers do not belong on boats.

So, if we can’t use the cheap stuff how should we choose an AC powered charger? The answer again depends a lot upon how the boat is used. For a cruising sailboat which depends upon periodic charges and recognizing that most cruising boats also rely on solar, wind and engine driven alternators for much of their recharge capability an onboard AC powered charger may seem unnecessary or at best a redundant luxury. There are, however some very real reasons to consider one. If space and budget allow, the ability to produce AC from an auxiliary generator has some distinct advantages. In addition to a redundant source of battery charging which is never a bad plan, many appliances operate more efficiently on generator produced higher voltage AC than on 12 volt DC or AC produced by an inverter. A sailboat particularly larger ones and a powerboat which spends a lot of time at anchor away from a shore-side source have a lot in common in terms of electrical requirements. Both need an adequate bank of deep-cycle capable batteries and a means to completely recharge them in a reasonable period of time. Since many of these vessels have refrigeration, freezers, water-makers and water heaters, all consumers of substantial amounts of power it is usually more efficient to replace their requirements with AC driven systems. Why not recharge your batteries at the same time? While a high capacity engine driven alternator is one way to do this you are using a sledge hammer to drive a tack. Why run your main engine to make a kilowatt or two of power when an efficient AC purpose built generator will do the job more quietly, and efficiently.

It is very common and perfectly acceptable from an engineering point of view to discharge/recharge your deep-cycle batteries in the 50-85% range for periods of time up to 30 days. But then, it is important to bring them all the way up to 100 %. Failure to do this will result in hardening of the sulfate formed as a normal part of the discharge process to the point where it can not be broken down by ordinary chargers and battery capacity is reduced. Continued repetition of this partial recharge cycle will result in premature battery failure. To avoid this requires a charging system with the ability to hold charge rate at the level recommended by the battery manufacturer for enough time to completely charge the battery bank. In the popular three stage chargers it is necessary that the absorption stage of the cycle is long enough so that not only is voltage driven up to the recommended level but that it is held there for sufficient time to also raise the specific gravity which lags behind voltage particularly in the thick plate batteries best suited for extended cycling applications. (See fig2)

Fig.2 courtesy Ratelco, Inc. voltages are per cell voltage multiply by 6 for battery voltage.

Many popular 3 stage chargers including the chargers in most inverters limit absorption time to 3-4 hrs, while acceptable for smaller battery banks, larger installations of 400-amp hrs and above require longer absorption times to completely charge the batteries. Two chargers which may be adjusted for absorption times appropriate to battery bank capacity are the very high quality models built by McCarron and Analytic Systems. While they have a higher initial cost, these industrial quality chargers are usually a once in a lifetime purchase and over the years will actually be less expensive than replacing several cheaper chargers. Additionally they have many important features which translate into faster more complete charging and greatly increased battery life, another substantial saving. And while it is true that this complete recharge can be accomplished with a properly regulated engine driven alternator it is going to take long engine run times to accomplish whereas the AC charger can be used when ever the AC generator is operating or shore-power is available.

For a boat which spends much of it’s time in a slip or as a live aboard some other considerations come into play when choosing an AC charger. While three stage chargers, assuming adequate absorption time, will give the fastest and most complete charge in the least time, they are best suited to charging batteries with out constant or transient loads for the reason that a three stage charger can not tell the difference between a discharged battery and a light bulb. If your boat has been sitting in its slip for a while and the batteries are fully charged and sitting at a float voltage, a load such as a light, pump or refrigerator can fool the charger circuits into pushing voltage above the level required for float charging, in other words you start charging a completely charged battery which results in heat and gassing with subsequent loss of electrolyte, plate warpage and premature failure. These problems are especially exacerbated in lead-calcium grid batteries (AGM and Gel)

Fig.3 Courtesy Analytic Systems

Fig.4 Courtesy Analytic Systems

For these situations chargers such as the Analytic Systems BCA series in two stage mode or the McCarron VMI are ideal . These chargers will carry the house loads up to their output capacity in amps with out raising voltage above the recommended float level with the damaging results discussed above.

For smaller battery banks requiring shorter absorption times there are many other chargers, mostly three stage available to the boater some good some not so. Professional Mariner LLC of Rye New Hampshire manufactures their reasonably priced Protech 4 line in sizes from 10 -50 amps and in various output voltages. These chargers have the advantage of being very flexible as to input voltage and frequency which makes them useable almost anywhere in the world including at the end of the last dock in a large marina with ever present low voltage, and are completely ignition protected. Be sure that what ever charger you choose that these features are present and look closely at the warranty.

One additional feature is available on some chargers. This is called the ‘equalize’ cycle. An equalize cycle is manually initiated by pressing a switch on the charger. Equalization should not be initiated until batteries are fully charged. Once equalize begins, the charger applies a limited current of usually not more than 10% of maximum (i.e. 4 amps for a 40 amp charger) for a predetermined period of time (some chargers have adjustable equalization periods) or until the battery voltage reaches a preset level above nominal (usually about15.6 volts for a 12V battery). The purpose of this cycle is to deliberately overcharge the good cells of a battery while allowing a weak cell to be fully charged. As this deliberate overcharging of the battery causes some water loss, it should only be performed once per month as discussed above, when battery capacity appears to be diminished or specific gravity readings vary by.005.

In addition, as the battery temperature is elevated by this cycle, a temperature sensor is supplied by some manufacturers to monitor battery temperature and modify charging voltage of all cycles according to battery temperature, as well as shutting the charger off if the battery temperature exceeds 120 degrees F or 49 degrees C.

A final decision is selecting the right charger size. As a general rule a charger with an output capacity in amps between 16 and 25 percent of bank capacity is appropriate. For example 100 amps divided by 6 or 4 equals 16 and 25 amp charger out put capacity. Flooded lead acid batteries should not be charged at more than 25% of capacity. It is claimed that gel and AGM types can withstand higher current charging but I am skeptical, having autopsied some of them which were subjected to these charging parameters. If you have a large bank say 1000 amp hrs then you could safely use a charger of 250 amp out put, but since that charger would be prohibitively large and expensive, the longer absorption times discussed above become even more important.