Marine batteries, their design, use, and charge/discharge abilities, have been a subject of intense interest within the recreational marine industry for decades. I clearly recall a meeting many years ago, when I worked as a boatyard electrician. I sat in the yard managers office with the manager and one of our long-time customers, a fastidious naval architect who had recently completed a roundtrip passage from New England to Bermuda aboard his 40-foot sloop as part of an organized race. In his hand he had a sheaf of papers which contained neatly written columns recording battery voltages, amperes being used, and the time that each reading was taken, which was roughly hourly, as well as an indication of when and how long the engine was used for charging purposes. He was less than content with the ability of his house battery bank to provide for the modest electrical needs of the vessel. It was equipped, of course, with cabin and navigation lights, communication equipment and radar (the latter only being used occasionally at night), as well as a few other small accessories. Even by the standards of the time, the late 80s, it was an electrically simple boat, yet, the batteries could keep up with its needs.
That meeting made a strong impression upon me and it served to reinforce my existing experience, which was a result of making my own offshore passages aboard sailing vessels whose battery banks seemed equally as anemic. In fact, I would later learn as I studied the subject of batteries and battery charging over the next 15 years, that the problem, as is often the case, was more complex than it appeared.
Fortunately, the world of batteries and the methods by which they are charged began to change dramatically shortly after that July afternoon meeting in the yard managers office. With the advent of first gel and then AGM batteries, it became possible to charge batteries much more rapidly when compared to the century-old technology of conventional flooded batteries. Vessel owners embraced gel batteries, only to find they didnt live up to their expectations. The reason, however, wasnt the batteries, it was the method used to charge them. While the battery technology had advanced rapidly, the charger technology lagged. Within a few years, however, the gap between advanced batteries and shore-powered, and to some extent alternator chargers, closed, and predictable, reliable performance was at hand. It changed life for many boaters by reducing considerably the time required to charge batteries, which often meant running the engine or generator for no other reason than to accomplish this task.
Running a generator or, worse, a propulsion engine for hours under these extreme light load conditions is detrimental to say the least. The batteries were getting charged, at the expense of the longevity or reliability of the engine, an undesirable scenario to be sure. Add to this scenario an inefficient charge source and the problem multiplies exponentially: batteries are discharged and not fully recharged over and over again, shortening their life dramatically. In spite of advances in shore-powered and alternator chargers, one of the reasons batteries of that era, and to a large extent the current one as well, suffered from chronic undercharging was because of the charge source, the stock alternator supplied with most engines. This is a story for another time, however, for the moment lets just say that off the shelf, internally regulated alternators that are supplied with most engines, regardless of their rated output capacity, are not designed to recharge large, deeply discharged house battery banks (when I say large I mean over 100 amp-hours and used in deep discharge house applications).
While gel and AGM batteries offered much needed and welcomed relief to beleaguered cruisers and their engines and generators, the electrical needs of many vessels have now outpaced the advantage that was once afforded by these SVRLA batteries (gel and AGM batteries both fall into the category of sealed valve regulated lead acid or SVRLA category). Now, however, thanks to advances made as a result of the increased demand for hybrid and electric automobiles, a variation on the AGM theme has been introduced by a company called EnerSys, the worlds largest manufacturer, marketer, and distributor of industrial batteries, which is the parent company of Odyssey Battery, a popular supplier of marine batteries (odysseyfactory.com) of Reading Pennsylvania. (They invented SVRLA technology in 1973.) Ive been a fan of Odyssey batteries for several years (I wrote a column about their impressive group 31, 1000 cold cranking amp start battery, which I tested to near destruction back in 2005, one of which has been installed in Bill Parlatores vessel Growler since then), their heavy duty construction and no-exaggeration capacity ratings make them attractive for both power and sail cruisers. Using a process called thin plate pure lead or TPPL, Odyssey batteries have broken through the charge acceptance rate barrier by enabling users to pump considerably more power back into a discharged battery at a rate once considered impossible.
Charge acceptance rate (CAR) is typically measured as a percentage of a batterys amp-hour capacity. Conventional flooded batteries, under ideal conditions, may have a CAR of roughly 25 percent, while gels and AGMs accept much higher CARs in the region of 30 to 50 percent. Its important to remember, however, that the CAR is highly dependent upon the batterys state of charge: a heavily discharged battery, one that has just 10 percent of its charge left, will typically have a CAR of nearly 100 percent.
The thickness of a batterys lead plates has a direct effect on its charge rate as well as its ability to deliver current. This is why dedicated starting batteries tend to be smaller and lighter than deep cycle batteries; they are made using many thin plates, which deliver a lot of power for a short period of time and consequently can be charged very rapidly. Deep cycle or house batteries, on the other hand, are made using less-thicker plates, which means they deliver less amperage, pound for pound, at a given moment and they take longer to recharge. Odysseys TPPL system utilizes, as the name suggests, extremely thin lead plates that can hold up to the stresses that previously required deep cycle plates be stronger and thus thicker. The TPPL system achieves this goal by utilizing a proprietary process for stamping, rather than casting, its plates and by utilizing extremely pure lead, 99.99 percent pure. Beyond that, the batteries utilize conventional AGM technology whereby the plates are packed into the case very tightly, separated by sheets of acid-soaked glass fiber, hence the absorbed glass mat designation. The thin plate technology enables the batteries to achieve CARs of upwards of six times their amp-hour capacity (for example, a 200-amp-hour battery, typical for an 8D footprint, could accept, if you could provide it, as much as 1200 amps of charge current) when discharged 50 percent, which is 12 to 15 times faster than charge rates hitherto accepted as fast.
With the advent of this improvement in battery technology its likely that engines and generators wont need to be run for as long as they once were to charge batteries and the output of charge sources. Its also likely that alternator and inverter/charger manufacturers will begin producing higher capacity models that will be able to take advantage of the improved charge acceptance rate.