What is the right battery to have on board because of your needs? We tried to understand this together with Sergio Monfardini, technical director of Mastervolt Italy. First of all, it must be said that battery models differ greatly, and to achieve maximum efficiency, on board, one would need a dedicated battery for each different type of service (charging curves, however, differ from model to model, and so does maintenance).
FOR THE CRUISE? THE AGMS
For the typical cruiser, a good choice is undoubtedly AGM batteries, models in which the electrolyte (i.e., the mixture of water and sulfuric acid) is almost completely absorbed by fiberglass elements, making them safer than liquid lead batteries in the event of breakage. They are maintenance-free and produce virtually no gas spills, which are harmful to health and the bilge. Hence the advantage of being able to locate them wherever you want on board, even in unventilated places. Because of their design, AGMs can be discharged quickly, thus providing a good boost for systems that require high current peaks: they are particularly suitable for engine starting and services. It costs roughly twice as much as a conventional lead acid battery, but a 12V/90 Ah AGM can develop a current comparable to that of a 180 Ah liquid lead battery. They also present a faster charging speed and heat up less. Beware, however, that the lower internal resistance may cause faster self-discharge when not in use. We therefore recommend that you disconnect the clamps if you are planning a long break or get a “smart” charger (perhaps a solar panel charger, if winterizing is not done indoors) that allows you to maintain a constant charge level. Although AGMs can withstand deep self-discharge (up to 70% of their capacity), should they go into sulfation it is more difficult to regenerate them because of the lack of liquid acid.
AGM AND GEL BATTERIES
As said earlier, every service, on board, would need a dedicated battery: the problem is that on a boat, where charging is often done directly from the engine alternator, you will need a good charge controller that doses the current according to the type of battery. You can “bypass” the step by equipping yourself with batteries that exhibit very similar charging cycles: such as AGMs and gel batteries (where the electrolyte is absorbed, precisely in a gel), which are suitable for utilities and continuous use. A reliable battery bank, therefore, will include AGMs for the starter and engine and gel ones for the services.
Classic liquid-lead batteries (either with topping up caps or sealed) can be used for both starting and services: cheaper, they require constant maintenance, topping up the liquid if necessary and avoiding too deep discharges (more than 50 percent of rated capacity). Regeneration is possible, but as time passes they will have less durability.
Lithium-ion batteries represent the top of the line: at high cost, they weigh up to 70 percent less than lead-acid batteries, presenting three times the service life (up to 2000 charging cycles) and their capacity does not vary with time. They always guarantee the same performance: suitable for continuous use, they are particularly suited for long, hard sailing (it is no coincidence that the Volvo Ocean Race’s new Ocean 65′ fleet is equipped with this type of battery).
LAST BUT NOT LEAST
A good rule of thumb would be to calculate, ammeter in hand, the power consumption of all your utilities to figure out the correct sizing of the battery bank. A “model” table on the Ampere consumption of individual services can be found at the bottom.