Wet
cellLead-acid batteries come in three basic types and many varieties. Starting batteries like those found in cars have many thin, porous or spongy plates that deliver occasional spikes of high current, down to about 80% of capacity in a short time, immediately followed by a fast and full recharge. Deep cycle batteries used as house batteries on boats have fewer solid lead plates that deliver less current over a longer time and can be discharged down to about 20% of capacity before slow recharging. Hybrid batteries are a compromise that delivers high current for starting and can be discharged down to about half of capacity. Batteries designed for marine use have stronger cases than automotive batteries and their plates are more rugged and more firmly attached. When buying a battery, heavier is always better and the best tend to be black all around with few or no labels or markings. wet cells, are the most common. They have liquid electrolyte, added through caps on the top, which should just barely cover the lead plates at all times. Some automotive wet cell batteries are maintenance free, having a valve that prevents loss of electrolyte by recombining the gases produced during recharging. Although fine for starting cars, these valve regulated lead acid (VRLA) batteries are less often found in boats because the electrolyte cannot be topped off if any is lost through total discharge, overcharging, or less than optimal recharging.
Gel
cellGel cell batteries have a thick, viscous electrolyte that cannot spill if the battery tips or cracks. Like VLRA wet cells, they are sealed, making it impossible to replace electrolyte that will be lost through gassing if the rate of charge is not precisely correct. Gel cells charge at a lower voltage than flooded cells and can be ruined if used with a wet cell charging system.
Absorbed
glass matAbsorbed glass mat (AGM) batteries have fiberglass mats glass mat between the lead plates to hold the electrolyte and prevent spills. These mats make AGM batteries the least susceptible to vibration. They charge at nearly the same voltage as wet cells. AGM batteries are sealed and cannot be topped off if electrolyte is lost during recharging. They take more current, during charging, which can overload the charging system but also means that they charge faster.
Both gel cells and AGM batteries can discharge at a higher rate than wet cells, thereby reducing
the need for battery capacity. Gel cells also charge slightly more efficiently than wet cells and AGM batteries charge significantly more efficiently. While unsealed wet cells can leak off up to 1% of their charge every day, a VRLA battery loses only about 2% a month. Golf cart batteries A good alternative to marine deep cycle batteries is golf cart batteries that are best obtained at warehouse clubs. They have heavy plates, well attached to tolerate the bumpy motion of a cart, and they will survive three or more times as many discharge cycles as a typical marine battery. They provide 6 volts, having only three cells, and so two must be wired in series. This is no problem if you change the cable connections and then cruise with the A-B battery selector set to “both” (as many recommend doing anyhow) but otherwise you might need another battery tray. Warehouse club golf cart batteries cost only one-third as much as marine deep cycle batteries, per amp-hour, over their lifetime.