More boaters are switching to this type for a vast performance improvement over flooded batteries. Sealed, valve-regulated AGM (Absorbed Glass Mat) batteries feature fine, highly porous microfiber glass separators compressed tightly between the battery’s positive and negative plates, which are saturated with just enough acid electrolyte to activate the battery.
During charging, precision pressure valves allow oxygen produced on the positive plate to migrate to the negative plate and recombine with the hydrogen, producing water. In addition to providing equal saturation across the entire surface of the battery’s positive and negative plates, the fibers in the dense glass mats embed themselves into the plates’ surface like reinforcing rods in concrete, providing more plate support and better shock and vibration protection than in conventional batteries.
Flooded batteries require maintenance—periodic inspection and topping-off with distilled water. They handle overcharging better than gel and AGM batteries, because of this hydrogen venting and because they are not sealed like the other types. They self-discharge at a higher rate (6 to 7 percent per month) and thus require off-season charging. Wet cells must be installed in an upright position and don’t tolerate high amounts of vibration.
Flooded batteries, unlike other types, use a reservoir of liquid sulfuric acid, and produce hydrogen and oxygen when the battery is being charged. Vented wet cells allow the gases to escape into the atmosphere, unlike gel and AGM batteries, which recombine the gases and re-introduce them to the system. Hydrogen is an explosive gas, so battery boxes and compartments must be vented to let the gas escape safely which poses the risk of explosion.
Sealed, valve-regulated (SVR) gelled-electrolyte batteries offer advantages over regular flooded batteries.
They self-discharge at only three percent per month, handle the highest number of lifetime charging cycles, are maintenance free, spillproof, submersble and leakproof. A pressure release valve keeps their internal pressure at a slightly positive level, but they can release excess pressure if needed. The SVR design nearly eliminates gassing, so they are safer to install around people and sensitive electronics (but gel batteries still need to be vented). Gel batteries, because they’re sealed, are manufactured to very high quality standards. They need carefully regulated smart charging to prevent damage.