There are three types of chemical marine batteries. These include the flooded batteries, gel batteries, and Absorbed Glass Mats (AGMs). These are rated depending on the energy output, which is measured in ampere hours. The output, together with the lifespan, is what typically dictates the price of the batteries. In a nutshell, how you use your boat, the type & size of your boat, and your budget are the key determining factors of which battery you should end up buying.
These are also called wet batteries, and are the cheapest and most common type of marine batteries. When the battery is being charged, the sulfuric acid produces oxygen and hydrogen, which escape through the vents. These batteries need periodic inspection and topping up with distilled water. The flooded batteries are known to self-discharge at a rate of 6%-7%, which is higher than with AGM and gel batteries. These batteries are difficult to maintain in boats as they tolerate minimum vibrations and need to be kept upright at all times.
The “gel” used here is a combination of pure water, fumed silica, phosphoric acid, and sulfuric acid. This gel is somewhat viscous, which prevents the battery from leaking if it’s tipped or if the case is damaged. This battery is also called a “recombinant battery” because the hydrogen and oxygen produced during charging is converted back into water by the pressure inside the batteries.
Absorbed Glass Mat (AGM) Battery:
These batteries are designed with glass mat separators that are saturated in an acid-electrolyte solution. Pressure valves allow oxygen that is produced on the positive plate during charging to move to the negative plates, where it recombines with hydrogen to make water. These batteries have better vibration and shock protection compared to gel and flooded batteries. Although they are more expensive than the others, the added cost is well reflected in the quality.