While you’d hardly think of specifying a particular chemistry for your car battery, it is quite common for marine batteries because they are available in three different chemistries that suit different applications. Most of our marine batteries fall into one of three categories: flooded, gel, or AGM. Your choice should be based on your needs (deep cycle vs. starting), the capacity and life span you’re looking for, and your budget.

All three types depend on the chemical reaction of lead and sulfuric acid to store and release electrical energy. Flooded, or wet, cell batteries provide high cranking amperages and are excellent for starting applications. They are generally lower in price and are available in standard sizes and terminal configurations. Compared to gel and AGM batteries, wet cell batteries are not sealed, (so they can be topped-off with distilled water) and they are better suited to accept overcharging. Properly charged and maintained, premium flooded deep-cycle batteries are capable of a few hundred to more than a thousand cycles, which can translate to many years of dependable service at an initial cost that’s substantially less than AGM or gel batteries with comparable amp hour ratings. However, they lose capacity and become permanently damaged if left discharged, must remain in an upright position, and are susceptible to failure in high-vibration applications.

Gel batteries (a.k.a. sealed, valve-regulated or SVR) are sealed and use pressure release valves to keep their internal pressure at a slightly positive value, yet they can release excessive pressure if needed. The impossibility of adding water, or gaining access to the battery’s inside, forces the manufacturer to make an extremely high quality battery with great consistency. When initially mixed from the various chemicals, the gel is quite liquid, and can be poured easily. The gel is drawn into each cell under a vacuum that eliminates voids and pockets of air that can result in dead spots on plate surfaces and loss of capacity and performance. Once it is in place, the gel becomes quite viscous, making the battery impervious to leakage in case the battery gets inverted or its case sustains damage. A downside to gel batteries is that there is a maximum charging voltage of 14.1 volts. Because of this requirement, the boat’s charging system must be adjusted for the battery to be properly recharged without dramatically shortening its life.

AGM (Absorbed Glass Mat) batteries feature a dense separator compressed tightly between the battery’s positive and negative plates. These specially designed plates are saturated with a precise amount of acid electrolyte. 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 in the surface of the plates, like reinforcing rods in concrete. This provides more plate support and better shock and vibration protection than in conventional batteries. The high packing density results in lower internal resistance, allowing AGM batteries greater starting power and greater charge acceptance than other types of deep cycle batteries. AGMs are spill-proof, as well as the most vibration/impact-resistant lead acid batteries available. They can operate in any orientation and won’t spill electrolyte if the case is broken. The low self-discharge and longer life expectancy (twice that of flooded cell batteries) should pay off in the long haul for boaters with serious battery requirements.

Flooded Batteries

Pros: Lower initial cost; Accepts higher charging voltages; Good deep cycle performance

Cons: Can spill corrosive battery acid; Must be installed upright; requires regular maintenance; sustains damage if left discharged; High rate of self-discharge; Requires off-season charging; Not suitable for high vibration.

Gel Batteries

Pros: Resist over-discharging, vibration, lack of maintenance; Leak proof when inverted or cracked; Can be installed on side; Tolerates low temperatures; Minimal gas release when charged properly; Low self-discharge.

Cons: Higher initial cost; Electrolyte cannot be replaced when overcharged; Need to be charged at 14.1 VDC max; Not ideal for charging with automotive or unregulated chargers.

AGM Batteries

Pros: Maintenance-free; Leak proof when inverted or cracked; Dual purpose (deep cycle or starting); Can be installed at any angle (except upside down); Resists shock and vibration; Minimal gas release when charged properly; Low self-discharge; submersible without damage; Long cycle life when properly charged; better performance for DC loads.

Cons: High initial cost; More weight per Ah than wet cells; Electrolyte cannot be replaced when overcharged.