What is the real differance bewteen GEL, AGM & Sealed lead Acid (VRLA) batteries????


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16 May 2001
West End, Surrey, UK
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copied from West Marine site.....

When it comes to batteries, we have turned to three leading manufacturers in the industry for our selection of starting, dual-purpose, and deep-cycle batteries. This year's catalog features three distinct battery technologies: flooded lead/acid batteries, gelled electrolyte, and absorbed glass mat (AGM).

While all of our batteries are constructed to levels of quality that are exemplary by marine standards, each technology has its pros and cons. Based on a collection of manufacturers' reports and independent tests, we've compiled the following information to help you understand each battery technology.

AGM Batteries
Absorbed glass mat (AGM) batteries were originally designed for military aircraft. Concorde Marine AGM batteries, like their military predecessors, are engineered to meet a number of similar conditions, including severe vibration and pounding shocks, immersion, and periods of sustained inversion.

AGM batteries feature a dense filling of absorbent glass matting packed tightly between the battery's positive and negative plates. Once the matting is in place, the battery is 90 percent saturated with acid electrolyte which, according to Concorde, allows the oxygen to readily migrate to the negative plate and recombine with the hydrogen gas to replenish the battery's water content. Sealed, positive pressure-relief valves within the battery re-direct excess hydrogen and oxygen vapors into the microfibrous glass mat, where it can be reclaimed as liquid and re-utilized, making it unnecessary to replenish water levels.

In addition to providing equal saturation across the entire surface of the battery's positive and negative plates, AGM advocates claim that the dense glass mat filling provides greater plate support, and more shock and vibration protection than conventional batteries. They suggest that this dense packing offers a lower internal resistance, allowing AGM batteries to recharge faster, discharge longer and generate greater starting power than deep-cycle batteries using other technologies.


No maintenance (except periodic external cleaning)
Sealed batteries cannot spill
Can be installed at any angle
Shock and vibration resistant
No gas release
Low self discharge (3% per month at 77°F)
Submersible without damage
Long cycle-life
Can be shipped via UPS

High initial cost
New technology-short time in marine market
More weight per Ah than flooded
Water cannot be replaced after overcharge

Gel Batteries
Our SeaGel batteries are made by East Penn Manufacturing, who we believe has the best technology and quality control in gel battery manufacturing. Originally a licensee of Sonneschein Prevailer batteries, East Penn now makes their own line of Deka gel batteries as well as the West Marine SeaGels. When customers ask us if it is worthwhile to upgrade to SeaGel batteries, we are extremely confident saying "Yes!", based on the low return rate of the batteries, and the field reports from our customers.

Like AGM batteries, a sealed gelled-cell battery is "recombinant". That is, the oxygen produced by the battery's positive plates, recombines with the hydrogen gas created by the battery's negative plates, forming water that's cycled back into the electrolytic gel. East Penn allows recombination to occur by sealing each cell of the battery with a pressure-relief vent, which holds the gases under pressure (3.4-4.0 psi), causing natural recombination to occur.

The "gel" is a mixture of sulfuric acid, fumed silica, pure water, and a phosphoric acid which produces a thixotropic gel that offers several-times the cycle-life of competitors' gel batteries, and three-times that of flooded-cell batteries.

Once mixed, the gel is drawn into each cell under a vacuum, which totally eliminates voids and pockets of air that can result in "dead spots" on plate surfaces.


No maintenance (except periodic external cleaning)
Sealed batteries cannot spill
Can be installed on side (with 10% capacity loss)
Low-temperature tolerant
Shock and vibration resistant
No gas release
Long cycle-life
Low self-discharge (3% per month at 68°F)
Can be shipped vai UPS

High initial cost
More weight per Ah than flooded
Water cannot be replaced after overcharge

Flooded Batteries
Though the newer battery technologies are impressive, a number of inherent advantages continue to make the high-quality traditional flooded-cell battery a serious contender in the deep-cycle battery marketplace.

Properly charged and maintained, premium wet-cell deep-cycle batteries, like those made for us by Trojan, are capable of as many as 1,000 cycles, which can translate to many years of dependable service-at an initial cost that's substantially less than comparable capacity AGM or Gel batteries.

Like the other battery technologies, flooded batteries depend on the chemical reaction of lead and sulfuric acid to create electrical energy. Unlike the others, flooded (or wet cell) batteries rely on liquid sulfuric acid to act as an electrolyte pathway between positive and negative plates. These plates produce hydrogen and oxygen when charging like the others but, rather than recombining the gases and re-introducing them to the system, vented wet cell batteries allow the gases to escape into the atmosphere. As such, they require periodic inspection and topping off cells with distilled water.

This is something of a double-edged sword: the occurrence of venting into the atmosphere increases the level of hydrogen gas around the battery which demands that the batteries be ventilated (since hydrogen accumulations greater than 4% are flammable). At the same time, only wet cell batteries allow water replenishment if batteries are overcharged.


Lower initial cost
Accepts higher recharging voltages
Less vulnerable to overcharging
Good deep-cycle performance with proper care
Less weight per Ah than gel or AGM

Must be installed upright
Requires periodic maintenance
Requires ventilation
Higher rate of self-discharge (6% to7% per month)
More fragile in high vibration environments
Corrosive battery acid can be spilled if battery case is ruptured or overturned.
No matter what battery you choose, we have some recommendations that will keep you from experiencing disappointment when you install a battery or multi-battery system in your boat:

Stay with one battery technology.
Never mix old batteries with new batteries in the same bank
Stay with one vendor if possible, and upgrade to either Trojan, East Penn, or Concorde over time.
Regulate your chargers (alternator, shore power, and environmental) according to the chemistry of the battery, and the temperature. This is especially important for Gel and AGM batteries, which can be damaged by overcharging.
Keep batteries clean, cool and dry. Check terminal connedtors at least annually to ensure a tight fit. Add distilled water regularly to flooded batteries.