The most common kind of battery in marine use today is the standard lead acid battery. Using an electrolyte consisting of sulphuric acid, these cells can store large amounts of electrical energy in a relatively small space. This energy is stored within lead grids mounted inside the battery. The lead explains the great weight of lead-acid batteries.
The battery selection is further divided along the lines of battery construction. Currently, there are three common battery technologies: Wet-cell, Gel, and AGM.
Wet Cells are the most common lead-acid battery-type in use today. They offer the most size and design options and are built for many different uses. In the marine business, they usually are not sealed so the user can replenish any electrolyte the battery vented while charging the battery. Each cell will feature a grid of lead plates along with an electrolyte based on sulphuric acid. Since the grid is not supported except at the edges, flooded lead-acid batteries are mechanically the weakest batteries. Since the container is not sealed, great care has to be taken to ensure that the electrolyte does not come into contact with you (burns!) or seawater (chlorine gas!), or an explosion from the hydrogen produced. The USCG requires that wet cell batteries be stored in an spill-proof box.
Gel Cells use a thickening agent to immobilize the electrolyte. Thus, if the battery container cracks or is breached, the cell will continue to function. Furthermore, the thickening agent prevents stratification by preventing the movement of electrolyte. As Gel cells are sealed and cannot be re-filled with electrolyte, controlling the rate of charge is very important or the battery will be ruined in short order.
Absorbed Glass Mat (AGM) batteries are the latest step in the evolution of batteries. Instead of using a gel, an AGM uses a fiberglass like separator to hold the electrolyte in place. The physical bond between the separator fibers, the lead plates, and the container make AGMs spill-proof and the most vibration and impact resistant lead-acid batteries available today. They are used in aircraft.
Basically, an AGM can do anything a Gel-cell can, only better. However, since they are also sealed, charging has to be controlled carefully or they too can be ruined in short order.
Gel and Absorbed Glass Mat batteries are relatively new but are rapidly gaining acceptance. There are some reasons to use them:
- Gel and Absorbed Glass Mat (AGM) batteries can dispense charge at a higher rate than flooded cells.
- Virtually no gassing under normal operating conditions: Unlike wet cells, gel cells and AGMs are sealed and operate under pressure to recombine the oxygen and hydrogen produced during the charge process back into water. You find AGM batteries on high-end yachts such as Hinckley, Hans Christian, Viking, Island Packet, and Broward.
- The ability to put Gel and AGMs in the bilges (they can even operate under water) also lengthens their lives: For every additional 15 degrees of heat over 77 deg F, battery life (regardless of type) is cut in half.
- AGMs can operate in any orientation (although you may lose some capacity that way) and even if a container is broken, an AGM will not leak. This is a feature particularly important to blue water cruisers who may encounter storms - you don't want to coat the inside of your boat with sulfuric acid if you ever get bounced arond. Proper (heavy duty) battery restraints are a must, regardless of battery type.
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Gel cells and AGMs require no maintenance once the charging system has been properly set up.
The costs of batteries is an issue:
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Wet cell 8-D - $150.00
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Gel - 8-D - $250.00
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AGM - 8-D $365.00 (West Marine has them for $501.00)
So choose the battery you feel is best for you and enjoy your boat.
