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Battery Types


The world’s most common big batteries are called flooded or wet cell, as they have enough liquid acid to cover the battery plates. The electrolyte is a diluted sulphuric acid, and the plates are a form of lead suspended in a grid, with positive and negative plates close to each other. By using thinner plates, and adding more of them to the battery, the additional surface area allows the electrochemical reaction to occur at an increased rate.

Frequently, thin-plate batteries are used for cranking an engine and more robust, thicker-plate batteries are used as a deep-cycling house bank (deep discharge can damage thinner plates). This older technology has charging limitations, makes it less desirable in certain applications and requires that batteries are stored and secured upright so electrolyte will not spill out of the vent caps.


Enter the AGM, or absorbed glass mat battery. This battery uses the same electrolyte but much less of it, and the plates are in closer proximity. Just enough sulphuric acid is present in an absorbent layer, in contact with the much thinner plates, allowing for a fast electrochemical reaction.

Illustrated is a spiral-wound configuration, but many styles of AGM mimic the case size and appearance of flooded batteries. The AGM is sealed with pressure-relief valves so the battery can be stored and operated on its side. And its recharging rate is increased (as is its rate of discharge).

Even with the new technology, all the traditional standards for a safe battery installation apply, as the AGM type can vent explosive hydrogen when distressed. This kind of venting often kills the battery, as not enough electrolyte remains behind and it’s not possible to add more.

  Roger Hellyar-Brook  

  Roger Hellyar-Brook  

Roger Hellyar-Brook runs a marine consulting business, repairing and upgrading boats of all types. He has spent more than 40 years in the marine industry and is the former manager of the systems program at The Landing School in Arundel, Maine.

Paul Mirto is a digital illustrator, longtime boater and former Coast Guardsman.

This article originally appeared in the July 2018 issue.



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Drawing of Fuel Gage System

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Exhaust Separators

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Illustration of Typical OEM alternator setup

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Alternator Pulley Side View Illustration

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Vacuum Bagging

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