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Battery Types and Applications

The biggest market for lead-acid batteries is the car industry. As such starting batteries (or cranking batteries) have developed into a cost and performance optimized mass product. Although special "marine" models exist, the batteries used on sailing vessels are usually simple starting batteries. They offer the best price to power ratio but have some serious practical disadvantages for usage on sailing vessels.

Batteries can be divided in two basic types depending on their application: starting batteries and deep-cycle batteries. The major structural difference between these types are the lead plates, which are solid in deep-cycle batteries and composed of porous sponge-like plates in starting batteries.

  • Starting batteries are designed to deliver quick bursts of high energy. For this purpose they have a large number of thin plates designed for a maximum active surface area. The plates are composed of lead in a sponge-like structure. This gives a large surface area, but if deep discharged, this fine sponge structure will be consumed rapidly. Therefore, starting batteries should not be discharged below 80% of their nominal capacity.

  • Deep-cycle batteries have thicker plates that can deliver less peak current, but can withstand frequent discharging cycles. The thick solid plates and are much less susceptible to degradation due to cycling. These batteries can survive a number of deep discharge cycles during which the capacity can be as low as 50% of the nominal capacity. Typical applications are the storage of electrical energy for supply systems. Generally deep-cycle batteries can also be used as starting battery when they are oversized by at least 25% compared to the required starting battery capacity.

  • So-called dual-purpose batteries are only a compromise between the previous types of batteries. The plates may be composed of lead sponge, but it is coarser and heavier than that is used in starting batteries.

Both types of batteries will be useful on board of sailing vessels: a standard starter battery for the inboard engine and some amount of deep-cycle battery capacity for the on-board power supply.

Valve-Regulated Lead-Acid Batteries

Today, most batteries that are sold are so called "sealed" or "maintenance-free" types. This types of battery is also known as Valve-Regulated Lead-Acid Battery (VRLA), which includes a recombination feature and valve regulating mechanism to allow for a safe internal disposal or escape of hydrogen and oxygen gasses produced while charging the battery.
The name "valve regulated" is misleading. These batteries are rather "recombinant" batteries: the oxygen evolved at the positive plates will largely recombine to water with the hydrogen ready to evolve on the negative plates, This way loss of water from the electrolyte is largely prevented. The pressure relief valve is strictly a safety feature that allows for safe dispersal of any excess hydrogen that may be formed during overcharge.

Valve-Regulated Lead-Acid batteries can be divided by the way they are built More specific by the way the electrolyte is made available to the battery plates:

  • Wet-Cell (Flooded): this is the traditional style battery. The liquid electrolyte is free to move in the cell compartment. These batteries are very sensitive to their operating environment. Both high and low temperatures will substantially decrease the battery performance. Despite the drawbacks of flooded cell batteries, they still offer the best price to capacity ratio.
    Flooded cell battery comes in 2 ways: serviceable and maintenance free. With the serviceable battery, the user has access to the individual cells to check the electrolyte and to add (distilled) water as the battery dries out. Maintenance free or "sealed" batteries have an excess amount of electrolyte in the battery to sustain the chemical reaction under normal use throughout the battery warranty period.

  • Gel-Cell: in this realization, the electrolyte is embedded in a gel-like substrate. The gel is a mixture of sulphuric acid H2SO4, water and SiO2). Gel-cell batteries can be operated "upside-down" and at lower temperatures compared to wet-cell batteries. The fixing of the electrolyte in the gel substrate increases the electrolyte resistance. Therefore, the high-current behaviour of gel-cell batteries is inferior to the high-current behaviour of wet-cell batteries They also must be charged at a lower rate (C/20) to prevent gas from building bubbles in the gel structure that will never heal an will cause a permanent loss of battery capacity.

  • Absorbed Glass Mat (AGM): the electrolyte of the AGM battery is immobilized by absorption in a fibreglass separator. This construction allows the electrolyte to be suspended in close proximity with the plate's active material. This results in a very low internal resistance, which yields excellent high-current behaviour and enhances both the discharge and recharge efficiency Since the internal resistance is extremely low, there is almost no heating of the battery under high-current conditions These batteries can be loaded at rates as high as Cx4, providing the cables are properly sized and the necessary power can be made available.
    AGM batteries also have a very low self-discharge rate (<3%/month). This means that they can sit in storage for much longer periods without recharging compared to standard batteries The plates of AGM batteries are tightly packed and rigidly mounted so tha they will withstand shock and vibration better than any standard battery Because of their inherent rigid structure they can even survive freezes of the electrolyte.

The advantage of the gel-cell and AGM batteries is that it impossible to spill acid even if they are broken. They can be oriented in any manner, unlike normal lead-acid batteries which must be kept upright to avoid acid spills and to keep the plates' orientation vertical. Since these battery systems are designed to be recombinant and eliminate the emission of gases on overcharge, room ventilation requirements are reduced.

Gel-cell and the AGM batteries typically cost twice as much as premium wet-cell batteries. However, in most applications these batteries will give double the life span and more deep-discharge cycles than wet-cell batteries compensating for the higher cost price.
Gel-cell and AGM batteries can be stored very well as they do not tend to sulfate as easily as wet-cell batteries. They come as sealed batteries, so there is no hydrogen gas escaping from these batteries and no corrosion problem. Most gel-cell and some AGM batteries require special charging rate, especially the deep-cycle models.

The following table summarizes the basic characteristics of Flooded-Cell, Gel-Cell and AGM batteries:

 Flooded-CellGel-CellAbsorbed Glass Mat
hazard of electrolyte spillyesnono
chock and vibration tolerancemoderatehighhigh
approximate price1.5 Euro / Ah2.0 Euro / Ah2.5 Euro / Ah
internal resistancemediumhighlow
bulk charge currentC/10C/20Cx4
approximate charge time10h20h0.5h
overcharge tolerancemoderatelowmoderate
self discharge rate (@ 25°C)~20% / month~10% / month~2% / month

Batteries for Marine Installation

On board of a sailing yacht two types of batteries will be installed: a starting battery for the engine and, depending on the required backup capacity, one or more deep-cycle batteries for on-board power supply.
On a rolling vessel, sealed gel-cell or AGM batteries are the most convenient for this purpose since they they don't have a liquid electrolyte and they don't have to stand upright for proper operation. These battery types are also more resistant to vibrations but they are typically 2-3 times more expensive than an equivalent flooded-cell type battery.

The starting battery should only be used to start the engine and should also be electrically separated from the rest of the power system.

The different batteries can be recharged separately with a battery charger powered from the mainland power line or from the alternator driven by the on-board engine. Diode circuits can be used to separate the two electrical systems and still allow combined recharging. However the better solution is to use two different chargers for each battery system.

Some vendors specialized on marine batteries:

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