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
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
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"
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.
The following table summarizes the basic characteristics of Flooded-Cell, Gel-Cell and
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
Most gel-cell and some AGM batteries require special charging rate,
especially the deep-cycle models.
| ||Flooded-Cell||Gel-Cell||Absorbed Glass Mat|
|hazard of electrolyte spill||yes||no||no|
|chock and vibration tolerance||moderate||high||high|
|approximate price||1.5 Euro / Ah||2.0 Euro / Ah||2.5 Euro / Ah|
|bulk charge current||C/10||C/20||Cx4|
|approximate charge time||10h||20h||0.5h|
|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: