The battery as
rechargeable batteries. The most
common type is the lead-acid
battery. A less familiar one is the
nickel-cadmium (NiCad) battery,
which can still often be found in old
emergency power systems. Due to
the high charge voltage required
by a NiCad battery, and the fact
that they are very environmentally
unfriendly, these batteries are not
suitable for use onboard a vessel or
Principle of the lead-acid battery
A battery is a device that stores electric
power in the form of chemical energy.
When necessary, the energy is again
released as electric power for DC
consumers such as lighting and starter
motors. A battery consists of several
galvanic cells with a voltage of 2 volt each.
For a 12-volt battery, six cells are linked
To achieve 24 volt, two 12-volt batteries
are linked in series. Each cell has positive
oxidised lead plates and negative lead
metal plates, and has an electrolyte
consisting of water and sulphuric acid.
During discharging, the lead oxide on the
lead plates is converted into lead. The
acid content decreases because sulphuric
acid is required for this process.
To recharge the battery, an external
power source - such as a battery
charger, alternator or solar panel - with a
voltage of around 2.4 V per cell must be
connected. The lead sulphate will then be
converted back into lead and lead oxide,
and the sulphuric acid content will rise.
There are limits set for the charge voltage
to prevent the release of an excessive
amount of hydrogen. A charge voltage
of more than 2.4 V per cell, for instance,
releases a lot of hydrogen gas, which can
form a highly explosive mixture with the
oxygen in the air.
The upper limit on charge voltage for a
12 V battery is 14.4 V, and the
corresponding value for a 24 V battery is
28.8 V at 20 °C. The relationship between
gravity of the water/sulphuric acid
mixture is as follows:
of the thickness and number of plates
applications. The maximum current
that can be delivered is determined by
the total plate surface. The number of
times that a battery can be discharged
and recharged – the number of cycles –
depends on the thickness of the plates.
A battery can feature either many thin
plates or a few thick ones.
The starter battery
A starter battery has many thin plates per
cell, leading to a large total plate surface.
This type of battery is, therefore, suitable
for delivering a high level of current over
a short period of time.
The number of times that a starter battery
can be heavily discharged is limited to
around 50-80. But as starting the engine
uses only a small part of the energy stored
(around 0.01 %), the battery lasts for many
years. This type of battery is generally
unsuitable for cyclic use.
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