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The battery as

power source

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.








0 %

11.64 V


± 100 %

20 %

11.88 V


± 80 %

40 %

12.09 V


± 60 %

60 %

12.30 V


± 40 %

80 %

12.51 V


± 20 %

100 %

12.72 V


0 %