Environmental Engineering Reference
In-Depth Information
an excessive pressure inside the cell can occur. Therefore the gas is released
through a relief valve in case of excessive pressure. However, water that is lost as
a result of this safety measure cannot be refilled. The benefits of this battery tech-
nology are reduced gassing, placing considerably lower requirements on the bat-
tery compartment, higher installation variability and prevention of electrolyte
leakage. However, the disadvantages are higher costs, higher requirements in
terms of overcharge protection and - in several applications - shorter technical
lifetimes compared to liquid electrolytes. However, particularly high-quality gel
batteries provided with tubular electrodes have shown satisfactory durability when
combined with suitable charging processes. AGM batteries showed better lifetime
in cyclic applications compared with conventional starter batteries for cars /6-34/.
The energy content of such batteries is indicated by their capacities and their
nominal voltage. The capacity is defined as the current quantity emitted by the
accumulator until reaching a certain end-of-discharge voltage and measured in
Ampere hours (Ah) (i.e. amperage (Ampere, A) multiplied by time (hours, h)).
The capacity is dependent on the discharge current, the temperature and the de-
fined end-of-discharge voltage. The nominal voltage is defined by the materials
participating within the electrochemical reactions and can be calculated for the
currentless equilibrium conditions based on thermodynamic coherence. While
NiCd and NiMH batteries show a nominal voltage of 1.2 V/cell lead batteries are
characterised by 2.0 V/cell and lithium-ionic batteries by 3.6 V/cell; the latter are
therefore used often for mobile applications. The nominal voltage defines also
how many cells have to be connected in series to obtain the voltage level needed
by the consumer.
2.1
2.1
2.0
2.0
0.13 x
I
10
0.13 x
I
10
1.9
1.9
1.8
1.8
4 x
I
10
4 x
I
10
1.7
1.7
0.57 x
I
10
0.57 x
I
10
I
10
I
10
6 x
I
10
6 x
I
10
2 x
I
10
2 x
I
10
1.6
1.6
10 x
I
10
10 x
I
10
1.5
1.5
20 x
I
10
20 x
I
10
1.4
1.4
16 x
I
10
16 x
I
10
1.3
1.3
0
0
20
20
40
40
60
60
80
80
100
100
120
120
Discharged capacity in %
Discharged capacity in %
Fig. 6.24
Typical characteristic discharge curves of a battery with tubular electrodes as a
function of the discharged capacity (standardised to nominal capacity of ten hours dis-
charge current
I
10
, end-of-discharge voltage 1.8 V/cell)
The useful energy of a battery is largely dependent on the relative discharge
current. Currents are expressed as units of the nominal current. The higher the
discharge rate is, the faster the voltage decreases. The extractable capacity and
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