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841
Figure 18.20
Cross-section through a tubular electrode of a battery after 3.5 years of operation.
The lead rod (light-coloured area in the centre) has almost completely disintegrated as a result of
corrosion. The diameter of the electrode is 8 mm. (Photo source: ZSW)
These large changes in volume act to loosen the active material. This effect
increases with increasing depth of discharge. Thus, deep discharge with low currents
has an additional negative effect on sulphation. Once the active material has become
loose, it can be separated from the electrode, for example, by gas movement, and gathers
as sludge at the base of the battery. If the volume below the electrodes that contains the
sludge is not large enough, there is a danger of short circuits between the electrodes.
The erosion effect is much less pronounced in sealed batteries, as the electrodes
there can be mounted under pressure. The pressure compensates the forces arising from
the volume change and increases the stability of the active masses.
The available active mass at the electrodes is reduced by the loss due to erosion.
This corresponds directly to a reduction in the accessible capacity. Accordingly, the battery
will be discharged earlier.
18.4.7.4.5 Short circuits
In addition to the danger of short circuits in the sludge volume of batteries with liquid
electrolytes, there are two further risks for short circuits.
The plate connectors from the positive electrode above the active material are also
subject to corrosion. This results in detachment of large corrosion flakes, which can fall
onto the electrodes and cause short circuits. This risk can be eliminated by including
separators that extend upward well over the electrodes.
Further, there is a risk for all battery types that dendrites (microscopic short cir-
cuits) may grow from the positive to the negative electrode through the separators. These
dendrites are so fine that they are usually not visible even when the battery is investigated
842
ELECTROCHEMICAL STORAGE FOR PHOTOVOLTAICS
in the laboratory. Their growth is accelerated by long periods at a low state of charge and
thus low acid concentrations. As described in Section 18.4.7.4.2, the solubility of PbSO
4
increases at low acid concentrations. For example, the solubility of PbSO
4
is 2 mg/l at a
sulphuric acid density of 1.28 g/cm
3
, and is already 35 mg/l at a density of 1.02 g/cm
3
.
The higher the solubility the higher is the rate of recrystallisation and hence the rate of
formation of large sulphate crystals and dendrites. The danger of dendrite growth can be
counteracted by thicker separators and by operating the battery at high states of charge.
In general, a short circuit is a defect that causes a sudden and complete breakdown
in the battery. Short circuits can occur from dendrites between the electrodes and the
sludge in flooded batteries from active material accumulated as a result of active-material
shedding under the electrodes. If the amount of material exceeds the free-electrolyte height
under the electrodes, short circuits occur through the “mud”. Microscopic short circuits
through the separator affect particularly the self-discharging properties of a battery.
18.4.7.4.6 Reverse charging
If a battery is subjected to a discharging current even after the battery has been fully
discharged, the potential changes its sign.
Reverse charging of an individual cell can occur in strings of series-connected
cells. The battery voltage and therefore the depth of discharge protection are controlled
on the basis of overall string voltage. A single cell within the string may have lower
capacity due to manufacturing deviations or due to accelerated ageing. In consequence,
the low-capacity cell can be over-discharged resulting in reverse charging.
In case of reverse charging, PbO
2
is formed at the original Pb electrode and vice
versa. Although this type of reverse charging can sometimes increase the capacity for
a short time, it is certainly detrimental to the cell lifetime in the long term. The main
cause of damage is the oxidation of additives in the lead sponge of the negative electrode,
which are included to maintain the high porosity of the electrode. If these additives are
destroyed, large lead crystals form in the negative electrode resulting in a loss of internal
surface area and therefore in an irreversible loss of capacity.
The danger of reverse charging can be reduced if the voltage of the individual cells
is monitored and used as the criterion to end discharging (rather than the total voltage of
the battery). Alternatively, it can be prevented by allowing charge equalisation between
the cells of a series connection [12].
18.4.7.4.7 Ice formation
Figure 18.21 shows the dependence of the freezing point of diluted sulphuric acid on
the electrolyte concentration. Ice must be prevented from forming in a battery under all
circumstances, as it is then practically impossible to operate the battery (in particular, a
frozen battery can hardly be charged) and it is possible that the cell housing may burst
(battery breakdown and contamination of the surroundings with sulphuric acid).
For operating modes with very low discharge currents, or if the deep-discharge
protection is inactive or non-existent, it is possible to reach extremely deep discharge,
SECONDARY ELECTROCHEMICAL ACCUMULATORS
843
0
10
20
30
40
50
Sulphuric acid concentration in water
[% weight]
−
70.00
−
60.00
−
50.00
−
40.00
−
30.00
−
20.00
−
10.00
0.00
Electrolyte temperature
[
°
C]
Solid-state phase
Liquid phase
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