Photovoltaic Power Generation - Renewable Energy: Technology, Economics and Environment - page 275

consequently also the useful energy are reduced. Fig. 6.24 shows the characteris-
tic discharge curves for different discharge currents. Amperage I is expressed in
units of the ten hours discharge current ( I 10 ), whereas the extracted capacity is
expressed in relation to the capacity available as a result of the ten hours current.
According to this rationale, only approximately 50 % of the capacity is available
if the current is increased tenfold; if, by contrast, the current is reduced by a factor
ten (in relation to the ten hours discharge current), about 30 % more capacity be-
comes available. But in general a battery fully discharged with a high current can
be additionally discharged with a much smaller current afterwards.
Capacities of lead-acid batteries are increased by approximately 0.6 % per K of
temperature increase and it is reduced accordingly for decreasing temperatures.
The nominal capacity is defined according to the manufacturer definition typically
for a reference temperature between 20 and 27 °C. However, the effects of wear
and tear and self-discharge multiply with increasing temperature. The optimal
operating temperature of lead-acid batteries applied within photovoltaic systems is
thus at about 10 °C.
Such batteries store the added electric energy at an Ah-efficiency (Coulomb ef-
ficiency) of approximately 95 to 98 %. If the battery is run in cyclic operation in
partial state of charge (< 80 % state of charge (SOC)) the Ah-efficiency amounts
to almost 100 %. The ratio of received to added energy (typical value 80 to 90 %)
is referred to as Wh-efficiency which results from the higher charging voltage
when compared to the discharging voltage. At 25 °C self-discharge amounts to 2
to 3 %/month. The rate is approximately doubling with each 10 K temperature
increase.
Charge controllers are of major importance for a safe and reliable battery op-
eration. They must protect the battery against deep discharge (according to battery
technology, discharged capacity use should not exceed 60 to 80 %) to prevent
premature ageing. In addition, charge controllers are responsible for the charging
strategy. For this purpose, voltage is limited in that way to enable quick battery
charging on the one hand, and to protect the battery against deterioration by gas-
sing or corrosion on the other hand. This is why the voltage needs to be limited.
Especially for batteries provided with gel or AGM electrolytes, excessive voltages
may create excessive gases which cannot be internally recombined and may thus
get lost through the relief valve. It is also of major importance to adapt the maxi-
mum charging voltage to the battery temperature. The maximum voltage is low-
ered with increasing battery temperature to prevent excessive gassing and corro-
sion.
For this reason high requirements are placed on charge controllers in terms of
reliability and the algorithms applied for deep discharge and overcharge protec-
tion. However, importance should also be attached to criteria such as self-
consumption, user-display to show the battery status and reverse polarity circuit
protection. Good charge controllers should allow for additional determination of
the battery's state of charge /6-9/, /6-30/, /6-33/, /6-34/.