Samlex Solar SCC-30AB Mode D'emploi page 22

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GENERAL INFORMATION: BATTERIEs
Effect of Temperature on Battery Voltage
The temperature of the electrolyte affects the rate of chemical reactions in the
batteries as well as the rate of diffusion and the resistivity of the electrolyte.
Therefore, the charging characteristics of the battery will vary with temperature. This
is nearly linear and the Voltage Coefficient of Temperature Change is normally taken
as -3 mV to -5 mV / ºC / Cell. Please note that the Voltage Coefficient of Temperature
Change is negative. This means that as the temperature rises, the charging voltage is
required to be reduced and as the temperature is decreased, the charging voltage has
to be increased.
All charging voltage set points are normally specified at 25 ºC / 77 ºF. In PV systems,
battery temperatures often vary up to 15 ºC from the 25 ºC reference. The Absorption
Voltage for a 12 V battery must then be adjusted as shown in the Table below or a
controller with Temperature Sensor should be used (assuming Voltage Coefficient of
Temperature Change as -5 mV / ºC / Cell or -30mV (.03 V) for a 6 cell, 12 V battery) :
BATTERy TEMPERATURE
25 ºC (Reference)
In case temperature compensation is not provided, the warmer battery at 40 ºC will
begin to heat and outgas at 13.95 V and will continue to overcharge until the non-
compensated Absorption Voltage set point is reached (14.4 V). In cooler temperatures,
the 10 ºC battery will experience severe undercharging, resulting in sulfation.
It is recommended that a battery charger / charge controller with a provision for
temperature sensing and compensation should be used if the battery electrolyte
temperature varies more than 5 ºC to 10 ºC (9 ºF to 18 ºF).
Self-discharge
The battery discharges itself even without any load connected to it. This effect
is caused by secondary reactions at its electrodes and proceeds faster with higher
temperature or in older batteries. Thermodynamic instability of the active materials
and electrolytes as well as internal- and external short-circuits lead to capacity losses,
which are defined as self-discharge. This loss should be small, particularly in respect of
annual storage. Self discharge (% of loss of capacity per month) for various types of
batteries is as follows:
• Lead Acid
• Ni-Cd
• Ni-Fe
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40 ºC
10 ºC
3% to 4%
6% to 20%
40%
ABSORPTION VOLTAGE
13.95 V
14.4 V (Reference)
14.85 V