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All charging voltage set points are normally specified at 25ºC / 77ºF. In solar systems,
battery temperatures often vary up to 15ºC from the 25ºC reference. The Absorption,
Float and Equalization Voltages must then be adjusted or a controller with Temperature
Sensor should be used. Table below shows example of adjustments for Absorption
Voltage of say 14.4V for 12V battery, (based on Voltage Coefficient of Temperature
Change as -5 mV / ºC / Cell or -30mV (.03V) for a 6 cell, 12V battery).
BATTERY TEMPERATURE ABSORPTION VOLTAGE (12V BATTERY)
40ºC 13.95V
25ºC (Reference) 14.4V (Reference)
10ºC 14.85V
Table 3.5. Absorption Voltage vs Temperature (example)
In case temperature compensation is not provided, the warmer battery at 40ºC will
begin to heat and outgas at 13.95V and will continue to overcharge until the non-
compensated Absorption Voltage set point is reached (14.4V). In cooler temperatures,
the 10ºC 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 5ºC to 10ºC (9ºF to 18ºF).
Loss of Battery Capacity at Low Temperatures
Batteries lose capacity in low temperatures. At 32ºF (0ºC), a battery will deliver about 70 to
80% of its rated capacity at 80ºF (26.7ºC). If the electrolyte temperature of the battery bank
is lower than 80ºF (26.7ºC), additional batteries will be needed to provide the same usable
capacity. For very cold climates, an insulated / heated battery compartment is recommended.
Freezing of Electrolyte
For applications with low ambient temperature, the lead-acid battery must also be
protected against freezing of the electrolyte. The risk of freezing depends on the
state of charge. The chart given below illustrates the freezing limit as a function of
the state of charge.
- 80
°
- 60 °
- 40 °
- 20
°
0 °
0 20 40 60 80 100
State of charge [%]
Temp
erat
ure
[
°
C
]
slushy until hard
- 80
°
- 60 °
- 40 °
- 20
°
0 °
0 20 40 60 80 100
State of charge [%]
Temp
erat
ure
[
°
C
]
slushy until hard
STATE OF CHARGE (%)
TEMPERA
TURE (ºC)
Fig 3.1. Temperature vs State of Charge
SECTION 3 | Description & Principle of Operation