SunXtender Technical Manual

Document No. 6-0100 Rev. H
Page 26 of 43
If the battery is exposed to cold climates, the state of charge should be kept at a maximum to
prevent freezing of the electrolyte. A fully charged battery will not freeze even under the coldest
weather conditions, but a discharged battery will freeze even when moderately cold. Table 6-2
gives the freezing point of electrolyte at various states of charge.
Frozen batteries are not capable of charging or discharging except at very low rates, and may
be permanently damaged by expansion of the electrolyte. If a battery becomes frozen, it should
be thawed by placing it at room temperature for at least 24 hours, and then charged in
accordance with Sections 5.4, 5.5 or 5.6 as applicable. However, if the battery container has
any evidence of cracking, the battery is no longer serviceable and should not be used.
Table 6-2. Electrolyte Freezing Point at Various Battery States of Charge
Battery
State of Charge (%)
Approximate Electrolyte
Freezing Temperature
100%
-70°C (-94°F)
75%
-47°C (-53°F)
50%
-25°C (-13°F)
25%
-13°C (9°F)
0%
-6°C (21°F)
6.4 Battery Sizing
The battery capacity for a PV system can be calculated using the following formula:
Capacity (AH) = Total Daily Load x Days of Autonomy x Design Factor
The Design Factor depends on the battery’s average temperature during the coldest time of the
year, as discussed above. Table 6-3 provides recommended Design Factors at various
temperatures.
Table 6-3. Recommended Design Factors for Battery Sizing
Lowest Battery Temperature
Averaged over 24 Hours
Design Factor
for
80% End of Life
Design Factor
for
50% End of Life
Degrees C
Degrees F
25 or above
77 or above
1.25
2.00
20 to 24
68 to 76
1.39
2.22
10 to 19
50 to 67
1.43
2.29
0 to 9
32 to 49
1.60
2.56
-10 to -1
14 to 31
1.84
2.94
-20 to -11
-4 to 13
2.23
3.57
-30 to -21
-22 to -5
2.84
4.54
-40 to -31
-40 to -23
4.17
6.67
Example:
For a 48VDC system, Total Daily Load of 30AH, 5 Days of Autonomy, and -8°C is the lowest
average temperature; the recommended battery capacity is as follows:
Battery Capacity = 30 x 5 x 1.84 = 276AH at 80% end of life. This requirement could be
satisfied with a PVX-2580L, which has a C/120 rating of 305AH. Four of these batteries in
series gives 4 x 12VDC = 48VDC.