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Sizing Tips
Batteries used in Photovoltaic applications face some of the most rigorous duty
cycles. The long term success of battery performance is directly related to the functional
sizing of the battery to the demand load, adequacy of charging current from the PV array,
and system minimum and maximum voltage set points.
There are many available PV battery sizing programs. A proper battery sizing should
include the following important factors.
Load Size
Convert each load into ampere hours (ampere load X duration time =
ampere hours). Determine the cumulative ampere hour amount. Loads occurring simultaneously are added together.
Number of Cells
Lead acid batteries are sized at a nominal 2.0 volts per cell. A 48 volt
system would typically require 24 lead acid battery cells.
System Low Limit
Based upon the lowest acceptable operating voltage of the equipment to
be powered, determine the low limit volts per cell. Use this voltage to choose the battery
size capable of supporting the current load to the desired time. Example: 42V divided by
24 cells = 1.75 v/c
System Charging Voltage Limit
The charge limit for gelled acid Dynasty batteries is 6.75 to 6.90, and
14.4 to 14.8 volts per battery respectively for 6 and 12 volt batteries. Various charge
controller types and array sizes may perform more successfully at different settings
within this range. Target the voltage setting to return 110% of the discharge ampere
hour amount. Limits set at lower or higher values will undercharge or overcharge the batteries
respectively.
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