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Battery Sizing
Worksheet
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Use this worksheet to
determine what size battery is required for your system.� Battery size is measured in amp-hours.
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This is a measure of
battery capacity.� Battery voltage is
determined by the number of "cells" in series.� All lead-acid
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batteries have a nominal
output of 2 volts.� Actual cell voltage
varies from about 1.7 volts at full discharge to 2.4
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volts at full
charge.� 12 volt lead-acid batteries
are made of 6 separate cells in one case. 6 volt batteries are made of
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3 cells in one
case.� Putting battery cells in
parallel increases amp-hour capacity, but does not affect voltage.
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�1. Solar array amp-hours per day required from the Systems Load
Worksheet, line 13:
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�2. Maximum number of continuous cloudy days expected in your
area:
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�3. Multiply line 1 by line 2:
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�4. Divide line 3 by 0.8 to maintain a 20% reserve after deep
discharge period:
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����� If no special conditions below apply,
skip to line 10:
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�Special
Condition #1: Heavy Electrical Load
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�5. Maximum amperage that will be drawn by the loads for 10
minutes or more:
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�6. Discharge rate of battery. If unknown, check with battery
supplier:
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�7. Multiply line 5 by line 6:
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�Special
Condition #2: High Charge Current
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�8. Maximum output amperage of
PV array or other battery charger:
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�9. Multiply line 8 by 10.0
hours:
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�10. Amp hours from line 4, 7 or
9, whichever is largest:
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�11.� If you are using a
lead acid battery, select the multiplier below which corresponds�
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������ to the battery's wintertime average
ambient temperature:
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Battery
Temperature�
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Multiplier
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80oF/26.7oC
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1
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70�F/21.2�C
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1.04
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60�F/15.6�C
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1.11
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50�F/10.0�C
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1.19
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40�F/4.4�C
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1.3
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30�F/-1.1�C
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1.4
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20�F/-6.7�C
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1.59
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�12. Multiply line 11 by line 10. This is your optimum battery size
in amp-hours:
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�13. Amp-hours of battery chosen. (Industrial Cell, GC4=220,
L16=350, etc.):
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�14. Divide line 12 by line 13. This is the total number of
batteries in parallel required:
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