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975-0171-01-01 41
4. Enter this information on the blank calculation worksheet (page 42).
Step 2: Calculate
Battery Size
5. Complete the rest of the worksheet; see Table 1-1, “Battery Sizing Example”
on page 41 for an example.
Size the batteries at approximately twice the estimated total amp-hour usage.
Doubling the expected amp-hour usage ensures that the batteries will not be
overly discharged and extends battery life.
Resist the temptation to skip this doubling step. More capacity is better since you
will have more reserve capacity, be better able to handle large loads and surge
loads, and your battery won’t be discharged as deeply. Battery life is directly
dependent on how deeply the battery is discharged. The deeper the discharge, the
shorter the battery life.
Troubleshooting If you find that the system shuts down when appliances with large motors are
started, the problem may be that this motor is too much for the battery. Even
though you calculated the amp-hour requirements appropriately, the startup of a
large motor makes high demands on the battery. You may find that adding more
amp-hours (in the form of extra batteries or replacement with a bigger battery)
solves the problem.
Table 1-1
Battery Sizing Example
This example illustrates how quickly your battery needs can escalate. To reduce
the required battery bank size, you can either conserve energy by eliminating or
reducing the use of some loads, or recharge more frequently.
Appliance
(A) Power
Consumption (Watts)
(B) Operating Time
per Day (Hours)
Daily watt-hours needed
for this appliance
(= A × B)
TV & VCR 200 W 2 hours 400 Wh
Small microwave
oven
800 W 15 min = 1/4 hour 200 Wh
3 lamps, 60 W
each
180 W 4 hours 720 Wh
Coffee maker 600 W 15 min = 1/4 hour 150 Wh
Hair dryer 1500 W 6 min = 1/10 hour 150 Wh
Total daily watt-hours of AC load 1620 Wh
× Number of days between charges 3
= Total watt-hours of AC load between charges 4860 Wh
Battery Ah used between charges (divide by 10 for 12 volt system;
divide by 20 for 24 volt system)
486 Ah
Recommended Battery Bank Size in Ah (multiply by 2) 972 Ah