Section 8 --- Electrical Systems - House
Cayman 2005
282
The working range of a deep cycle battery is between 50 and 100% state of charge (SOC). Deep
cycle batteries should not be cycled below 50% state of charge. Discharging a deep cycle battery below
50% state of charge shortens the life of the battery. Deep cycle batteries use an amp hour rating which
is usually calculated over a 20 hour discharge interval. For example: A deep cycle battery with a rated
capacity of 100 Ahrs. is designed to release current at the rate of 5 Amps per hour. Multiply a 5 Amp load
over a 20 hour discharge period equals the rated 100 Ahr. capacity. These discharge figures are calcu-
lated with the battery starting at 100% state of charge with the battery at 80º F when the discharge
cycle begins. However, increasing the discharge load applied to the battery from 5 Amps to 10
Amps on a 100 Ahr battery does not yield ten hours of discharge time. This is due to the internal reac-
tions which occur when a battery is discharging. Actual discharge time for a 10 Amp load may be
closer to eight hours of discharge time. Increasing the load applied to the battery to 20 Amps will not
yield five hours discharge time but may be less than three hours. It might be understood as a point of
diminishing return.
Calculating applied loads to an inverter to approximate run time from the battery amp hours
available is not an equal trade up when voltage is inverted and amperage is calculated. When the
inverter is used to operate an AC load it uses approximately ten times the DC current needed from the
battery when inverting 12 Volts DC to operate the 120 Volt AC item. There is also a small efficiency
loss of about 10% when inverting. For example: When using the inverter to operate an AC electri-
cal item, which has a current draw rating of 2 Amps, the inverter will use over 20 Amps DC power
from the batteries.
Determining Current Consumption:
First determine the amount of current used by an AC item. For example: The television is rated at
200 watts at 120 Volts AC. Calculate watts to amps. Divide 200 watts by the operating voltage of 120,
this equals 1.6 Amps. Multiply 1.6 Amps AC current by a factor of ten the inverter will use, this equals
16 Amps DC battery current. Add the revised 10% efficiency loss figure, this calculates to a total of
17.6 Amps DC. If the battery bank capacity is rated at 500 Ahrs., actual elapsed time to the suggested
50% state of charge would net viewing time for the television at approximately 13 hours in ideal condi-
tions.
The run time figure will vary greatly with the actual state of charge of the battery bank when the
discharge process begins. Ambient temperature, combined with other working loads, such as lights and
parasitic loads applied to batteries, affect run times. Calculating the exact run time is not precise
due to all the variables and equations involved; however, an approximate time figure can be obtained.
Proper battery maintenance and charge cycles affect battery performance. Observe the battery condition
with hydrometer and voltage readings. Use only distilled water when filling batteries. To achieve
the highest quality of battery performance and longevity maintain the batteries in their proper operat-
ing range.
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