Calculating Run Times: Calculating run time figures when operating 120
Volt AC electrical items with an inverter can be exponential. This is due to bat-
tery characteristics. Flow characteristics of electrons vary with different battery
types and chemical compositions. Deep cycle batteries are generally designed
to slowly release a majority of their charge capacity. Deep cycle batteries are
rated in amp hours (Ahrs) with the discharge occurring over an extended peri-
od of time before the battery is charged. Engine starting batteries are designed
to quickly release large amounts of current for short durations, without deplet-
ing battery reserves. Commercial type batteries bridge the gap of deep cycle
and engine batteries. Commercial batteries release medium amounts of current
over a longer period of time, but they are not designed to cycle their charge
capacity.
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. Example: A deep cycle
battery with a rated capacity of 100 Ahrs. is designed to release current at the
rate of five amps per hour. Multiply a 5 amp load over a 20 hour discharge
period equals the rated 100 Ahr. capacity. These discharge figures are calculat-
ed with the battery starting at 100% state of charge with the battery at 80º
Fahrenheit when the discharge cycle begins. However, increasing the discharge
load applied to the battery from five amps to ten amps on a 100 Ahr battery
does not yield 10 hours of discharge time. This is due to the internal reactions
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 to operate the 120 Volt item. There is also a small efficiency
loss of about 10% when inverting. For example: When using the inverter to
operate an AC electrical 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 cur-
rent used by an AC item. For example: The television is rated at 200 watts at
120 Volts. 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
Electrical Systems - House
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WINDSOR 8•219
Battery Charge Time
& Consumption Rate
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