Electrical Systems - House --- Section 8
The Executive 2005
369
Battery Charge Time & Consumption Rate
Calculating Run Times:
Calculating run time figures when operating 120 Volt AC electrical items with an inverter can be
exponential due to battery characteristics. Flow characteristics of electrons vary with different bat-
tery 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 dis-
charge occurring over an extended period of time before the battery is charged. Engine starting batter-
ies are designed to quickly release large amounts of current for short durations, without depleting bat-
tery 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. 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.
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