Battery Charge Time & Consumption Rate - Monaco Cayman 2006 Manual

Battery Charge Time & Consumption Rate
Calculating Run Times:
Calculating run time fi gures when operating 120 Volt AC electrical items with an inverter can be exponential
due to battery 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 period of time
before the battery is charged. Engine starting batteries are designed to quickly release large amounts of current for
short durations, without depleting 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. 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 fi gures are calculated 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 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
fi ve 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 effi ciency 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 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% effi ciency loss fi gure, 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 conditions.
The run time fi gure 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 fi gure 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 fi lling batteries. To achieve the highest quality of battery performance and longevity maintain
the batteries in their proper operating range.
Cayman 2006 Electrical Systems - House • Section 8 — 201