Electrical Systems - House --- Section 8
Cayman 2005
281
If the pump never runs out of pressure, and is operated at or below its 15 GPM level, it will
hold 12 psi. However, a pump that is connected to a water tank with a finite capacity will start to lose
the ability to hold pressure as the level of water in the tank drops. Think of siphoning water from a buck-
et. As the level of the water drops, the volume of water exiting the siphon slows down.
When the tank is full it is capable of feeding more “pressure” to the pump inlet due to gravity, and
the pump always has enough water available to maintain its rated pressure and volume. However, if
the water tank gets low the pump will not have enough water volume coming in to maintain 12 psi at
15 GPM. If the loads are removed from the pump by closing the valve on the outflow, even with low
pressure in the tank the pump will eventually pressure up to 12 psi. It will just take it longer to get
there. When the valve is opened the pump will sustain 12 psi for a brief period, but since the tank is no
longer feeding the pump as fast as needed the pressure will eventually drop. This analogy can be restat-
ed by replacing the pump with a battery, pressure with voltage, volume with amps, outflow valve with
a switch, water with electricity and the water tank with the battery electrolyte.
The level of the tank could be thought of as the rate of the reaction occurring in the electrolyte. When
the battery is fully charged the electrolyte has an excess of reactions taking place to feed the battery
terminals. This tapers off with time as the electrolyte is spent, so maintaining voltage becomes possi-
ble. With no loads the discharged electrolyte will be capable of producing close to the rated voltage,
but only after a period of time has elapsed for enough of a reaction to take place to bring the voltage
back up. Hopefully, this explanation will clarify why a battery measured at rest can indicate close to its
rated voltage but will not run a load.
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.
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