Another way of looking at this is to use the analogy of a water pump
(a battery is like an electric pump). The pressure in PSI (pounds per square
inch) that a pump delivers is like a battery’s voltage. The volume of water in
GPM (gallons per minute) is like the electrical current. Let’s look at a 12 PSI
pump with no loads (the pump is running but the outflow valve is turned off).
The pump will run and the internal pressure of the pump will build up to some
point higher than 12 PSI. When the valve is opened and the water is free to
flow into the loads, the pressure will drop to the rated output pressure of 12
PSI, but only if the load is not too big. If the pump is designed to maintain 12
PSI at 15 GPM, and a load demanding 20 GPM is connected, the pump will not
be able to keep up and the pressure will get sucked down to a lower PSI. If the
load is reduced or removed, the pump will catch up and return to its rated 12
PSI pressure. If the pump has an infinite source of water, such as a lake or the
water utility (this is like the grid, no battery), the pump will never run out of
pressure. If the pump never runs out of pressure, and is operated at or below 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 bucket. 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 anal-
ogy can be restated 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 occur-
ring 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 possible.
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 make clarify why a battery measured at rest can indicate close to its rated
voltage but will not run a load.
Electrical Systems - House
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