A simple way to think of it is to use the old "water" analogy. Water
pressure being analogous to voltage, and water
flow being analogous to amperage.
If you have a water pump (or are hooked up to dockside water via a pressure regulator
fitting), the pump will kick on when it senses the water pressure is below some set threshold that the pressure switch senses - say 3 psi. Once the pressure is up to the cutoff limit - say 6.5 psi, the pump will stop.
So if you turn on one faucet, the pump won't turn on unless the pressure drops to 3 psi or lower. Once it does, the pump will kick in. In this water example, your accumulator (pressure vessel) acts like a battery in a DC electrical system
. It stores energy in the form of compressed air. When you open a faucet, the pressure forces water through the plumbing
So - water ~ electricity, pump ~ battery, water pressure ~ voltage, water flow ~ amperage.
Now, compare that to our electrical system
. The major conceptual difference between the two (and it's minor) is that with electricity, the losses are higher, and are usually dissipated as heat. And since a battery will self-discharge over time, you have to maintain a "float voltage" on them to keep them fully charged (pressurized). The analogous thing in a boat's water system is water leakages.
Older water pump technology was basically on/off, so the accumulator tank handles things until the pump kicks in. Newer variable speed pumps can now start up at a low level and "share" the load with any accumulator tanks
in the system.
Now back to electricity. When you turn on a deck
light, current starts flowing from the battery bank thru the wiring
to the light. In this case, the voltage drop (water pressure drop) is slight, but it's there. Think of a modern battery charger as a variable speed water pump - it can sense the voltage level, and put out current to keep the voltage level up. If you start up a big load system like start the engine
or use the windlass
, then you're usually drawing more current from the batteries than the charger can match, so you'll have to keep running the charger for quite a while to replenish the amps that were drawn down from the batteries.
Like Bill says, it's unlikely with today's technology that you'll get batteries to last 15 years. With water pumps, basically the only wear and tear is from impurities in the water which cause build up of scale over time, and some wear and tear on o-rings, seals
, diaphragms and the like. With batteries, there's a chemical reaction every time the batteries are discharged, and another every time they're charged. And, like in life, nothing's free. No chemical reaction is 100%, so there are losses over time. Sure, you can equalize your batteries, or even pulse-charge them in an attempt to crack sulfate crystals, but you never reach 100%, so eventually the batteries will be no longer usable.
Wait for a portable fusion generator