Electrical concept question

[Stu Jackson]

Originally Posted by noelex77

Actually that’s not what I said.
This is what I said



But if you are now happy that solar panels output can be measured instantaneously in amps and over time in amp hours rather than the original

No swim necessary, It's simply true.

The ONLY thing you can measure from a charging INPUT is AMPS.

The amount of TIME it happens becomes amp HOURS.

Not sure what's so hard to understand?

The West Advisor: Solar Power

[Stu Jackson]

Originally Posted by Hartley18

FFS..

Just to correct the record: Solar panels and other charging devices output in "watts", people!!... That's Amps x Volts. Over time, it's called "watt-hours".

Only if the voltage is constant (which for a charging device it never is) is it ever correct to talk about AH. And, although, like many charging devices, it may have a "max current output" rating it is never correct to say a solar panel puts out a fixed number of amps (because it simply doesn't - the current output is dependent upon voltage proportional to light level hitting the cells).

To get meaningful amps/amp-hours out of a source of supply, you need something to fix the voltage at a set level. An MPPT can do this; so does a (very) large battery.


We now return you to your regular program....

Programming department here!

You are actually and completely correct.

But W = A * V ALSO means that

A = W / V

So, with a reasonably constant voltage, it's pretty much the same thing, isn't it?

So, given a "nominal" 12V system, albeit with varying voltages from say 11.5 V to 12.8 V, and higher when charging, we come to the very same conclusion.

In fact, the popular battery monitors actually measure watts and watt hours, so we're also in total agreement.

You and others on this thread may be interested in Maine Sail's "Installing a Battery Monitor" article, and my associated "Gotcha Algorithm" article, which pretty much covers the whole measurement and charging and acceptance issues. See --- Wiring & Installing A Battery Monitor - SailboatOwners.com

We can "argue" all day and night () about whether it's better for the OP to run his fridge during the day or at night or whenever he chooses to, and whether it's better to do so when charging or when not.

But the basic, overall, 24 hour period use of POWER that is STORED in the house bank simply doesn't change (very much). Granted, there are efficiencies and other issues, well brought up by these responders to this thread, but here's the result:

"Breaking In" New Wet Cell Batteries

All I'm saying is that real world, parsing the small differences in efficiencies, it really doesn't make much difference. That's the WHOLE purpose of having STORAGE batteries.)

I just came back from a 2 day mini-cruise, and ran my fridge constantly (on 7 and 5 out of 7 on the thermostat). Fridge ran all night, keeping our food at a reasonable temperature, and kept the ice in the evaporator frozen. We have a small solar panel (11Watts) which doesn't contribute much during a cruise, but works fine for offsetting the wet cell battery inherent drain during the week and actually adds a few amp hours during the week, since it ADDS watts at a certain voltage over time to the house bank. I don't keep our boat plugged in to shorepower when I'm not there. Simple.

Thanks for watching, and I agree, back to Yan Can Cook. Oh, sorry, that's CNG or propane... Let's not go there...

Thanks again...

Stu

[mitiempo]

I still say it is best to run the fridge often enough to keep the food below 38 degrees F so it doesn't go bad - day or night, charging or not.

I agree - that's what batteries are for.

[noelex77]

Originally Posted by Stu Jackson

That's the WHOLE purpose of having STORAGE batteries.)

What we are talking about in this thread is charging and battery efficiency. If your batteries waste more energy this needs to be replaced. You need larger solar panels a bigger wind generator or longer engine running time.

The batteries do not produce energy only store it.

If your consumption is higher than production once all the factors have been taken into account (including battery/ charging inefficiency) the batteries can not compensate for this, only average it out over a longer period. If there is still a deficit you will need to run your engine longer to replace this energy. A situation common to many boats.
The savings mentioned in this thread are small. For most boats we a talking about a few AHrs a day, but its still helpful.