Originally Posted by Jody
The sticker on the back of my little dorm fridge says:
115V AC 60HZ ...... 1.25 amp
According to lapeer20m, that means my fridge requires 143.75 watts
143.75 watts @ 12 volts = 14.3 amps
So basically the refrigerator that I own may be too much of a drain on the batteries to use with an invertor, but shouldn't be too much of a drag when plugged into a campground receptacle or powered by a generator.
I am a professional firefighter and a Bluegrass musician... and this electrical stuff is new to me. Thanks for sharing the knowledge.
Ummm....143.75 watts divided by 12 = 11.97 amps; not 14.3 amps. The really simple way is to take the rated amp draw on the AC side (assuming it's not a 220v) and multiply by 10; it's surprisingly accurate and much easier math. So the 1.2 AC amps on the sticker becomes 12 amps on DC...just about what we got when we did it the long way.
It may well work for you. The information I post here is only so that folks get a chance to learn about this stuff so that you're in a better position to make choices that are best for you. Every one has a different modis operandi and what works for one person may not for another; and vice versa.
At least with some basic info...like the fridge draws 12 amps on DC...you can figure out how long the thing will run on a particular battery. That is, if you go buy a battery rated at 105 amp-hours the fridge looks like if will run for 8.75 hours continuously (of course, the refer cycles on and off and you have to make your best guess as to what percentage of the day the thing actually runs). In the real world you can't go that long because you're drawing a higher load than the load the manufacturer used to rate the battery [Typically it's a 20-hour rating, meaning in the case of a 105 amp-hour battery it will sustain a 5.25 amp load for 20 hours; if the load is higher than that the battery will lose charge faster in an accelerating manner. This is Peukert's Law.] and two that means you're taking your battery to "dead" in order to run the fridge the maximum length of time.
If asked to optimize system operation I size everything so the batteries will not drop below 50% of their rated capacity to sustain the anticipated daily load of the system. This ensures the battery pates don't get physically damaged (deep discharge traction batteries can go down to 80% but they aren't common so I'll stick to the batteries we normally buy from Wal-Mart, Interstate, etc). But if you're buying (relatively) inexpensive batteries from Wal-Mart and such and you get to run the fridge longer you may not care about ultimate battery life; they're cheap enough to replace that perhaps it isn't an issue for you.
I don't advocate that approach because it's not the "best" in terms of effciency, overall cost, or longivity but I also don't dismiss it out of hand because in some situations it's the best solution. It takes a fair amount of money to implement a truly great battery system and then it takes dedication to monitoring the system (and fairly expensive equipment to do that); in the long run it may not be worth all the investment in time and money if the end goal is just to run a refrigerator on an inverter for weekend trips.
In the end only the person (or persons) using the bus (and therefor the systems in it) can decide what is "best" for them. I can only hope to provide some of the tools that allow you to do that.