keeping hooked up to shore power |
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fwunder
Senior Member Joined: 04 Oct 2013 Location: New Jersey Online Status: Offline Posts: 1676 |
Topic: keeping hooked up to shore power Posted: 21 Sep 2015 at 10:03am |
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Beat ya to it furpod!
Kill-A-Watt has been on for 30 minutes. I'll let it run all day and see, but this is what I see so far. Batteries (dual sixers) are topped off. Only breaker on is converter. I used 10.6 watts @ $0.10/KwHour. Shore power cord to 50 ft. heavy extension to house. fred |
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2014 RPod 178 => MyMods and Buying Habits
2008 4Runner Limited 4.0-liter V6 Yes, those are wild ponies dining on grass while dumping tanks! |
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furpod
Moderator Group - pHp Joined: 25 Jul 2011 Location: Central KY Online Status: Offline Posts: 6128 |
Posted: 21 Sep 2015 at 9:37am | |
I gotta "killawatt"When we get back from the mini mini, I will plug it in.
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WillThrill
Senior Member Joined: 04 Jul 2014 Online Status: Offline Posts: 298 |
Posted: 20 Sep 2015 at 11:17pm | |
It's true that the converter does need power to run itself, but it isn't much, certainly not 200 watts. A completely electric house only has an average draw of 1,000 watts, and a Pod with nothing but the converter keeping the batteries topped off and the smoke and propane detectors running is not 20% of a house, more like 2%. My point here is that once the batteries on the Pod are fully charged, there is no way that the Pod will draw even close to 2 amps at 120 volts, which would be 240 watts. That would be about 25% of all the power we consume for our all electric home (except the furnace). No way. For those who are confused for reading this, I'll make it simple for you: watts are always constant; volts and amperes can fluctuate. Here's an example. A 'typical' marine battery has about 75 amp hours of capacity, and it's a 12 volt battery. If we multiply those together, we get 900 watt hours. That means we can run a 10 watt device (regardless of whether it uses 120 volts AC or 12 volts DC) for 90 hours before the battery is completely dead (which damages the battery a lot). If this is a 12 volt device, then we just use the power as it comes off of the battery. If this is a 120 volt AC device, you must connect an inverter to the battery, which inverts the 12 volt DC power to 120 volt AC power. The inverter uses some power all on its own, and some efficiency is lost in the inversion process, but it isn't earth shattering (it's about 10%). So you might draw 12 watts of power from the battery instead of 10, but you get the point. Now pay careful attention to this. If you were drawing 12 watts of power from an inverter connected to a 12 volt battery, the inverter would be supplying .1 of an amp on the output side going to the device, and it would be drawing 1 amp on the input side. The reason for this is because 12 watts divided by 120 volts AC is .1, but 12 watts divided by 12 volts DC is 1.0. Amps can change, but the watts stay the same, always. Going back to my original point, once the batteries on the Pod are fully charged and everything that can be turned off is, the wattage being drawn by the Pod will go way down, like below 50 watts, probably below 30. If you're constantly drawing 30 watts of power, that's 21,600 watt hours in a month (30x24x30). The national average of $.12 per kilowatt hour (1,000 watts for one hour) multiplied by 21.6 is $2.59. So a constant 30 watt draw would cost you $2.59 a month. That's not a big deal for most people. As Furpod already pointed out, he can't tell a difference in his electric bill when his Pod is connected and when it's not. If it were $20 or more, he probably would. I hope this clarifies this issue. |
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"Not all those who wander are lost." Tolkien
2014 Hood River 177 2005 GMC Envoy XL |
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techntrek
Admin Group - pHp Joined: 29 Jul 2009 Location: MD Online Status: Offline Posts: 9059 |
Posted: 20 Sep 2015 at 10:15pm | |
Correct - the converters just aren't designed to be energy efficient. The problem gets multiplied because it is plugged in 24/7. Even a 4 watt night light uses almost 3000 watts in a month.
That is mixing up the draw of the converter while plugged in with the draw of the pod while plugged into the TV. The converter needs energy to run itself, plus the converter is at best 93% efficient (usually less) while supplying 12 volts to the pod, plus the battery is only about 85% efficient when charging.
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Doug ~ '10 171 (2009-2015) ~ 2008 Salem ~ Pod instruction manual
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Tars Tarkas
Senior Member Joined: 14 Jan 2013 Location: Near Nashville Online Status: Offline Posts: 1446 |
Posted: 20 Sep 2015 at 9:55pm | |
So this is why I asked.... for some reason I always make the kinds of mistakes with Ohm's Law at least one of you is making. And I remember when it cost three cents to mail a letter, so whether it costs a dollar or seventeen dollars a month to keep the pod plugged in (just figuring minimum usage (the charger/inverter, the propane detector, and the fridge mobo)) seems like an absurd question.
I think the take away is that it's closer to a dollar a month to keep the pod plugged in. If that's right, my thoughts are that unless you're dealing with sub-zero temps, it doesn't seem worth pulling the battery and putting it back in later. And that's assuming most of us already have another battery charger/minder. Even paying for a switch and a cable to disconnect the battery in place seems like over kill. That's me. This doesn't seem like a critical issue, so I'm pretty sure whatever makes sense to whoever is okay. And I'd have a different attitude about this if I was putting my pod in storage for months at a time, but as I said earlier, I keep my pod in the back yard and get into it for one thing or another fairly often. And we'll take it to the Smokies, or somewhere, even in January or February. TT |
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2010 176
FJ Cruiser |
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WillThrill
Senior Member Joined: 04 Jul 2014 Online Status: Offline Posts: 298 |
Posted: 20 Sep 2015 at 8:26pm | |
2 amps at 120 volts is 240 watts. There is no way the Pod is drawing that much power with nothing significant unless it's bulk charging the batteries. That would be a 20 amp draw from a TV (at 12 volts), and most TVs are not equipped to deliver 20 amps to the travel trailer. Once the batteries are fully charged, it takes less than 12 watts of continuous power to maintain them. Watts are the constant, not amps. When you check out inverters, you see that a 150 watt inverter almost maxes out a cigarette lighter plug because those are generally only equipped to deliver 15 amps at 12 volts (180 watts). |
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"Not all those who wander are lost." Tolkien
2014 Hood River 177 2005 GMC Envoy XL |
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techntrek
Admin Group - pHp Joined: 29 Jul 2009 Location: MD Online Status: Offline Posts: 9059 |
Posted: 20 Sep 2015 at 7:40pm | |
I did mess up the number on the LP detector + fridge. That is a half amp at 12 volts so it is almost nothing of the 2 Ah difference at 120 volts.
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Doug ~ '10 171 (2009-2015) ~ 2008 Salem ~ Pod instruction manual
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codycountry
Senior Member Joined: 26 Jan 2014 Location: Wyo Online Status: Offline Posts: 168 |
Posted: 20 Sep 2015 at 7:24pm | |
I just plug in my dual battery system once a month for a day on my house regular 120v plug in, it's not a RV plug in just a regular plug in, but no matter if it's 30 below zero, or 80 above zero F --- one day keeps things charged. Seems to work well and the batteries like it and stay at full charge for when I need the juice.
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techntrek
Admin Group - pHp Joined: 29 Jul 2009 Location: MD Online Status: Offline Posts: 9059 |
Posted: 20 Sep 2015 at 6:54pm | |
I'm talking about amperage measured at 120 volts, and it includes the amount the converter is using to run itself, conversion inefficiency from 120 to 12 volts, and the the charging inefficiency into the battery. None of that is a factor when you are off-grid, then you only need to worry about the LP and fridge control board draw.
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Doug ~ '10 171 (2009-2015) ~ 2008 Salem ~ Pod instruction manual
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WillThrill
Senior Member Joined: 04 Jul 2014 Online Status: Offline Posts: 298 |
Posted: 20 Sep 2015 at 6:50pm | |
Your calculations are that assuming that 2 amp hours for the Pod are 240 watts. That would be true if the Pod was using 120 volt power, but it isn't. The 2 amps are at 12 volts, which is just 24 watts. So your figures are off by a factor of 10. If the Pod used 240 watts of power just sitting there, it would completely drain a marine battery in around 3-4 hours. |
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"Not all those who wander are lost." Tolkien
2014 Hood River 177 2005 GMC Envoy XL |
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