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Came in the mail today, connected up right away.  Reading instructions leaves me wondering.  It shows, with my CPAP running, I am using .81 to .75 amp, that is with the tank heater ramping up.  Is there a function that shows how many amps I use over a given time?  I can take the .81 times 8 hours and get 6.48.  Does not sound right, how do I figure out how much I use nightly?  I wanted to average it over 4 or 5 nights but it does not show an amount used over time?  I am trying to figure out as noted in some of my recent posts just what size/how many batteries I need. 

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One thing to recognize is that the Kill a Watt is measuring amps at 120 volts. Your trailer batteries are a 12 volt system, so when producing 120 volts through an inverter the amp draw would be ten times higher if the inverter was 100% efficient, which it is not. Probably safe to figure 12V amperage at 11 - 12 times your measured amp draw.

So on your 12 volt system you'll be drawing between 9 and 10 amps. Do that for 8 hours and you've pulled 72 - 80 amp-hours out of your batteries. 

As far as how to get the meter to give you total usage over time, I can't help because I've never owned one. Sorry. 

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Just looked up the manual for the P4400 Kill a Watt meter. The last of the five buttons below the display toggles between total consumed Killowatt-hours since power was applied and time. Once you plug in your CPAP it should start to monitor this value. The next morning press this button and it should give you the total KW-hours consumed overnight. 

Once you've got that value we can help you with understanding how to apply that to your battery needs. You could even do it several nights in a row to see how consistent the draw is, but I'll bet it is pretty constant. 

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14 minutes ago, Sehc said:

Read the instructions. The meter will do all you want to know.

Instructions that came with it is pretty bland.  Does not explain how to figure out how many amps during a set time.

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23 minutes ago, mptjelgin said:

Just looked up the manual for the P4400 Kill a Watt meter. The last of the five buttons below the display toggles between total consumed Killowatt-hours since power was applied and time. Once you plug in your CPAP it should start to monitor this value. The next morning press this button and it should give you the total KW-hours consumed overnight.

Ok, I see that.  I was expecting it to show amp usage over time.  I'll run it a few times and come back with the info, thanks!

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2 minutes ago, NDBirdman said:

Instructions that came with it is pretty bland.  Does not explain how to figure out how many amps during a set time.

Measuring total Killowatt-hours over time will get you what you need. Dividing watts by volts will get you amps. 

For instance, if you measure that your CPAP uses 1.0 KW-hours in one night, you can convert that to amp-hours by dividing by the voltage. 

Since you are ultimately interested in 12 volt batteries, I would first multiply KW-hours by 1000 to get you to simpler watt-hours, so now you're looking at 1000 watt-hours of use. Divided by 12 volts, you now have 83.3 amp-hours pulled from a 12V system. I mentioned inverter efficiency in an earlier post, and that still applies here. So I'm going to bump this by 10% for efficiency loses, giving you about 92 amp hours use overnight. 

So in this hypothetical example your CPAP would draw 92 amp-hours out of your batteries in one night's use. 

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Uh... I'll watch it for a little bit to see what it burns in killowatts.  I'll post that here and ya'll can tell me what's up.  My head now hurts, that is waaaaaay over my understanding.

Thanks for the info.

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Birdman, the Kill A Watt meters I've seen DO HAVE A FUNCTION THAT RECORDS "ENERGY" IN KILOWATT HOURS (I cant say if yours does or not) and that's the best figure to show HOW MUCH ENERGY THEY CONSUME OVERNIGHT.

Once you tell us the "energy" (that's X watts for X time, IE Watt Hours) they use overnight we can size how many amp hours of stored battery energy you need to power the CPAPs while sleeping.

To your specific question "  I can take the .81 times 8 hours and get 6.48.  Does not sound right, how do I figure out how much I use nightly?"

 If the CPAP draws 0.81 amps at 120 VAC and if it ran say 8 hours, that's "APPROXIMATELY" .81 X 11 =  8.91 12 VDC battery amps for 8 hours OR 71 Amp Hours and if you have two running that's approximately 142 Battery Amp Hours used up overnight

 If you required 142 Battery Amp Hours and don't what to draw the batteries down over 50% you would need to start off when you go to bed with at least 284 or lets say 300 Amp Hours for good measure.

 Since you will use energy from say 5 PM to bed time when the solar isn't producing it keeps looking like 400 Amp Hours of stored battery energy is where you may end up BUT ONLY THE EXACT FIGURES WILL DETERMINE THE ANSWER. As I told you often my running ONE CPAP and a small 120 VAC fridge overnight works with 450 battery amp hours and you may come in at less, but at this point and based on what you have said I dont envision you getting by with too much less then 300 Amp Hours to allow for safety and other night loads (water pump or vent fans or especially a furnace!!!!)

Remember, Amps is current flow, Amp Hours is a measure of ENERGY. Typical Deep Cycle Batteries are rated at Volts and Amp Hours since we need to know how much "energy" they can store ………… Your meter may well have a function to tell you how many Watt Hours or Kilowatt hours the appliance draws over time...…….

Fun chat glad to help, let us know

John T 

 

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So far, I checked every morning, it does say 0.18 (9 hours per meter), one short night I had 0.14( at 7.5 hours per meter).  I made sure it was 0., it is.

Edited by NDBirdman

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Ok. So 0.18 KW-hours is the same as 180 watt-hours, which is an easier unit to work with. 

To convert this to amp-hours on a 12 volt system (which is how most deep cycle batteries are rated) you would divide this number by the voltage, which is 12.

So you are using 15 amp-hours of power in a 9-hour period of time. A pair of 6-volt deep cycle lead-acid batteries is generally rated around 220 amp-hours, though you really only want to use about half of that, so say 110 amp-hours.  This would seem to show that you could run your CPAP for several nights on a good pair of batteries, but you also need to understand that you'll have other draws on the batteries, including lights, fridge, furnaces, etc. 

A couple of other things to think about - Your CPAP is currently plugged into a 120V wall outlet, correct?  In order to do this in the RV you'll have to have an inverter to convert the 12V DC to 120V AC.  Inverters have some efficiency losses so your 15 amp-hour draw could go up to 17 - 18 amp-hours as some power is "wasted" by the inverter. 

Earlier you'd mentioned that the CPAP was drawing .75 - .81 amps with the tank heater heating up.  Your overnight usage is indicating an average amp draw of far less than that (you are averaging .17 amps over the 9 hour period).  Are you seeing a big drop in amps once the tank is heated up?  If not these numbers aren't adding up. 

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2 hours ago, mptjelgin said:

other draws on the batteries, including lights, fridge, furnaces, etc.

Earlier you'd mentioned that the CPAP was drawing .75 - .81 amps with the tank heater heating up.  Your overnight usage is indicating an average amp draw of far less than that (you are averaging .17 amps over the 9 hour period).  Are you seeing a big drop in amps once the tank is heated up?  If not these numbers aren't adding up. 

These batteries will only be used for 2 CPAPs, not connected up for camper use.  I have a separate battery for that, won't be connecting them so I'm good there.  I will use the meter on my wife's CPAP to see is she is drawing the same, it's a different/newer model.

I have not looked to see what it is after tank heats up, guess I should one night to see what it is doing. 

So if my wife's is about the same as mine, then a pair of deep cycle 6vs, connected to a suitcase style solar 100 watt panel, we are good to go?  Going this route, what would be a good size inverter?  Should be a fairly small one?

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If your wife's CPAP is similar, then you'l be drawing about 35 amp-hours each night. A fully charged set of batteries should support that for 3 nights with no recharging.  In good conditions a 100 watt solar panel should return 40+/- amp-hours to a set of batteries on a good sunny day, so you are in the ballpark. You would be wise to monitor your charge levels daily to make sure you aren't falling behind due to cloudy days. If you overdraw the batteries they will fail prematurely. 

As far as inverter size, your CPAP seems to be drawing about 100 watts max. This is based on your observed maximum of .81 amps at 120 volts. If your wife's is similar you are looking at 200 watts total. It is always a good idea to size inverters with some "head room", so I'd be looking for a minimum 300 watt inverter, and wouldn't hesitate to go as high as about 600 watts. Small inverters in this size range are relatively inexpensive. 

I would get a pure sine wave inverter. They are more expensive but produce power that is similar to what comes out of the wall socket. Your CPAP's may have electronics that are sensitive to the quality of the power provided. The less expensive "modified sine wave" inverters might not make your machines happy. And again, in this size ranges even pure sign wave inverters are not terribly expensive.  

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16 hours ago, NDBirdman said:
20 hours ago, NDBirdman said:

So far, I checked every morning, it does say 0.18 (9 hours per meter), one short night I had 0.14( at 7.5 hours per meter).  I made sure it was 0., it is.

Birdman, now that you gave us ACTUAL ENERGY use we can size your battery bank

NOTE that below is using the figures you provided above, earlier you indicated "the CPAP draws 0.81 amps at 120 VAC" which equated to 71 Amp Hours over eight hours THOSE TWO FIGURES ARE QUITE A BIT DIFFERENT ??? I cant say from here how often or how long the tank heat and humidity function draws energy, perhaps that could explain the big number difference from your two posts ????

1) 0.18 KWH/1000 = 180 Watt Hours

2) Using 12 battery volts, 180/12 = 15 Amp Hours of energy drawn from your battery bank (Battery full charge is 12.6 but as it discharges voltage drops)

3) Using two CPAP's and assume they draw near the same energy that's 30 Amp Hours but the Inverter is NOT 100% efficient so if it were say 85% efficient that 30 Amp Hours equals 34.5, at 90% efficient it equals 33 Amp Hours drawn out of your battery bank

4) Since if using AGM or Flooded Lead Acid you don't want to draw down over 50%, you need to start the CPAP's with at least 70 rated  Amp Hours of energy storage and to be on the safe side Id prefer at least 100 when the CPAP's start

5) You spoke above using a pair of deep cycle 6 volt batteries. If so and if they were similar to Trojan T-105 that series combination would give you 225 Amp Hours (112.5 USEABLE) at 12 volts so ifffffffffff you actually used 34.5 Amp hours total per night, you can get around three nights of use. Of course lesser Amp Hour batteries would give you less  

6) INVERTER The brick converters you posted were rated at 90 watts but lets use say 200 Watts total for two CPAP's so to be safe and allow for expansion Id suggest say a 400 Watt or larger AND I HIGHLY RECCOMEND A PURE SINE WAVE INVERTER. Some equipment has problems with MSW (cheaper to buy) but most ALL equipment does fine on PSW

7) SOLAR  You mentioned a 100 Watt Panel. Ifffffff it is exposed to good bright sunlight at a direct angle (ideal best condition but NOT typical) using say 13.6 charging volts, that would equal around  7 battery charging amps and over lets use 4 hours that's 28 Amp Hours of energy (42 Amp Hours over 6 good sun hours) which is close to that 30 Amp Hours you use up each night   NOTE this is ONLY an approximation NOT exact or typically practical. Unless the sun was bright direct angle to the panels I more often saw 5+ never 7 the theoretical and on many days I didn't see 6 good hours of sun...……..

Hope this helps, you have asked good questions and we have tried to help the best we can based on the info?? you provided.

NOTE It appears mpt and I agree (a good thing right) HOWEVER we have based our analysis on the figures you gave us so we are only correct if your figures are. They seem on the low side to me but if they arent correct let us know ………..

John T   Longgggggggggg retired n rusty Electrical Engineer and NOT any Solar expert so nooooooooo warranty lol Post back any follow up questions and we will be glad to help

Edited by oldjohnt

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Put meter on wife's CPAP last night.  She only sleeps 7 hours and up like the energizer bunny... LOL  Anywayz, she used .13 KWH last night.  Looks like a pair of 6v Deep Cycle golf cart batteries would do us good.  Now with her So-Clean running, I may put the meter on that thing, I bet it eats the energy.

Maybe a 200 watt suitcase solar charger just to make sure the charge is done quicker/fuller?

Edited by NDBirdman

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Sounds like they use a similar amount of energy.  A 200 watt setup would obviously give you a nice cushion and could be handy those days when the sun doesn't really want to shine!

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22 hours ago, NDBirdman said:

Looks like a pair of 6v Deep Cycle golf cart batteries would do us good.  Now with her So-Clean running, I may put the meter on that thing, I bet it eats the energy.

Maybe a 200 watt suitcase solar charger just to make sure the charge is done quicker/fuller?

Birdman, I have to agree as any solar is good solar but more solar is better lol

An advantage of more solar can (in certain situations) be you don;t have to discharge your batteries to as low of a level. While 50% max is a figure you hear about, its better for the battery and life expectancy if you ONLY draw them down say 20% to 30% NOT 50%....IE use up less of its rated Life Cycles 

I often find myself camped under a total shade canopy and/or its rainy n cloudy for days in which case I don't harvest anywhere near what I would iffffffffff the sun was bright n shiny PLUS the panels were tilted and pointed to the sun throughout the day. Even if two of us calculated (based on your figures) the 100 watt panel will suffice I SAY GO FOR 200 and I bet you wont regret it   

  VERY FUN CHAT YALL

 John T

Edited by oldjohnt

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3 hours ago, NDBirdman said:

Now with her So-Clean running, I may put the meter on that thing, I bet it eats the energy.

If it sanitizes and sterilizes as it claims there's a good chance it draws considerable MORE (Subject to how long it operates and at what power) energy (heating water or steam etc)  then the CPAP. The 200 Watt is sounding better all the time lol I have no idea how much energy it consumes, throw that meter to it n let us know  

John T

Edited by oldjohnt

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Welp, went and do'd it.. .LOL  The meter has been on the SoClean, meter run time is 154 hours.  What surprised me, and makes me wonder, is for the whole time, It used 0.03 kwh.  This thing only runs for about an hour a morning then sits idle.  The only thing drawing power the other 23 hours is a digital clock/timer.  Judging from this, I'd say the amps this thing uses per run/morning is a non-player in the grand scheme of things.  You can hear it run for that hour, sounds like a small pump running.  Makes me wonder, is this thing actually doing anything?  My better half says she can tell the difference so who am I to judge it.

Thanks ya'll!

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From what I've read the SoClean generates and uses O3 (ozone) to clean the CPAP.  It may not taken much energy to generate the ozone, so perhaps your readings are correct.

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Birdman, Since I have no other specs or data to dispute your measurements, I'm gonna take you at your word, especially if it ONLY runs say one hour per day. Based on that your two CPAP's and the So Clean can still work almost three nights before the batteries need re charged HOWEVER a good daily re charge back up to 100% SOC  is much much better which the 200 Watt Solar you spoke of can supply unless there's like no sunshine for days lol

 Best wishes keep us posted

John T

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