Fun with a Trimetric & CPAP, lights draw - Fiberglass RV


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Old 11-07-2015, 11:19 AM   #1
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Trailer: '88 Scamp 16, layout 4
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Fun with a Trimetric & CPAP, lights draw

Last weekend, as part of my never ending quest to spend money and time creating the perfect (for me) FGRV, I installed a Trimetric TM-2030RV($$) with the appropriate giblets to make it work. Previously I had installed two 6V golf cart batteries ($$) on the tongue in a custom made box ($). I have also swapped out all the 12V lights for LED fixtures ($), installed a TF-130 Truckfridge($$$$$$$$), and a new PD converter($$) along with new appropriately sized wires and fusing.

Although we had camped a couple of times before installing the TM, once with shore power and once without, all I had to monitor the batteries was a cheap digital voltmeter. It worked but I wanted a more accurate and scientific way to see what was going on. That and my eventual plan is to install some solar so the TM will fit right in as part of a complete solar system.

Last week I ran the fridge a while so I could fully recharge the batteries so the TM could see a cycle and set the battery percentage function. Without going into it too much the TM uses voltage and amperage to tell when it thinks the batteries have reached full charge and then monitors usage to determine percent of full at any point, pretty neat.

So far I have recorded the draw of a few things, the LED lights, 150W inverter, and my CPAP machine. I had read that LED lights use 1/5 of an amp, and dang if that does not square almost exactly with what I am seeing. I turned on lights one at a time and watched the amp reading. It was almost exactly .2, .4, .6, .8 and then 1 full amp with five lights burning.

In contrast to the lights which did not reveal much new, the CPAP proved very enlightening. Not necessarily for what the CPAP used while running, but for when it was not. I had previously set my CPAP up with the inverter and tested it with the battery out of my tractor in an effort to be ready for power outages so I knew it worked and did not draw much.

That was confirmed in spades today. What was most interesting were the "parasitic" draws of the inverter and the CPAP plugged in not running. The inverter alone turned on with nothing plugged in drew .27 amps and the CPAP plugged in but not running another .27-.29 amps. Total amps for the inverter and CPAP running was just 1.35 amps.

This confirms why the company's recommendation for an inverter to run my CPAP was for the 150W unit. I questioned this originally since the CPAP name plate says 2.5A at 110V or 275 watts leading one to believe a 300 watt inverter was called for. The actual draw (with inverter) is 1.35A at 12.8V (battery volts showing on the meter) or 17.28 watts. That does not seem right so please check my math/assumptions but that is what I see.

If there is a take-a-way to this rambling post it is to utilize an appropriately sized inverter, and turn things off/unplug when not in use (duh).
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Old 11-07-2015, 08:29 PM   #2
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A nice analysis.

Am I recalling correctly that your CPAP does not have the capability of running on 12 volt? By far, that is the easiest solution as most CPAP's are 12 volt systems with a 110 volt plug and a "brick" to lower the voltage to something less than 12. For my sons CPAP I was able to purchase a 12 volt adapter for around $25. I have checked the draw but with a CPAP there are many variables, so many that when in use the draw that you measure may not be relevant to anyone else due to the prescribed pressures your machine is set for. Of course then there is the variable of how long you wear it and finally if it shuts down upon determining that the mask has been removed and what draw it has at that point.

If your unit is an old one and if you have medical coverage to pay for a new one it maybe worth while investigating. My son's was replaced after five years and there were some significant improvements in that time. I saw the cost of the units and know this is not something you will go out and buy on a whim.

With our dual six volt batteries and a 120 watt solar panel we have managed to stay 85% charged for two weeks in mid summer. I cannot capture enough sunlight to get the batteries fully charged, a shade issue mostly. Even though I have a TM I am not sure if the CPAP or charging portable devices is the biggest user.
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Old 11-08-2015, 08:46 AM   #3
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Originally Posted by Paul Braun View Post
Am I recalling correctly that your CPAP does not have the capability of running on 12 volt? By far, that is the easiest solution as most CPAP's are 12 volt systems with a 110 volt plug and a "brick" to lower the voltage to something less than 12. For my sons CPAP I was able to purchase a 12 volt adapter for around $25.
Yes, mine is just old enough to be 110V only. I think the next model after mine was 12V with the brick. I have thought about a new one but this one works so well and I am used to it so hate to rock the boat. What I would really like is another one altogether so I have a spare, but the insurance company will not go for that and as you note, like anything medical it is high dollar.

Staying around 85% in real world conditions sounds pretty good to me. I have three, 100W panels and have room for all three on the Scamp (side A/C so no roof air in the way). But my plan is to mount just two for now and leave the other as a portable unit if the trailer is shaded or I just feel like catching some additional rays. Anyway, it is all fun with the Scamp and I don't even have to leave my back yard!
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Old 11-08-2015, 09:38 AM   #4
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Timely....

thanks for all that Wolf....I really enjoyed your post as I too have been working on understanding/quantifying the charging and discharging process in my 12V system.

My system is 2 GC batteries, I deleted the converter and installed a 3 stage charger, I have a 40W panel on the roof and another 40W panel that is "deployable"...small, protable 150W inverter

The tools I use to measure is a panel meter and a hydrometer...unfortunately the readings I get from both often contradict each other !!! that was a surprise...(as an example the the panel can read 12.3V while the hydrometer check can indicate 1.260...like yesterday!)

Clearly knowing how much power is stored in your batteries at any given time is way more complicated than how much gas you got in the tank of your TV ! (I wish)

I sort of knew that getting a TM would be the solution but resisted it (hate to spend more money)....you've convinced me that I should either get and install one....or quit worrying/trying to get an accurate measure of battery bank capacity at any given time. But, very much like you, I'm "having fun with my RV parked in the dirveway" so I'll have to get busy and look at sourcing a TM. Thanks for posting.
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Old 11-08-2015, 09:56 AM   #5
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Originally Posted by Franswa View Post
The tools I use to measure is a panel meter and a hydrometer...unfortunately the readings I get from both often contradict each other !!! that was a surprise...(as an example the the panel can read 12.3V while the hydrometer check can indicate 1.260...like yesterday!)
I need to get a new hydrometer, I have an old one but have no idea where it is. A hydrometer is the IMHO the best way to get and accurate reading of a batteries individual cells, and by extension the whole battery (bank). I think you add up all the individual cell readings to get the battery total voltage. In your case 1.26 * 12 would be 15.12V! Also, for the most accurate readings batteries should be tested at rest, that is some period after charging or discharging. The TM shines in that once set, it should tell you in real time what is going on with your system.
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Old 11-08-2015, 10:30 AM   #6
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Really...????

I don't know about your formula there.....two GCs = 6 cells.....so with your formula my batteries would be at 7.5 volts ??? something not quite right there...

when I checked them yesterday according to some charts (there are lots on the internet and strangely they don't all agree...that should tell you something right there!).....12.3V = 60-70%....and 1.260 = 80-90%......and that's is the confusion/discrepancy I was referring to

all I can say is it's a darn good thing somebody got the idea of stringing wires up on poles a long time ago.... cause if we all had to power our houses with 12V it'd be a full time job just to keep track of what was going on !!!!
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Old 11-08-2015, 06:40 PM   #7
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Name: Peter
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what is a trimetric TM???

Quote:
Originally Posted by Timber Wolf View Post
Last weekend, as part of my never ending quest to spend money and time creating the perfect (for me) FGRV, I installed a Trimetric TM-2030RV($$) with the appropriate giblets to make it work. Previously I had installed two 6V golf cart batteries ($$) on the tongue in a custom made box ($). I have also swapped out all the 12V lights for LED fixtures ($), installed a TF-130 Truckfridge($$$$$$$$), and a new PD converter($$) along with new appropriately sized wires and fusing.

Although we had camped a couple of times before installing the TM, once with shore power and once without, all I had to monitor the batteries was a cheap digital voltmeter. It worked but I wanted a more accurate and scientific way to see what was going on. That and my eventual plan is to install some solar so the TM will fit right in as part of a complete solar system.

Last week I ran the fridge a while so I could fully recharge the batteries so the TM could see a cycle and set the battery percentage function. Without going into it too much the TM uses voltage and amperage to tell when it thinks the batteries have reached full charge and then monitors usage to determine percent of full at any point, pretty neat.

So far I have recorded the draw of a few things, the LED lights, 150W inverter, and my CPAP machine. I had read that LED lights use 1/5 of an amp, and dang if that does not square almost exactly with what I am seeing. I turned on lights one at a time and watched the amp reading. It was almost exactly .2, .4, .6, .8 and then 1 full amp with five lights burning.

In contrast to the lights which did not reveal much new, the CPAP proved very enlightening. Not necessarily for what the CPAP used while running, but for when it was not. I had previously set my CPAP up with the inverter and tested it with the battery out of my tractor in an effort to be ready for power outages so I knew it worked and did not draw much.

That was confirmed in spades today. What was most interesting were the "parasitic" draws of the inverter and the CPAP plugged in not running. The inverter alone turned on with nothing plugged in drew .27 amps and the CPAP plugged in but not running another .27-.29 amps. Total amps for the inverter and CPAP running was just 1.35 amps.

This confirms why the company's recommendation for an inverter to run my CPAP was for the 150W unit. I questioned this originally since the CPAP name plate says 2.5A at 110V or 275 watts leading one to believe a 300 watt inverter was called for. The actual draw (with inverter) is 1.35A at 12.8V (battery volts showing on the meter) or 17.28 watts. That does not seem right so please check my math/assumptions but that is what I see.

If there is a take-a-way to this rambling post it is to utilize an appropriately sized inverter, and turn things off/unplug when not in use (duh).
I use a CPAP and let me tell u it was not cheap, thankfully my wife's medical paid a good portion of it, right now not using as I'm doing the yearly face burn which is not pleasant. We have a 1000W inverter as when I'm sleeping with 12V cpap everything else is on charge to like camera batteries, computers, phones, laptops, etc, etc, and the running of the Cpap but it also has it's own battery and is good for 1 1/2 nights not plugged in so it gets charged now and again to.
Stude
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Old 11-09-2015, 06:06 AM   #8
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I generally use a 10 to 1 rule of thumb for comparing amperages. So 2.5A at 120VAC is 25A at 12VDC. But your CPAP placard shows watts which should give a more accurate number for your machine. My only thought was that I wonder what sort of start-up loads there are. I believe CPAP machines have a small compressor motor in there. Some motors have a surge load at startup (like A/C compressors) that the placard rating needs to account for (to size the fuse/breaker). Surge loads (aka inrush current) are gone in a blink of the eye and you usually need a meter designed to capture them. Even so, it’s such a small, brief load I think you can ignore them except in terms of trying to relate the CPAP placard to what your meter is showing.
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Old 11-10-2015, 09:10 PM   #9
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Just a note about measuring battery state of charge, Victron makes a nice meter, BVM 702. Cost is about $175.


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Old 11-10-2015, 11:57 PM   #10
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Name: Jim
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My CPAP power consumption

As has been noted, power consumption of a CPAP does vary a lot -- depending on model, power source, settings and etc.I use a Phillips Resperonics REMstar Pro C-Flex+ System 1 at home and in my Scamp.At home it is ac powered using the factory power brick.The brick outputs 12 Vdc to power the CPAP so powering directly from a 12 Vdc trailer battery is pretty easy.Rather than buy the Phillips 12 Vdc to CPAP adapter I put together one using a generic adapter and the appropriate little round plug to save a few bucks.
Powering directly from the battery with the adapter takes considerably less power than using the battery to power a 120 Vac inverter and then using the 120 Vac brick to power the CPAP. The inverter uses power to convert from 12 Vdc to 120 Vac and the brick uses power to convert from 120 Vac to 12 Vdc with none of this power actually used in the CPAP.The direct dc to dc connection avoids all this waste.
The other major factor in power consumption is whether you are using heat or not. Heating takes lots of power and will really limit CPAP use with battery systems.
Some approximate current and power data for my system are:

120V ac Power consumption @ 10.0cm H2O
standby (resting) 0.04A 1.5W
blowing, no heat, no c-flex 0.22A 14.9W
blowing, ramp 6.0cm, no heat 0.14A 8.8W
10.0 cm, heat #2 0.84A 56.5W
C-flex working, heat#2 0.09-0.47A 4.5-26W
C-flex working, no heat 0.09-0.16A 3.5-10.9W
0.06 KWHr -- one night C-flex, some heat

The c-flex function varies the pressure as you breathe in and out and since pressure is one of the things affecting power consumption, the CPAP’s power consumption varies. (It is also not really appropriate to measure these changing currents with a meter -- this is not rigorous technique, just to get the feel I needed)


CPAP- trailer battery power 12.62 Vdc
standby/startup 0.01A 0.1W 12.59V
10.0 cmH2O, no heat 1.0A 12.2W 12.36V
6.0 cmH2O no heat 0.52A 6.5W 12.43V
10.0, no heat, C-Flex 0.1-0.6A 1-4W 12.49V
then 10.0, no C-flex (off face) 0.72A 9.2W 12.39V
10.0, no C-Flex, heat #2 4A 47W 11.77V
10.0, no C-Flex, heat #1 4A 47W 11.77V

(Note voltage drop at CPAP (e.g. 12.62 to 11.77) as current increases results mostly from trailer wire size/length and wire connector resistance - which has since been improved)
cmH2O is centimeters of water, an air pressure measurement/setting on the CPAP
Since I run at 10 with C-Flex and no heat, my power consumption is a relatively low 4-5 A-hr per night, and I can go a number of nights with my single 27F battery.
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Old 11-11-2015, 05:22 AM   #11
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Good analysis of your unit. Should be helpful as an overall guide. CPAPs come up so often on the board I think this post might be a good one to pin somewhere.
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Old 11-11-2015, 07:10 AM   #12
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If you use almost any battery charge level indication you must take into account the battery temp.
The TM2030 has a sensor option for this adjustment.


Sent from my SCH-I605 using Fiberglass RV mobile app
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Old 11-11-2015, 07:54 AM   #13
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Originally Posted by redbarron55 View Post
If you use almost any battery charge level indication you must take into account the battery temp.
The TM2030 has a sensor option for this adjustment.


Sent from my SCH-I605 using Fiberglass RV mobile app
I was wondering if anyone was going to mention battery temperature
50 some years ago in my service station days commercial grade hydrometers were temperature compensated and had a thermometer built in.
You matched the circular chart with the temperature to get an accurate specific gravity reading of the acid.
Same held true for the ones we used for antifreeze
This inexpensive one if as advertised sounds pretty good.
Temperature Correcting Battery Hydrometer: BatteryMart.com

Joe
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Old 11-11-2015, 08:03 AM   #14
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Also when checking the "water level" it must always be checked with a fully charged battery. The acid takes up more space when it is charged. If you fill the battery when it is in a discharged state then it is likely the cells will overflow and you will make a mess and lose acid, not just water.
If the plates are not covered add just enough water the cover the plates and then charge.
After charging add water to full and then finish charge.
Many batteries become weak and the battery boxed rotted away due failing in this procedure. The acid is lost and diluted and the overflowed acid makes a mess.
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