CPAP on battery power issues? - Page 2 - Fiberglass RV

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Old 08-04-2014, 03:02 PM   #15
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Name: Jorah
Trailer: Casita
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So does an inverter compensate for, what would normally be, too low of a voltage for the CPAP?
Is that how I got so much out of the car Batteries I was using?

"Never confuse motion, for action."
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Old 08-04-2014, 03:36 PM   #16
Name: RogerDat
Trailer: 77 Scamp 13
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Originally Posted by hariph creek View Post

Solar is still going to have to wait. We get by on a pretty modest income. $75-$100 dollars is not a small amount of "discretionary spending" for us. Add in enough solar capacity to be sufficient and a controller thing and...? As with most things, the initial cost isn't the whole cost.
I hear you, I too have to make discretionary spending choices with some care. I was thinking if solar could extend use of current battery instead of battery upgrade which might cost more. Do solar before battery upgrade.

On the other hand you know how much power you will have in a dual battery set up, and how long that will run the required equipment so the battery option being a more precise solution sounds like it might be the better use of funds.

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Old 08-04-2014, 04:40 PM   #17
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I would suggest you re-evaluate the priorities, Solar panel (60 Watt to 100 Watt) first. This will keep the 74 to 80 amp hour deep cycle battery on your trailer charged daily. Later if you still fee the need for expense of 6 volt deep cycle batteries the solar panel will still be useful.
Byron & Anne enjoying the everyday Saturday thing.
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Old 08-04-2014, 06:06 PM   #18
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A small suitcase solar system costs quite a bit less than 2 6 volt batteries and lasts a lot weight penalty or installation, either.

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Old 08-04-2014, 08:34 PM   #19
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Originally Posted by Byron Kinnaman View Post
I would suggest you re-evaluate the priorities
I agree. CPAPs are for life and breath situations.

Dig out the manuals for the unit and the meter. Learn how to test your battery. Is it getting a full charge? Is it holding a charge? Find out if there is a low voltage cut off on the CPAP. If the manual does not say call the manufacturer.
Then test your battery usage at home while you have a connection to the power grid as a backup.

Yeh, I'm doing a bit of fear mongering here. But you have to ask yourself what would you do in a power outage situation at home? Personally I am losing faith in our infrastructure. A few years back a good chunk of the east went down for a few days. An ice storm this winter left a number of sections of my city of 3+ million without power for over a week. Not to mention that 9/11 taught us all that the western world is a target for terrorists.
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Old 08-05-2014, 12:28 AM   #20
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My inverter provides good quality sine wave AC power until the lower cut off voltage is reached. It will still kill your batteries. Probably faster than running on DC but the CPAP is probably more sensitive to low DC voltages. What the inverter is doing is providing a stable AC source that is converted to a stable DC source. Bypass that and you have a unstable DC source. You could also use a DC to DC converter; that would take an unstable (decreasing ) DC input and produce a stable DC output. This might be slightly more efficient than using an inverter for this function but at a greater and IMHO unnecessary cost. Small inverters are cheap.

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Old 08-05-2014, 10:12 AM   #21
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Most newer Respironics CPAP machines will draw between five and six amps per hour on 12VDC. Eight hours of use will draw down you battery system by nearly 50 amps. Powering your machine off an inverter will take even more due to the power overhead of the inverter itself.

First you need to know the Amp Hour (AH) rating of your batteries at 20 hours. This is the standard and should always be used when comparing different battery systems. In order to get an AH rating, the battery has to be drained down to zero over the course of a specified amount of time. The amount of amperage that it took to get it down to zero, over that specified amount of time constitutes the AH rating. Because of the Peukert effect (the faster a battery is drained, the less overall amperage is available), if you discharge a battery over the course of 100 hours, the AH rating looks higher that if you discharge that same battery over the course of 1 hour. So, there has to be a standard. For deep cycle batteries the standard rating is 20 hours. So, if a battery has a rating of 100AH @ 20 Hr rate, then that battery was discharged over 20 hours with a 5 amp load. So, that weird 20Hr rate that you see after the AH rating on batteries, tells you that the rating in question is the realistic, common rating; rather than an over-inflated number to make the battery look better than it really is. Of course, we NEVER want to drain our battery below 50% in actual use.

Second you have to have some way to replace the amps that you've used overnight. Simply put, you have got to recharge your battery(s). There are really only two practical ways to achieve this - with Solar or thru your onboard converter/charger. Other than solar, you've got to have an external source of electricity; hookup or generator.

Being in the medical field, I know the CPAP machine really is a life-line. As you well know, it does much more than just give everyone a peaceful night's sleep. With that in mind, you need to come up with a method to assure that you can use your machine thru the night (EVERY NIGHT) without having to worry so much about how much power it is consuming.

With tight financial resources, you will need to prioritize. If your battery is rated at 100 AH, most likely you will bring it down to at or near 50% with just your machine. This will work (barely) but your battery will suffer for the max draw down being placed on it. If you had 200 AH you could also run some lights and the fan. An additional battery would probably provide that buffer, but without some way to recharge them youíre still losing ground.

The other choice is coming up with a method to recharge your existing system by either buying a generator and carrying extra gasoline around with you or buying a small solar system with the idea to expand it in the future as monies allow.

Have faith that there are ways out there to camp comfortably.
Steve and Tali - Dogs: Reacher and Lucy and our beloved Storm and Maggie (waiting at the Rainbow Bridge)
2008 Outlaw Oliver Legacy Elite & 2014 Outlaw Oliver Legacy Elite II
2017 Silverado High Country 2500HD Diesel 4x4
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Old 08-05-2014, 10:28 AM   #22
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Good post Steve.
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Old 08-05-2014, 11:53 AM   #23
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Put another way, if you consume 48 amp hours (6 amps for 8 hours is 48 amp hours, for example), you will need to replace ~48 amp hours plus some for normal system losses. A 100 watt solar panel will give you about 7 amps (100ų147) of 14 vDC charging current. That means you would need at least 7 hours (48ų77) of good sun. Just to make our lives miserable, battery chargers donít return the max amount right up until the battery is recharged, instead it runs full out for about 80% recharge then tapers off to approach 100% which means it takes significantly longer than 7 hours (in this example) to return to full charge. Many/most/some panels recharge in partial sunlight but not at their rated output. This is part of why I use a generator (Honda 1000) when boondocking. It doesnít recharge any faster than a solar panel in full sun, but it runs the same rain, shine or shade, at no more than an idle. That and I rarely have more than a patch of sun in my campsite and Iím not going to spend my time chasing it around to maximize a panelís output.
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Old 08-05-2014, 01:27 PM   #24
Name: RogerDat
Trailer: 77 Scamp 13
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While those small Honda generators are a good option in terms of dependability they are probably not cost effective if working within a tight budget.

Given the budget considerations and the draw outlined in the post by Steve (Outlaw) the least expensive option for multi night safe camping is second battery plus solar with some way to monitor the current state of charge in order to bail to a campsite with power if the battery charge in late afternoon indicates insufficient power to get through the night.

My thinking is a second battery gives you two nights with possibly a third if you abuse the batteries by discharging them past 50%. Even if solar only replaces 1/2 of daily usage in two days it will have still have provided enough for a 3rd night. I will say this stuff is too expensive to purchase it twice. Figure out your power needs for solar and don't purchase until you can purchase enough panel to provide a good margin of surplus to deal with cloudy or inclement weather.

That ability to monitor available battery power before going to bed and knowing the power draw of the device is critical to making an informed decision that will protect ones health without it becoming an urgent issue in the middle of the night.
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Old 08-05-2014, 09:48 PM   #25
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Jorah, One aspect of your original post that has not been addressed yet by others is appropriate wiring. To set the stage, the original owners of our EggCamper BOTH needed CPAP while boondocking their way up the Alaskan Highway. They mounted three 12V batteries on the tongue to power a 1000W pure sine wave inverter that they both plugged in to at night. They were on the move most days, so they relied on a high amp alternator installed in their tow vehicle (Hundai Sante Fe) to recharge the camper battery bank while driving. I don't know all the power flow data details. That's just how they rolled, and that's how we purchased it.

Now back to the wiring. They installed a pair (+/-) of dedicated 8 gauge tinned marine grade wires (expensive) from the battery bank on the tongue directly to the input terminals on the inverter. I'm guessing it is about a 13' run from battery terminals to inverter terminals. I'm also guessing it was not easy to thread that heavy gauge wire around corners, behind walls and cabinets, etc.

My point in sharing all that is, when the original owners made this very important modification, they were just as serious about the quality of the wiring as they did the quality of batteries and inverter. I'm guessing it can make a big difference in CPAP performance. So you might want to spend some time researching and getting advice from this forum about the most appropriate wiring for your situation, too. Lots of knowledge to be had on this forum. Best of luck....
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Old 08-05-2014, 11:21 PM   #26
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Name: Jeremy
Trailer: Compact Jr and Teardrop Trailer
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I use a CPAP also. I have a 115 amp hour deep cycle battery to run mine in my teardrop trailer. I can get up to 5 nights of use with this battery if I minimize my light usage. My CPAP will shut off if my battery drops below 11.5V. Also I have notice if it is above 80 degrees or below 40 degrees it will affect my battery life.

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Old 08-06-2014, 10:15 AM   #27
Name: John
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The main issue here is the dropout voltage of the CPAP vrs the cutoff voltage for the battery. Typically the battery is considered used up at 10.5 but the CPAP cuts out at 11.5 so even though you have battery power left the voltage is too low for the CPAP. The other issue is the wire gauge used to supply power to the CPAP. You could easily loose another 0.5V there.

I'd do a couple of things. First with a fully charger battery and the CPAP running measure the voltage both at the battery and the CPAP. Make sure to do this while the CPAP is running at its highest level which may take a while to reach. This will give you the voltage drop. The only way to reduce this voltage is a larger gauge wire along the path.

Now let say this drop is 0.5V then you know that anytime the battery get to 12V your CPAP will drop out. What can you do?

One solution is what's called a Buck-Boost DC to DC conveter. These will take in 10 to 15V, some are 10 to 30V, and output a clean 12V out, it's adjustable so you could up that a little. Now your CPAP is always getting 12V all the way down to the battery cutoff point.

Nothing comes for free and there is a downside to these. They are typically only 75-85% efficient so you'll loose some capacity.

Here is one good for 100W, I'll see if I can find another one that's less expensive.

Buck-Boost DC/DC converters 12 volts to 12 volts, 24 volts to 24 volts, 24 volts to 19 volts, 12 volts to 6 volts. DC/DC conversion
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Old 08-07-2014, 12:30 AM   #28
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Trailer: 1984 19' scamp
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CPAP on battery power issues?

Is it possible to run on dc the first night to conserve battery power, and off an inverter the second night (assuming the inverter can handle more dc drop than the cpap)? I realize the inverter has more losses, but maybe it will still run the cpap on ac at 10-10.5 vdc battery power?

It still won't get you five days, but might help some.

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