Deep Cycle Batteries and CPAP

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TeaTime2
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Deep Cycle Batteries and CPAP

Post by TeaTime2 » Thu Aug 07, 2014 9:47 pm

So this is my first time bringing a CPAP to the playa, and my camp does not do generators.

From looking things up, I've deduced that having a DC converter with a deep cycle marine battery should do the trick. However, my electrical engineering skills are lacking. Hence, questions:
-how do I choose the right kind of battery?
-How do I recharge the battery on the playa (If it's off of someone's generator, what cables/other devices would I need?)
-How feasible is the solar solution and what parameters and devices and cables do I look for there?
-What else am I missing?

Thank you all

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FossaFerox
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Re: Deep Cycle Batteries and CPAP

Post by FossaFerox » Thu Aug 07, 2014 9:57 pm

Do you have the specs on the CPAP machine's power consumption? What's the plug look like? Even the model would be helpful.
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Re: Deep Cycle Batteries and CPAP

Post by Captain » Thu Aug 07, 2014 9:59 pm

I would suggest combining two 6 volt golf cart batteries for a single high amp hour 12 volt output. Golf cart batteries have high amp hours and can go pretty low in charge and still be usable. This is what fulltime RV'ers use for their house batteries.

To recharge from a generator you would be a converter, this would convert AC to the DC need by the battery, but you would also need a charge controller.

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Re: Deep Cycle Batteries and CPAP

Post by Captain Goddammit » Thu Aug 07, 2014 10:03 pm

It probably draws like 30 to 50 watts. The spec sheet will list maximum power consumption but average will be less.

There is extensive discussion and testing on the 6-volt vs. 12-volt battery issue. There's no real evidence that two 6-volt batteries in series work any better than two 12-volt batteries in parallel.
All you need is a car battery charger.
Last edited by Captain Goddammit on Thu Aug 07, 2014 10:15 pm, edited 1 time in total.
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Re: Deep Cycle Batteries and CPAP

Post by FossaFerox » Thu Aug 07, 2014 10:14 pm

I'm mainly curious if the actual machine runs on DC. The CPAP machines I've seen use a brick to convert to 12V DC, so he may be able to save some money and cut some efficiency losses by skipping the inverter. If he can run it straight off a 12V deep cycle battery (or a pair of 6Vs, is that really more sensible?) it might shift the equation away from recharging solutions to bringing a couple of batteries while getting something like 30 hours of use per battery...
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Re: Deep Cycle Batteries and CPAP

Post by TeaTime2 » Thu Aug 07, 2014 10:34 pm

FossaFerox wrote:Do you have the specs on the CPAP machine's power consumption? What's the plug look like? Even the model would be helpful.
The converter is a 110W max consumption, has both a cigarette lighter plug and a set of battery clips. Can run from a 12V or 24V DC power source.

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Re: Deep Cycle Batteries and CPAP

Post by FossaFerox » Thu Aug 07, 2014 10:53 pm

I meant the CPAP machine itself. If the internals are running on DC (likely) then you'd be going from DC to AC back to DC which would mean annoying losses, though if you already have an inverter I guess it isn't that big a deal, I just wanted to save you the inverter cost if possible.

Anyway, to charge it off a genny or solar (which is pretty inefficient, would not recommend) you do need a charge controller which would be troublesome, though someone else could help you sort through it. As for picking a battery, what you care about is it's capacity (Amp Hours at a given voltage, total capacity is actually voltage * amp hours). I'd focus on how you're going to recharge anything as step one, then find a battery that will work well with that as step two, then pick the one with the biggest capacity as the final step.
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Re: Deep Cycle Batteries and CPAP

Post by AntiM » Fri Aug 08, 2014 5:47 am

My friend ran his CPAP off an inverter hooked to a marine battery. Topped off the battery charge with a trickle solar panel in the daytime. Worked well.

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Re: Deep Cycle Batteries and CPAP

Post by rmc50 » Fri Aug 08, 2014 1:07 pm

I have several friends who use CPAP units and camp. My experience has been that a standard 12V deep cycle battery lasts two or three nights before it needs to be recharged.

If you can run the CPAP unit directly off 12V it would be a lot better. If you have to use an inverter to create 110 volts and then plug in the adapter cord the inverter will consume as much power as the CPAP will.

I would highly recommend bringing some way to recharge it, You can buy small generators (1000 watts) that are really quiet. You could run it during the day and turn the stereo on and no one would even know a generator was running.

A 50 watt solar panel would probably keep up as well.


Rod

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Re: Deep Cycle Batteries and CPAP

Post by aknod » Fri Aug 08, 2014 1:13 pm

My friend had his CPAP last year, and the battery charger for it pulled 10-12 amps. I think he might have gotten an extra/larger battery just in case, but the amperage of his charger was high enough that our EU2000 kicked off a couple of times when any other load was on it. I'm sure there are lots of different batteries and chargers, though. There should be a label on the charger that shows the output voltage and amperage, as well as a label on the CPAP machine that shows input voltage and amperage. If you can post those numbers it would be fairly easy to do the math on power needs.

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Re: Deep Cycle Batteries and CPAP

Post by Captain Goddammit » Fri Aug 08, 2014 1:58 pm

An inverter won't necessarily consume as much power as the CPAP does.
There are good ones and crappy ones.
Pure sine wave inverters are generally more efficient.
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Re: Deep Cycle Batteries and CPAP

Post by Papa Bear » Fri Aug 08, 2014 2:18 pm

I'll be bringing a CPAP this year, and have tested it a few nights running here at home.

For my model (a Philips), at least, it runs on DC and uses a power brick to convert from 110V AC. It was trivial to order a 12V DC adapter cable and hook it up directly to a 35 AH deep cycle battery, thus skipping the inverter. Cost all of $15 for the cable, I think? I'm planning just to occasionally use a standard battery charger hooked to the camp generator.

From what I remember, the "official" battery solution from the manufacturer was a 14 AH battery, and reviews suggested it would last 2-3 nights before needing recharging.

Depending on what kind of structure you're in, you might want to go with an AGM battery, so you can keep it inside with you without worrying about fumes.

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Re: Deep Cycle Batteries and CPAP

Post by FossaFerox » Sat Aug 09, 2014 8:46 am

You can find 85 AH 12V batteries at Costco for not a lot of money, so if 14 AH is good for at least two nights, 85 should get you 12 nights worth of sleep. Though I'd still bring a backup. That's for one particular machine, though, your model might fair differently.
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Re: Deep Cycle Batteries and CPAP

Post by Captain Goddammit » Sat Aug 09, 2014 12:56 pm

I've got a pair of 110 aH batteries from Costco, I think they were about $80-$90 a piece.
CPAP power draw is mostly the humidifier. I've never used a CPAP myself but I'm guessing if you need a humidifier anywhere, BRC is the place. So, I'd go a little heavy on the battery supply.
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Re: Deep Cycle Batteries and CPAP

Post by rmc50 » Sat Aug 09, 2014 3:49 pm

Captain Goddammit wrote: Pure sine wave inverters are generally more efficient.

I have no idea where you got that information from but I challenge it.

A cheap inverter uses a two phase switch: +110 to -110. This will work for a lot of devices, especially those that utilize switching power supplies (most modern electronics). They don't do so well with transformers and induction motors (series wound motors do fine).

Most better inverters use a "modified sine wave". The edges are still square, but they have a zero phase. = +110 -> 0 -> -110 -> 0
Transformers and motors are happier with this waveform.

A good inverter can achieve efficiencies up to 90%, but that is generally at full power and the efficiency falls off dramatically as the used power decreases. For example, a Xantrex Prosine 1000i (sine wave inverter) uses about 22 watts just sitting at idle. A Xantrex Prowatt 250 (modified sine wave) will draw about 1 watt at idle. These numbers are from the Xantrex data sheets. Xantrex is also one of the "better" inverter brands.

If you need 200 watts, you should get a 300 watt inverter (you want a little "head room). If you use a 1000 watt inverter the efficiency will be very bad.

Rod

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Re: Deep Cycle Batteries and CPAP

Post by FossaFerox » Sat Aug 09, 2014 4:01 pm

Thank you Rod; please hang out more often. You have useful knowledge and I appreciate your explanations.
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Re: Deep Cycle Batteries and CPAP

Post by maladroit » Sat Aug 09, 2014 6:16 pm

Pure sine wave inverters are expensive --> Expensive power handling equipment will likely include efficiency as an important design metric --> Pure sine wave inverters will probably be more efficient than a no-name Chinese inverter.

The logic is not far off, but rather than efficiency being a typical characteristic of pure sine inverters, it really just boils down to the old you-get-what-you-pay-for adage.

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Re: Deep Cycle Batteries and CPAP

Post by GreyCoyote » Sat Aug 09, 2014 7:25 pm

How you define efficiency and how you use the inverter are key.

A modified sine wave inverter design can be made to be very efficient. 92%+ is common into a RESISTIVE load. If you are just running lights or gadgets, use one of these. The amount of reactance in these loads is nil and the highly distorted inverter waveform carries no penalty. All of the AC power made can be used efficiently by the load so spending money to fix a non-existant problem is silly.

The metrics change however when running reactive loads. Run a modified sine into these types of loads and they fight back. These loads do not want anything but a pure sine wave and will be unhappy and inefficient on anything else. They will pull excessive current and have poor power factors. You run the risk of damaging the load while at the same time pulling heavy current from the batteries. For these types of loads, you need a sine wave inverter. While the inverter section of these units is less efficient, the resulting power can be used much more efficiently by the load. Overall, the system efficiency goes up. Way up.

Its important to look at the entire system when talking about efficiency. Always look at the power input verses work done of the entire system. Dont get hung-up in numbers that cant be compared. A 92% efficient modified sine inverter driving a reactive load that is reduced to 50% efficiency by the distorted waveform isnt saving you power.

tl;dr: Modified sine inverters are for mostly resistive loads. Sine wave inverters are for reactive loads. Use the right tool for the job and everyone will be happy.
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Re: Deep Cycle Batteries and CPAP

Post by Captain Goddammit » Sat Aug 09, 2014 10:38 pm

I've got a 2000 watt Xantrex sine wave inverter. Maybe tomorrow I'll hook it up and actually measure it's idle power draw.
I can tell you from experience that trying to run fluorescent lights, LEDs, microwaves, power tools, oh jeez what else... a lot of things just don't work right, if at all, on modified sine wave power. I got the info that sine wave inverters are more efficient from actual practice with them.
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