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I am an absolute novice in regards to electrical engineering, and so I am hopeful your expertise can guide me in the right direction. I know this is an unusual post for this forum, and so I thank you in advance for your consideration.

First, let me provide some context to my question. I am a quadriplegic, and I use a ventilator to support my breathing. The ventilator setup includes a humidifier to keep my lungs moist. In my particular situation, I'm unable to be without humidification for any significant period of time. I do use a heated wire with the humidifier which decreases condensation in the ventilator tubing. Both the ventilator and humidifier are attached to my wheelchair, and each has its own power cords. The ventilator also has its own battery component, but the humidifier does not. As you might imagine, needing to be connected to a power outlet greatly limits my mobility.

Given all of that, I would love to be able to power my humidifier with some type of battery pack - although I don't know how or if it's possible. With the advances in battery technology, I'd like to think it's possible. My initial idea was to tie into the batteries that power my wheelchair, but the people who service my chair told me those batteries would be depleted quickly from the humidifier.

Here are the electrical specifications for the humidifier, along with a couple of other pictures:

humidifier electrical specification

Humidifier 1

Humidifier 2

So, I guess I have several questions. Where would you begin if you needed to resolve this problem? What capacity battery would I need in order to run the humidifier for several hours? Is such a battery readily available or would it need to be designed specifically for this purpose? Could the connection be grounded? Is there any other issues that I might be overlooking?

I would be very grateful for any knowledge you could share with me.

Many thanks, David

starbucksguy
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    That's not a trivial amount of power. You're going to need tens of kilograms of battery to run it for several hours. – Ignacio Vazquez-Abrams Mar 17 '15 at 19:50
  • You would need to attach or pull around on a trolley a car battery or two and an AC inverter (to convert DC battery voltage to the 115V AC) to power the unit. – KyranF Mar 17 '15 at 20:11
  • There are some commercial battery or rechargeable products out there that would probably be cheaper. – Passerby Mar 17 '15 at 20:24
  • I looked at the [HC500 manual](https://www.fphcare.com/CMSPages/GetFile.aspx?guid=fb23576c-397f-4e4c-96aa-8953c28f26d3), and it's clear that electrical efficiency was not a high priority in its design. Ease of use, and minimizing the possibility of contamination/infection are obviously much more important. That said, have you searched for alternative units that might have lower power requirements? – Dave Tweed Mar 17 '15 at 20:26
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    Just running the numbers, if you needed 10 l/min. of air at 40 mg/l absolute humidity, and given that the heat of vaporization for water is about 2260 J/g, that means that you'd need about 15 W of heat delivered to the water. The trick to getting good efficiency will be to minimize the heat that is lost to the environment. – Dave Tweed Mar 17 '15 at 20:42
  • Thanks everyone for taking the time to look think about this and respond. @DaveTweed what range of power requirements would be optimal for me to for when I'm researching other units? Good efficiency is another great point. Thank you! – starbucksguy Mar 17 '15 at 22:20
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    I really don't know. I just wanted to figure out what the basic requirements were based on the physics of the situation. The "heated wire" option, if you are using it, is obviously intended to minimize condensation in the delivery tube. There may be other design subtleties that dictate using extra energy, too. But I'm thinking that if the heating chamber and the delivery tube were suitably insulated, you could get the power requirements down to on the order of 30-40W. – Dave Tweed Mar 17 '15 at 22:39
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    +1 for a well written and unique question. I love this community. – Greg d'Eon Mar 17 '15 at 23:35
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    I would seriously suggest obtaining an humidifier designed to run on DC. Using an inverter to convert the battery's DC to AC is highly unefficient and would decrease your battery life even more. –  Mar 18 '15 at 09:38
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    From an engineering design viewpoint, a chemical steam generator may make more sense for mobile applications. Hydrogen peroxide decomposition is quite energetic (3 kJ/g). – MSalters Mar 18 '15 at 09:41
  • Would you please let me know the volume of the liquid tank and the required amount of time to boil/finish all the liquid in the full tank. Also is it boling the water or just evaporating is enough for you? Depending on your answers I may suggest a 12 Volt around 4 Amp water heater(70-80C) or 12 Volt 8 Amp water boiler. Even can add a thermostate and send for free. –  Mar 18 '15 at 12:53
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    I would look into an insulated hose cover (http://www.snugglehose.com/) to reduce or eliminate the power draw from the heated wire. – Gabe Mar 18 '15 at 16:49
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    @Korhan the water chamber holds 15.8 fluid ounces (480 ml), and it typically takes ten to twelve hours for the water to completely evaporate. The temperature of the air entering my lungs is kept between 37 and 38 degrees C, though I'm not sure what the actual water temperature is. This humidifier has a heater wire socket and a temperature probe socket that integrate into the ventilator tubing. This, unfortunately, wouldn't be available on a small water heater. I do appreciate your thoughts tho! – starbucksguy Mar 19 '15 at 02:48
  • @Gabe I never heard of the Snuggle Hose, but it looks like a great option for insulting the ventilator tubing. Thank you very much for sharing! – starbucksguy Mar 19 '15 at 02:50
  • @MSalters Could you suggest an online resource where I could learn more about chemical steam generators? Thanks! – starbucksguy Mar 19 '15 at 02:56
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    @starbucksguy: Sorry, not a chemical engineer myself so I can't really be more authoritative than Google. And of course you wouldn't need as much steam as a typical industrial application. The reason I suggested it was that a H2O2 solution wouldn't be much heavier than the water you already have to carry around, and the catalyst to decompose it would literally be less than a gram. However, this device would produce some extra oxygen (>300 grams or so in 12 hours, 250 liter) and I'm not medically qualified to state whether that's acceptable. (It would enrich air to about 25% I think) – MSalters Mar 19 '15 at 12:14
  • I wonder what the implications would be to have a separate humidifier designed that was more power efficient and meant to run off batteries. Something like an ultrasonic humidifier that needs less wattage. Something like: https://www.youtube.com/watch?v=9GBoF8aw7VA – PlugTrade Mar 20 '15 at 23:46
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    I'd like to suggest, besides getting a better humidifier, NOT adding an inverter, and here's why: Most devices that run on AC actually use DC, but have an AC to DC converter built in. This converter isn't 100% efficient, neither is an inverter, so assuming the HC500 actually uses DC internally, you'd be plugging in a battery (DC) to convert to AC, for the HC500 to convert _back_ to DC, losing power every step of the way. If you look more closely at the datasheet, there's a good chance with some slight modifications you can remove the AC to DC parts, and then wire a battery almost right in. – TheEnvironmentalist Mar 22 '15 at 07:20

12 Answers12

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My wife has an HC-150, which is smaller than yours because she only uses it at night. However, as Olin said, a smaller unit might be an option if you only need it for a few hours.

The other answers talked about the peak current ratings, but what's important for battery capacity is the average current. My wife's unit peaks at around 90W, but only runs at around a 33% duty cycle at a cold start, and much less than that once warmed up, which means an average power consumption of well under 30W. She easily runs both her humidifier and CPAP from one marine deep cycle battery for 9+ hours on campouts. It's heavy, but not so heavy you couldn't mount it to a wheelchair.

The best way to learn your actual average power consumption is to get a kill-a-watt and measure for several hours under typical conditions. Or just take the experimental approach, buy a battery and the most efficient inverter you can afford, and see how long it lasts while you're safe at home. You probably also want some sort of level indicator so you don't discharge too far.

Note that peak power is still important when selecting your inverter, because it has to be able to handle that power, plus some margin for efficiency and error, even if just for short bursts.

Something else to consider if you're wanting this for moving around your own home might be some sort of docking/charging station so when you move to a new room you can unplug and replug yourself. Search for robot charging station for some ideas.

My daughter has cerebral palsy, and I know how important mobility is for her. I wish you the best of luck in finding a good solution.

Karl Bielefeldt
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    Very helpful thoughts, @Karl Bielefeldt. I looked into the HC-150 and found a 100 V model, but unfortunately, it doesn't doesn't offer the heated wire and temperature probe ports. I didn't realize the importance (and drawbacks) of that feature in my original post. I am going to order the kill-a-watt device you suggested; it looks like something that is just great to have. Thank you for your help! I wish your daughter all the best. – starbucksguy Mar 19 '15 at 07:06
  • You may also be able to reduce the power usage of the humidifier by wrapping it in something thermally insulating so less heat is lost through the sides. – John Meacham Apr 01 '15 at 07:18
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The basic problem is that this humidifier needs a lot of power. It needs 184 W worst case from the specs you provided, so let's say 210 W into a inverter. Let's say it has to sustain this for 2 hours, so that's 1.5 MJ. So far this is all just basic physics, nothing clever electronics can get around.

Let's put the 1.5 MJ figure in perspective of a car battery. 210 W at 12 V requires 17.5 A. For two hours, that's 35 Ah, but you can't drain such a battery all the way down without damaging it. In car battery terms, you'd need one rated to 50 Ah at least. That's not going to be small or light.

Today there are other battery technologies that are both smaller and lighter for the same energy storage. For example, the 20 Ah 3.3 V prismatic lithium cells from A123 are about 500 g each. (210 W)/(3 V) = 70 A total, so four 20 Ah cells can in theory handle this for one hour. However, this needs to be derated for various reasons, so figure 5 cells at least for one hour duration, so 10 for two hours. That's 5 kg or 11 pounds, although that doesn't count the inverter, the charger, and the mechanics to hold it all.

So, yes, it could be done at significant expense, would take significant space and weight, and cost a great deal. A better answer is to spend the money instead on a better humidifier, one that was designed for efficiency and battery backup use. I haven't looked what is out there like that, but that's where I'd start.

Olin Lathrop
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    Thanks for walking me through the basic physics of this @OlinLathrop - I definitely need that. Another user also pointed out that a better humidifier would be the best solution, and so I will look into that. But I appreciate you suggesting the 20 Ah 3.3 V prismatic lithium cells from A123 as well. Thanks again! – starbucksguy Mar 17 '15 at 22:46
  • Four 3.3V 20Ah cells in series will yield 13.2 volts at 20Ah, so to get a 12V 60Ah battery he'd need three 12V strings in parallel, which is *12* cells. – EM Fields Mar 18 '15 at 12:07
  • @EMFi: (210 W)/(3.3 V) = 64 A, which four 20 Ah cells could theoretically provide for one hour. However, this does point out that I forgot that this is intended for 2 hours duration, not one. Going to fix that now. – Olin Lathrop Mar 18 '15 at 15:18
  • You might also look for sources of inverters with 3.3V inputs. – EM Fields Mar 18 '15 at 18:00
  • @EMFi: You are apparently only considering connecting all the cells in *parallel*, which adds their currents at the same voltage. You can string batteries together in *series*, which adds their voltages add at the same current. The 10 cells I mention therefore would be overall a roughly 30 V 20 Ah battery intead of the 3 V 200 Ah battery you are envisioning. Since it's pretty certain you know this, it's hard to say what your point is other then trying to be contrary. – Olin Lathrop Mar 18 '15 at 18:17
  • @OlinLathrop: He explicitly said three parallel banks, each having four in series. That is indeed 12. You seem to be working from total stored power, neglecting the fact that you need an integer number connected in series, and an integer number of banks. If four cells for one hour are not enough, you can't just add one for five in series unless the inverter can handle a 20VDC supply, nor can you add the extra one in series with the four, nor does it make sense to put three individual cells in series with one parallel pair. Maybe you're lucky and the inverter works across a wide range Vin – Ben Voigt Mar 18 '15 at 18:48
  • @Ben: Yes, I'm working with the total amount of stored power. Nobody specified any particular inverter, so we can assume any input voltage range we want. I also didn't specify how exactly the cells would be hooked up. That wasn't the point. The overall number of cells required, and therefore the weight, is. In this case I'd probably put the 10 cells in series and have the inverter work from 20-35 V, so yes, I can add individual cells as I like. Or put in your terms, there is nothing here saying we can't use a single bank. – Olin Lathrop Mar 18 '15 at 19:23
  • Problem here is that you earlier said that 5 cells would do the job, without knowing anything about the inverter, but now it's 10 cells and you admit that you still don't know anything about the inverter. Classic case of lifting yourself on your own petard. – EM Fields Mar 18 '15 at 19:54
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    @EMFi: The original 5 cell figure was just a slipup, which I fixed. We're not stuck with any particular inverter since we get to spec it or design it. The discussion was only to show the rather large size and weight required due to the basic energy requirements. A 185 W 120 V inverter that runs from 20-35 V is certainly possible. It might even be available off the shelf - I don't know since I haven't looked - but it doesn't matter since that's not the point. Actually whether you end up using 10 cells or 12 cells doesn't change the point much either. Again you are just being contrary. – Olin Lathrop Mar 18 '15 at 20:56
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    @OlinLathrop: On the contrary, all I did was point out an error you made, which got your knickers in a bunch, and now what you're trying to do is muddy the water with a lot of diversionary crap to make it seem like you're squeaky clean, you cheater. – EM Fields Mar 19 '15 at 12:45
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If your humidifier runs on AC and you need to power it from a battery, then you'll need to interpose an inverter between the battery and the humidifier in order to convert the battery's DC output into the AC that the humidifier needs.

From the spec's, the worst case power input to the humidifier is in 230V heated wire mode, where the power dissipated by the humidifier will be 184 watts.

Assuming 60% efficiency through the inverter means that in order to supply 184 watts to the humidifier, the battery must supply about 307 watts to the inverter.

For a 12 volt battery, that's a drain of about 26 amperes so if you want to run the humidifier for an hour before the battery goes flat, it'll have to have a capacity of 26 ampere-hours. For a two hour run its capacity needs to be 52 ampere-hours; for a 3 hour run, 78 ampere hours, and so on.

Also, battery capacities are usually specified with the time variable based on the current taken at C/10 or C/20, where C is the capacity of the battery in ampere-hours. Taking current at a rate higher than that incurs a running time penalty, about -10% at C, for lead-acid, if I recall correctly.

In any case, peruse the battery manufacturers' data sheets for something more definitive and, by the way, deep-discharge batteries - if you go lead-acid - are probably what you should be looking into.

Just out of curiosity and certainly not meaning to be offensive in any way, how do you manage to post here if you're quadriplegic?

EM Fields
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    Thanks for taking the time to formulate your very helpful response, EM Fields. I honestly didn't consider the use of a converter; this really is all new to me. My main takeaway from everyone here is that my goal is achievable, especially with a more efficient humidifier. To answer your question, my neuromuscular condition allows limited use of my thumb and index fingers. I use a trackball mouse in conjunction with an onscreen keyboard called [WiViK](http://www.wivik.com) to operate my computer. – starbucksguy Mar 17 '15 at 23:02
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    Your condition certainly doesn't affect your beautiful English. – EM Fields Mar 18 '15 at 20:10
  • I concur; it's rather impressive. – Lightness Races in Orbit Mar 19 '15 at 11:12
  • You can find inverters on Ebay and in many stores that sell car, boat or solar power equipment. You need a 12V to 120V AC or 12V to 230V AC inverter rated for at least 300W. It should be easy to find and not too expensive. You can power it with a normal car battery, but make sure you get one that's powerful enough to supply 26 A for an extended time. If you can't find a good enough battery, use two or more connected in parallel. The batteries are heavy, but not heavier than that you can get someone to mount it/them on the wheelchair itself. The inverter doesn't weigh much. – Minthos Mar 22 '15 at 12:08
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If you are interested in exploring further DIY solutions, you might consider a nebulizer rather than the humidifier you're using now:

We have been using the nebulizer on long trips with saline or sterile water. The nebulizer doesn't use as much electricity and is smaller. We have a Die Hard Power inverter that plugs into the lighter in the car, and we plug the nebulizer into it and run it as i mentioned with saline or sterile water. That might work for you. (source)

If it's important that the humidified air be heated, this won't meet your needs, but if you can use room temperature water atomized into the ventilator air then it might work pretty well, and would significantly reduce your power usage. You'd need to keep the heated line or add a water trap, though. If you worked on it further, you could control the nebulizer so it only added water on inhale, and turned off on exhale. This would reduce condensation and wasted water, as well as preserving energy. You might be able to find a nebulizer that fits your tubing as-is, thus making it easy and inexpensive to try. There are a number of portable, battery operated ultrasonic nebulizers that are fairly inexpensive that could be tested.

Another resource to check out are CPAP machines. There are some with attachable humidifiers which have a DC powersupply. Just buy the humidifier attachment, then look at the power supply and create a battery pack and regulator that provides the same DC power. These would be easier to power with batteries. Like the nebulizer there are probably some tradeoffs, but it is probably worth checking out.

I wonder if it would be better to have an atomizer tip or ultrasonic vaporizer right at the entrance to the tracheotomy with power and a small water supply tube going up the air tube. With proper timing, a burst of moisture would go with each inhale breath, generating nothing for the exhale. Maybe a little heater if the water needed to be warm.

Adam Davis
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  • That is such an interesting resource you tracked down, @AdamDavis. Just when you think you've scoured the internet, something great pops up. I actually use a nebulizer several times a day (in conjunction with the humidifier) to administer medication, and so it would be really easy to experiment with this suggestion - especially during the summer months. I am definitely going to give it a try. Thanks so much for tracking down the thread you referenced. – starbucksguy Mar 19 '15 at 07:27
  • If it needs to be heated then a separate heating element could work. – ratchet freak Mar 22 '15 at 18:10
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Natural gas = 13,900 Wh/kg

Lithium battery = 180 Whr/kg

A humidifier is a heating unit. Batteries are not the most efficient way to carry energy to heat water. Natural gas is Muuuuch more efficient. Perhaps there are humidifier units using gas avaliable.

Besides, nowdays, they use vibrations to humidify water, atmospheric water vapor production can perhaps be adapted as a lung humidifier, they consume around 10 watts.

bandybabboon
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    This is thinking along the right lines for a portable heat source, though I think natural gas or camping gas are too complex and risky. An alcohol burner would be a simpler choice - a small "spirit lamp" with a string-like wick gives around 100-150W heat. Either way, integrating it safely with electronic control and pump are still a lot of work. –  Mar 22 '15 at 19:35
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You have a bunch of excellent answers here already. Several people have already calcated the hourly energy requirements for your humidifier. Both the lead acid and the much lighter lithium (e-bike battery) options have been presented. The inverter is a must to bring the battery's voltage up to 120 and to AC. Unfortunately, an efficient inverter will big, somewhat expensive, cause a loss of at least 15% energy, be bulky, add electrical cords and take up room.

I would encourage you to investigate if any company makes a 12 volt humidifier. Perhaps the army uses them out in the field where there is no electricity and only batteries? Most housebound appliances can now be purchased in a 12 volts DC form, so why not a humidifier. If one does not exist, you might be able to convince a company that builds humidifiers to build you a 12 volt DC version. Surely there are many others people yourself looking for independence.

Just a thought to make your project easier.

Good luck.

Filek
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    Thanks for your thoughts, Filek. I will investigate other humidifier options. Unfortunately, the likelihood of a company building a new version is quite low. At the same time as I posted this question here, I have been in communication with a company who manufactures a portable suction machine. I asked if they could provide me with a diagram or other information to help me modify their unit with a touch switch, allowing me to turn the machine on and off. They refused and shunned all future correspondence. So the outpouring of help here is wonderful. – starbucksguy Mar 19 '15 at 07:51
  • Poor customer service! Depending on its motor, converting it to run directly off of 12 volts might be a very easy change for them. I suspect that the manufacturer is hesitant to make any changes for the end user because of how important the humidifier is more your health. They may be worried that you don't know what you are doing and might hurt yourself and then blame the company. A retail store manager that sells humidifiers might have an easier time convincing the manufacturer to make the change. Also, a good electrician might be able to change it to a 12 volt device. – Filek Mar 19 '15 at 22:30
  • It looks like Adam Davis has found a DC powered humidifier that is used in conjunction with a CPAP machine. Likely a 12 volt device so they do exist. Avoiding the inverter makes your project much more efficient, probable cheaper (good inverters cost $), less heat produced, saves space, etc., lots of good reasons to explore this option. Also, your humidifier requires a lot of energy and thus require significant battery capacity (as calculated in other answers). Dad acid batteries will add a lot of weight and take up significant room leaving the lithium ion battery as the ideal choice. – Filek Mar 19 '15 at 22:37
  • Continued from previous comment... The problem is that virtually all inverters are created for lead acid batteries (in fact I have not seen an inverter made for lithium battery voltage). They don't work for the lithium batteries because the voltages are a little different. So another reason to look for a 12 volt (or any directly DC powered humidifier as you can wire batteries are in series to increase voltage). – Filek Mar 19 '15 at 22:43
  • This is great information: "I have not seen an inverter made for lithium battery voltage." There is so much in this thread for me to think about. I do have a call into the manufacturer, and I feel like now I can raise the pertinent points with them. I'll try to keep everybody here updated as I make progress. – starbucksguy Mar 20 '15 at 07:29
  • Yes keep us informed and keep asking questions. Good luck! – Filek Mar 20 '15 at 07:34
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Your unit takes about 180W in 'heated wire mode' and about 120W without. Add on the overhead of a regular mains inverter, let's say 80% efficiency, that becomes 225W for heated mode. This is in the capacity range of electric bike batteries, for instance here's a 360Wh battery for GBP200. So each 5Kg e-bike battery would get you about 1.5 hours, which would decline as the batteries get older.

The problem then becomes one of managing the battery(s) - making sure they charge properly, feeding the inverter properly, if you have multiple batteries when to switch between them, how to spot they're getting low, etc. The battery management solutions for electric bikes and other equipment may help here.

However, a bigger problem is one of reliability. If your laptop battery dies, you just turn it off. If your ventilator battery dies, that's potentially life-threatening. Designing a system to cope with that (backups, failsafes and so on) will considerably add to the cost and weight. Selling such a device, or perhaps even giving one away, would also need approval from the FDA or equivalent regulatory authority - more time and expense. They'll want to make very sure that the system won't kill people, and rightly so.

So you could ask a friend to help and you might get away with it, by agreeing to take the risk yourself. But it really depends how long you can cope without power - if the answer is 'very little' I wouldn't want to risk it.

user1908704
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  • It sounds like he wants to do this just for himself, so I don't think regulatory authorities are the issue so much as if the system fails and damages his humidifier or other equipment, having used unapproved power sources might make it out of warranty. There's also the issue of how and where to attach this significantly sized battery to his wheelchair. – I. Wolfe Mar 17 '15 at 21:12
  • @user1908704 I really appreciate your lengthy response. I should first clarify that while reliability is important because I'd like to have a dependable unit, it's in no way life-threatening. My ventilator has an integrated battery system that functions independently of the humidifier. As I.Wolfe mentioned, damaging the humidifier would be the greater risk for me. Acknowledging those risks, are you saying the e-bike battery could provide around 90 minutes of power with an inverter? – starbucksguy Mar 17 '15 at 22:39
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You can extend the battery life by reducing losses in the system.

One loss you mentioned is condensation in the tubing counteracted by a heated wire. This heat will escape through the walls of the tubing and be lost. If you add insulation to the tube then the wire won't need as much power.

The other I see is the condensation on the walls of the humidifier itself and the "hot surfaces warning. This will let heat escape; again insulation will help. Though it may create a fire hazard if heat is trapped in the wrong place. Insulate with caution!

ratchet freak
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    Patching these sources of energy loss does not mean that the device has a control mechanism in place to take advantage of it. – Scott Seidman Mar 18 '15 at 15:41
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    @ScottSeidman that's why the disclaimer and fire warning – ratchet freak Mar 18 '15 at 15:44
  • If there is no such control mechanism, insulating will not impact battery life, either. – Scott Seidman Mar 18 '15 at 15:48
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    Ratchet, thanks for raising the insulation issue again and creating discussion on it. @ScottSeidman, could you tell me more about the type of control mechanism that would need to be present in order for the insulation to benefit battery life? The humidifier does have a probe that monitors the temperature of the air I breathe in; would that be a type of control mechanism? – starbucksguy Mar 19 '15 at 07:39
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    Yup, that would do it. – Scott Seidman Mar 19 '15 at 10:39
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Just to throw out a fairly extreme theoretical... (this wouldn't make much sense unless you wanted a total solution including powering a wheelchair -- the humidi-vent itself could be done much more easily with lithium batteries if you up-volt, but it'd be relatively inefficient [which isn't really a major problem, just kind of "feels bad" to do])

It's possible you could source LiFePO4 batteries from China as a longer-term portable solution. You could take 77 3.2V LiFePO4 batteries at 100Ah each and find a regulation method to avergae output at 230V (from testing LiFePO4 LR6 batteries, it seems manufacturers tend to call ~3V the cut-off point when advertising Ah). You shouldn't run them below 3V (only because voltage-under-load drops off a cliff around that point), but you would also want to keep in mind that LiFePO4s start with a quick "surge" at a significantly above-spec voltage (~3.9V) -- this'd probably ruin whatever's drawing power from it, so it really needs to be regulated.

Anyway - so assuming 300Wh per battery, that'd come to 23.1KWh. Assuming the unit draws 35W on average, this'd give you ~3 weeks' power (factoring in self-discharge). If it powered wheelchair treads as well (throwing out a rough guesstimate with incomplete info, here), and assuming you in motion (at walking speed [~3.7mph] on fairly flat terrain) ~10% of a 24h cycle, we'd add ~12.5Wh to the 35Wh from the humidi-vent for a guesstimated run-time of ~a week and a half.

-But for the Hell of it, let's assume you're on some kind of hiking-similar trip (keeping in mind this battery type is relatively good with normal outdoor heat and sunlight) and you're moving 60% of the time on rough terrain at walking speed for an average hourly consumption of 240Wh (275 after humidi-vent). Guesstimated run-time there would be a bit over three full days, allowing for a decent weekend trip.

I'd estimate the cost of the batteries at around $7,700 (this is not a conservative estimate and assumes you get a pretty solid deal). You'd need a regulator(~$100), bank frame/wiring (~$500), and charge method (~$500-5000). You'd probably also need some kind of hoist or sliding method to remove the bank for charging (alternately, a charging frame might be able to fit over the batteries, but you'd have to leave the wheelchair). With someone's labor (~25-75h), I'd guess this project'd come in at ~$11k. However, you could cut the number of batteries in half and run @ 115V, which'd roughly cut all costs in half (since the battery bank would come in at a bit over a couple hundred pounds if size were halved, it may be possible to only need a couple people to move it manually assuming it had good grip points), where you'd just need to decrease the originally-given run-times by ~55%. The project'd be silly if it didn't power a powered wheelchair itself, though - but you gain significantly in efficiency by not needing to up-volt. This bank will be massive, like 2-4 electric forktruck batteries (but the bank doesn't necessarily have to be one solid block since you effectively have 77 "modules"), and weigh ~500lbs (though assuming the load's well-centered and secured, you could probably get a little crazier with driving on angled surfaces - unless it's slick, where a rollover could definitely be lethal). Depending almost entirely on how the charger's rigged up, you should be able to get 1500-4000 charge/discharge cycles out of this bank before dropping below 80% of original capacity (I believe a LiFePO4 solution would last you >15 years before even noticing it doesn't hold as much charge, and it would likely be able to last a lifetime if you're willing to put up with needing to charge it every week or so).

Addt'l notes: Since discharge per cell will be extremely low (given the load will be split among a relatively huge number of cells), run-times are likely ~10-20% under-estimated (you should be able to get significantly over 100AH per battery). They should run relatively cool and aren't prone to exploding. You also wouldn't have much at all to worry about with regard to maintenance. I didn't factor in the weight of the bank in estimating power-chair draw, and I have only a faint idea on what a power-chair draws (and especially faint when thinking what your mystery device draws), and don't even know if the wheelchair's rated for the additional weight. Better compromises could likely be made to reduce the size of the bank at the cost of run-time - that'd be a discussion to have with someone who doesn't happen to have a few free minutes without appropriate knowledge and isn't just looking for a distraction. :P

All the best -- sounds like a fun project. You might be able to find a local professional willing to take up the project just for the experience.

ETA: A well-designed regulator would be able to cut off power to everything but the humidi-vent once average voltage per battery dips below 3V. Ideally, it'd switch to 115V and down-volt, allowing you to draw fully from the batteries (LiFePO4 isn't TOO picky about deep discharge like, say, a lead-acid is, but voltage output plummets once discharged below 3V, so switching to 115V is necessary). I'd guess this'd give you days of additional run-time on the humidi-vent in case of emergency. A complex (heavy, large, expensive) charger inside the chair could allow for solar solutions or a simply plugging in to the wall, but I'd think the costs there outweigh the benefits.

Kluge
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  • Fairly extreme is right. 77 batteries... Good job I guess on thinking it all out, but not a practical solution I'd say. – I. Wolfe Mar 20 '15 at 16:00
  • Very interesting theoretical solution, but factoring in my history of roll-over incidents, this is not something I'd likely pursue! Just kidding. The weight, though, would be one definite deal breaker in terms of being able to access the wheelchair lift on my van - which I believe is rated at 400lbs. – starbucksguy Mar 24 '15 at 21:02
  • @Kluge - only wrt your LiFePO4 deep discharge comments - I'm well aware that some resellers claim that 100% DOD does not harm LiFePO4 lifetimes but this does not appear reliable. Information on effect of high DOD varies with source but the general indications are that to get the sort of long lifetimes that LiFePO4 can achieve you definitely need to limit both % DOD and terminal voltage. Dropping below 3V is an extremely bad idea. Resellers here offering long lifetime warranties require 80% DOD absolute maximum using "gas gauged" battery monitoring and recommend no more than 70% DOD. – Russell McMahon Mar 24 '15 at 22:24
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The answer to your question is yes, you can power your humidifier from a battery. All you need is an inverter that takes the battery voltage and converts it to the voltage (and current) required by the humidifier.
Using the specs provided, you need (minimum) a 120v, 2A (240W) output; 12v 20A input inverter; and a marine "deep discharge" 12v, 400CCA battery.

The battery and inverter can be mounted on a small trailer (anything with wheels) so that it can be easily attached and towed by the chair. To make things simple, I recommend duplicating this so that one unit would be charging while the other is being used. Good luck!

Guill
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You might consider using a smaller battery with continuous recharging via a fuel cell. See: http://en.wikipedia.org/wiki/Fuel_cell

Some issues of fuel cells are: (1) heat generation...more efficient fuel cells tend to run at higher temperatures, and (2) oxygen consumption.

A fuel cell could be run on replaceable propane or natural gas cartridges, resulting in water vapor as the by-product. This would consume oxygen, and so you would not want to use this in a confined space. Fuel cells lack the noxious by-products one would be wary of from traditional fuel burning generators.

Unfortunately, I don't know where to find a suitable fuel cell. In terms of batteries, I would start by investigating sealed batteries used for aviation. The quality and range of environment used for batteries of modern medium and large aircraft is very high.

HopeItHelps
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If you can tolerate having a piece of equipment outdoors that supplies a power cable inside, then there is a much cheaper solution. A lawnmower engine driving an car alternator is a heck of a lot cheaper than the inverter and batteries. Since gasoline is 9700wh/ liter; the total system for supplying 12 volts is also a heck of a lot less weight. Even taking into account the efficiency losses; a 5hp lawnmower engine could run the equipment and charge batteries at the same time so it could be arranged to operate a "gasburner" that generates exhaust intermittently.

steverino
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