Let's see if I got this right

[mettleramiel]

Okay, so my boss asked me for help setting up a remote power system in order to power his oxygen tank when he goes camping. I think that I have most of it figured out, but I thought that I would ask you guys first.

Alright, his machine is rated at 500 watts, so if I divide that by 120 I get the amps that it uses (4.6), and that means that a battery rated at 10 amphours will run it for 2 hours and 10 minutes. Am I right so far?

Ok so of I am right, my question is that I need the system to be able to run at night only (that's the only time that he uses the machine) so about 8-9 hours. So if I get a 100 amp hour battery bank, what size solar panel will I need in order to charge it?

Thanks alot, guys.

[DanB]

No.. that's not the whole story.  You would be right - if you had a 120V battery.  If it's a 12V battery, then the current drawn will be 10X that - so you'd have about 13 min.  Also figure inefficiency in the inverter and the rest of the system so perhaps 10 min at best.

[RP]

Does the oxygen system have a plug for battery use (like from a car)?

[Volvo farmer]

Yikes! I came up with four or six 220Ahr batteries and about a KW of solar. 5-6 grand! :-0  Keep in mind, if you want batteries to last, They should only be discharged to about half their Ahr rating. For infrequent use, I suppose you could go 80% DOD.

I think your first step should be to get one of those Kill-a-watt meters and actually measure how much power this oxygen machine uses in an 8 hour period. Even though the plate says 500 watts, actual consumption could be much less.

[dinges]

Not quite sure what an 'oxygen system' entails. Nor do I know how dependent your boss's life is on the system.

Most electronics components I order are clearly labeled as 'not intended for life support systems' (or nuclear applications). Same may go for other stuff like inverters & batteries.

Keep in mind the possible liability questions that could arrise should the system you designed/built fail.

In all likelihood, the system you are now trying to design already exists and can be bought. Not only that, but it may be engineered in such a way as to minimize the chances of anything going wrong.

Maybe I'm being too cauteous, but, if you're in the USA or a country with similar liability legislation, I'd be VERY reluctant to design/build such a system for anyone but myself.

[mettleramiel]

Ah, I see, so the amount of amps being drawn changes with the voltage available. Makes sense. I thought everything seemed a little too easy.

He is not dependant on the machine (can't remember what it is called) to live. It is for sleep apnia (no idea how to spell it) if it failed, he would just wake up.

So I guess that the solar panel idea is out of the question. I'm still a little lost as how to determine what size panel you need when designing a system. Volvo said 1KW, so is double the wattage being used usually a good guess?

Thanks alot for your help guys. Very helpful.

[mettleramiel]

Oh, and one more quick question. Since the amps being drawn goes up when the voltage goes down, is that why 24 and 48 volt systems are used over 12 for larger systems?

[windjunky]

Take a look at "Battery Basics", it's a little book on lead acid batteries.  Wish I knew the publisher/Author but you can find it at Amazon for about $15 I think.  It points out that not only is there an Amp Hour rating on every battery, there is also a max amp rate that each battery has.  If you exceed this rate (say .5 amps/hr) then you will get less than the listed draw down out of your battery.  Can't recall the name of this parameter but it is something like an "amp-rate".

WindJunky

[wdyasq]

I think you need to get a bit more educated before you start engineering life support systems.

BUT - he's your boss....

Ron

[nanotech]

Watts stays constant.

Watts = Voltage X Amperage

So as voltage goes down, amperage must go up and vice-versa.

Volvofarmer's bit about the 1KW of solar would be to compensate for the fact that your boss would be using his air pump for 8 hours at 500W, or 4KW/Hrs.  Daytime sunlight is generally only good for about 4-5 hours, so to make up for any clouds, etc he suggested 1KW worth of solar which would equal your 4KW/Hrs.

And since you generally only want to run batteries down to no less than 50% before charging them all the way back up, you'll need minimum of 8KW/Hrs of storage capacity.

8000. W / 12VDC = 667 Ampere hours
      
8000. W / 24VDC = 333 Ampere hours
      
8000. W / 48VDC = 167 Ampere hours
      
      

So if you're able to set up a 48V solar system, you'd need 8 Trojan T-105 batteries hooked in series at a cost of $105 each plus cables and lugs, and you're looking at $1,000 just for your storage.  And solar power is running about $5 a watt, so an additional $5,000 for the solar panels.

It can be done, but it's going to be expensive.

[jimjjnn]

My mother used portable oxygen tanks filled from her main O2 tank when she had to go out. My daughters both worked for a Medical mfg of O2 concentrators for home use that were 120volts. Later they made portable units that had a battery and wallwart that was 12volts. I don't know what the power required was.

I would suggest your Boss look for a portable unit as it will be cheaper than the solar system you are envisioning. I would add an extra battery to take along too.

[mettleramiel]

OK, NOW I think I got it. So, let's say that I have one 12v 200ah battery and i use 10 amps for 1 hour. Now I have 190ah. To charge it back up, I will need 120 watts (12v x 10a) making the smallest panel possible a 24watt panel with 5 hours of sunlight. Is this right?

Thanks to everyone for all the help with my boss's machine. It was just an idea, he was planning on setting up power there eventualy anyway, now he'll just get it sooner.

[wdyasq]

And you have losses - 24 watts at 12V is not the same as a 12V-24W panel that puts oru 24 Watts at maximum voltage - about 20V. So will only get about 60% of the panel rated wattage in. You are not right.

Ron

[mettleramiel]

well, I was working it out as if this was a perfect world. I realize that there are line losses, losses dues to less sunlight day to day, ect. I know that you always have to plan for more than you intend to use with any renewable energy system.

[pepa]

Mattleramiel, I have the same problem that your boss has, except that I have to have oxygen 24/7. Last year I went on a deer hunt for five days on an island that only had one duplex outlet for the entire camp area. I took enough bottle oxygen for the entire trip, along with one 220 amp hour battery and a 700watt inverter to run my nebelizer and Sullivan  nasal cpap system. The one battery provided me with all the power I needed for the entire trip with daily charging. Have your boss try running the machine on an inverter and battery system to see how the battery will hold up and you can work out recharging the battery form that point or take extra batteries, Hope this helps, (just because we are down doesn't mean we are out of the fun) pepa.

[ghurd]

I connected one of those.

From memory... It was rated lower, maybe 125W?  It had an occasional surge to maybe 125W, but averaged something like 40~50W?

I'd suggest he looks into other models or brands.  Something wallwart powered would be my first choice.

Sizing PVs is how much power is made compared to used per day.

For something like this, add extra PV and battery.  Don't cut it too close.

I would not camp on a golf course, I'd be where there are trees. Not the best for solar maybe.

Spare parts would be a great idea.  An extra inverter with easy to change fuses, spare fuses, etc.  100% high Quality parts.

And a backup plan, just in case.

G-

[thirteen]

You might call a medical supply company and maybe find what they have for a battery operated system.  They may have something you could use or modify to fit your needs. Just an idea.