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I have an oxygen concentrator that is 120V. The data plate says 3.0 amps and the specs say 280 watts. My question is this. I want to use it in an RV off the grid but without running my generator all night. I have two Interstate 12V AGM batteries I think they are rated at 75 amp hours each.

If I use an inverter will these batteries be enough to power this unit for a full night? If so, what size inverter do I need? Sorry about the basic nature of this question but I am an electronic dummy.

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closed as off-topic by duskwuff, Bence Kaulics, laptop2d, Dmitry Grigoryev, Daniel Grillo Aug 11 '16 at 12:22

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  • \$\begingroup\$ 12V * 75Ah * 2 / 280W ~= 6.4h, but only with 100% conversion efficiency. \$\endgroup\$ – Ignacio Vazquez-Abrams Mar 17 '14 at 0:13
  • \$\begingroup\$ Just for the sake of argument, what about just taking a bottle of oxygen? I don't know what the purchase/rental costs are in your location but it could be a simpler option. \$\endgroup\$ – John U Mar 17 '14 at 9:34
  • \$\begingroup\$ A bottle is an option. \$\endgroup\$ – user38794 Mar 22 '14 at 16:23
  • \$\begingroup\$ If a bottle is an option then you can get a medical regulator with a CGA 540 fitting that will fit welding oxygen tanks. The regulator is cheap, see Amazon. The CGA 540 fitting allows you to buy oxygen. You can't buy a filled oxygen bottle with a CGA 870, without a license. \$\endgroup\$ – jonk Aug 10 '16 at 6:23
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Your question is "what capacity of batteries do I need to power a 280W 120V load from 12VDC lead-acid Absorbent Glass Mat (AGM) batteries for a given length of time?"

The rated power draw is 280W. The batteries will discharge to something like 10.5V at the end of discharge, so let's say that the average voltage is 11.2V. Further, let's assume the inverter is 75% efficient. That means the average current will be \$280W \over 11.2V \cdot 0.75\$ = 33A. That means that if the batteries can actually safely (without damage) deliver 75Ah each, the operation time will be about 4.5 hours.

You really need to look at the detailed data for the batteries to see what they recommend for discharge "depth" to see if that assumption is true, but it's unlikely it will operate for more than 4-5 hours. For 8-10 hours of operation you'd need about double that capacity of batteries. AGM batteries generally allow significantly more discharge depth than conventional lead-acid batteries, but it still might be only 80% of the rated amount.

Note that this assumes that the oxygen concentrator presents a steady 280W load. If the load is intermittent then 280W likely represents the maximum load, and you may get considerably more operating time out of a pair of batteries. There is an inexpensive gadget called a Kill-A-Watt (see Amazon etc.) that can be used to measure the consumption, and it may give you a more reliable number than relying on the nameplate watts. I would definitely make that determination before buying the batteries.

Here's a portable oxygen concentrator that claims two hours at 2l/min continuous flow (the mode I presume you'd be using at night), using a 98Wh lithium battery, so the average consumption of that product is only about 50W.

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    \$\begingroup\$ Yes, the kill-a-watt is an excellent suggestion as it will tell your exact power needs which might be significantly different than what is on the specs (which might just be max power use). This might just save you from buying batteries you don't need. \$\endgroup\$ – Filek Mar 18 '14 at 3:52
  • \$\begingroup\$ Thank you for your answer and suggestion. The Kill-a-Watt numbers for nine hours were as follows: amps 2.8-3.1, watts 240-307, va 326-375, pf 72-81, kwh 2.55. As I stated earlier I am a true armature at electrical issues but this seems like quite a bit of power required. \$\endgroup\$ – user38794 Mar 22 '14 at 14:56
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You will need an inverter rated at more than 280 watts - 400 - 500 watts would be good, and should provide some headroom for possible motor starting surges.

If the thing draws 3 amps at 120 volts, the inverter will draw somewhat more than 30 amps at 12 volts, depending on the inverter efficiency - say 35 amps as a rough estimate. If the unit draws 280 watts continuously, you're looking at 350 ampere-hours to run it for 10 hours. It is commonly recommended that you not discharge a lead-acid battery more than about 50%, so you'd need about 700 AH of battery - a bank of six "golf cart" deep cycle batteries might be adequate.

Then you have to think about recharging the batteries, and running the oxygen concentrator during the day....

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  • \$\begingroup\$ The info provided by the oxygen concentrator is a little confusing. 120 volts at 3 amps means 360 watts, yet they put 280 watts on the stamp. Perhaps one is max wattage and the other is average or typical. At any rate, at the 280 watts, you would need 77 amp hours out of each of the 3 batteries to get 10 hours (assuming 100% efficiency of the inverter which is wishful thinking). The point is, it just can't be done with the 3 batteries if the 280 watt value is to be believed. \$\endgroup\$ – Filek Mar 18 '14 at 3:44
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    \$\begingroup\$ Thank you for your answer. The Kill-a-Watt numbers for nine hours were as follows: amps 2.8-3.1, watts 240-307, va 326-375, pf 72-81, kwh 2.55. As I stated earlier I am a true armature at electrical issues but this seems like quite a bit of power required. \$\endgroup\$ – user38794 Mar 22 '14 at 14:57
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As others have indicated about 30 amps on the 12 volt side so takes a lot of battery power for the night. though while driving it could easily run off the alternator at 30 amps. you would be best off getting a small quite generator like the 1000w inverter ones. Wouldn't cost much more than 1 set of batteries to run it for the night.

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