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Old 04-01-2012, 10:14 AM   #1
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Battery Draw Question - Oxygen Concentrator

My dad has a lifechoice oxygen concentrator that he uses every night, and he wants to ensure he has plenty of battery capacity to run it at night when we are dry camping. He tested it by hooking the unit up directly to a 12 volt deep cycle group 24, and he said that after a full night (8 hours) his battery was at 75% charge.

I found on the website for the unit that it should draw about 40watts. What I don't understand is this: If I calculate the amp draw using 12 volts (the way he had it hooked up) - 40 watts @ 12 volts =3.3 amps drawing from a 70ah group 24 - it should last only 21 hours right (assuming we draw the battery down to dead)?

Now if I do the same calculation using 120 volts: 40 watts @120 volts = .33 amps drawing from a 70ah battery - it would last 212 hours? That just seems completely wrong.

Can anyone help me understand this?
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Old 04-01-2012, 10:24 AM   #2
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What formula is your dad using to determine the battery had a 75% charge?
(the battery is considered fully discharged when voltage drops to below 10 volts or so)
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Old 04-01-2012, 10:24 AM   #3
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Your numbers are correct, but you can not get 120 volts from an RV battery. You can't use the 120 volt calculation and apply those numbers to a 12 volt battery.

Also, draining a battery to less then 50% will shorten the life of the battery. If the battery is rated for 70ah you really only have 35ah of useable power.
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Old 04-01-2012, 10:25 AM   #4
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current draw

Quote:
Originally Posted by Nightwatch View Post
Now if I do the same calculation using 120 volts: 40 watts @120 volts = .33 amps drawing from a 70ah battery - it would last 212 hours? That just seems completely wrong.

Can anyone help me understand this?
One more step convert from 120 to 12. The current draw at 12 V is 10 times that at 120. Actually more accounting for loss in the inverter.

Your time on battery will be less than 21 hours. Add lighting, furnace, hot water your time will likely be 15 maybe 12 hours.
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Old 04-01-2012, 10:35 AM   #5
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Lumatic - it ended up about 12.4 volts after the 8 hours of use, so that's about 25% discharge give or take (haven't looked at a chart in a while)

Azyfly - yep - don't have any intention to draw down past 50%. Fortunately, he just upgraded to AGMs for the house batteries - 2 AGM's in series at 6 volts for the main house batteries, but he is going to bring along 2 interstate group 24's that he will charge separately with a generator and/or the alternator directly to use on the concentrator. So they won't be connected to the AGM bank at all. He's just going to install a 12 volt receptacle in the cabin of the TT, and hook up the 12 volts sitting outside. Will charge them in the bed of the truck as he drives.
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Old 04-01-2012, 11:14 AM   #6
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Lumatic - it ended up about 12.4 volts after the 8 hours of use, so that's about 25% discharge give or take (haven't looked at a chart in a while)

Azyfly - yep - don't have any intention to draw down past 50%. Fortunately, he just upgraded to AGMs for the house batteries - 2 AGM's in series at 6 volts for the main house batteries, but he is going to bring along 2 interstate group 24's that he will charge separately with a generator and/or the alternator directly to use on the concentrator. So they won't be connected to the AGM bank at all. He's just going to install a 12 volt receptacle in the cabin of the TT, and hook up the 12 volts sitting outside. Will charge them in the bed of the truck as he drives.
Beat me to it!!

I'll bet there are some marine boxes -- with carrying handles & tiedowns -- that would make this easier, safer and allow storage with dedicated power cables attached for stowage.

.
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Old 04-01-2012, 01:22 PM   #7
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It would be a good idea to measure the actual current draw in operation. I have found most name plates read high, possibly to allow for starting surges etc.
3.3 amps would be about 26 amp hours per night. Certainly doable if you don't go wild running everything else.
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Old 04-01-2012, 02:11 PM   #8
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Figure that your batteries have about 12 watt hours per pound usable capacity at a draw of about 1 watt per pound. Higher draws will reduce the available capacity.

A group 24 weighs about 45# so that's a bit over 500 watt hours. At 40 watts (which is close to the 1 watt per pound), that's a bit over 12 hours. This is using the 50% depth of discharge for most cost efficiency idea (see smartguage.co.com) - (makes sense, the 1 watt per pound is the 20 hour rate and 12 hours is about half of that 20)

You'd need 8 to 12 hours with a good multiple stage battery charger to recharge the battery.

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It would be a good idea to measure the actual current draw in operation
good idea. Don't forget other loads as well such as alarms, control boards, etc.
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Old 04-04-2012, 10:47 PM   #9
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So - it appears by conclusion that your dad should be fine as long as the RV batteries are charged each evening and that no other devices (furnace fans, etc.) are used.

The question - what if the RV batteries fail while he is asleep?
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Old 04-04-2012, 11:52 PM   #10
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Nightwatch

You may find that it is more practical and cheaper to bring oxygen bottles than to increase battery capacity.
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Old 04-05-2012, 06:13 AM   #11
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Nightwatch

You may find that it is more practical and cheaper to bring oxygen bottles than to increase battery capacity.
My brother had both when he came home from the hospital. He lives in an area prone to power failures. Sal.
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Old 04-05-2012, 10:11 AM   #12
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Increased batt capacity and increased O capacity sounds good from here.
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