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04-01-2012, 09:14 AM
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#1
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2 Rivet Member
Evergreen
, Colorado
Join Date: Jul 2010
Posts: 68
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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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04-01-2012, 09:24 AM
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#2
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Rivet Master
1971 25' Tradewind
1993 34' Excella
Currently Looking...
Estancia
, New Mexico
Join Date: Feb 2005
Posts: 7,742
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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)
__________________
Sail on silver girl. Sail on by. Your time has come to shine.
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04-01-2012, 09:24 AM
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#3
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Site Team
2002 25' Safari
Dewey
, Arizona
Join Date: Mar 2005
Posts: 15,606
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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.
__________________
Richard
Wally Byam Airstream Club 7513
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04-01-2012, 09:25 AM
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#4
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More than one rivet loose
Currently Looking...
Los Alamos
, New Mexico
Join Date: Feb 2006
Posts: 3,756
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current draw
Quote:
Originally Posted by Nightwatch
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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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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Michelle TAC MT-0
Sarah, Snowball
Looking for a 1962 Flying Cloud
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04-01-2012, 09:35 AM
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#5
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2 Rivet Member
Evergreen
, Colorado
Join Date: Jul 2010
Posts: 68
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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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04-01-2012, 10:14 AM
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#6
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Vintage Kin
Fort Worth
, Texas
Join Date: Nov 2006
Posts: 8,014
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Quote:
Originally Posted by Nightwatch
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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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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04-01-2012, 12:22 PM
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#7
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Rivet Master
1961 24' Tradewind
1969 29' Ambassador
1970 21' Globetrotter
Jamestown
, Tennessee
Join Date: Jul 2008
Posts: 1,783
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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.
__________________
Rick Davis 1602 K8DOC
61 tradewind, plus a few others
13 Ram 2500 TD
99 Dodge TD 577K miles
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04-01-2012, 01:11 PM
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#8
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Rivet Master
1975 29' Ambassador
Reno
, Nevada
Join Date: May 2003
Posts: 1,351
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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.
Quote:
It would be a good idea to measure the actual current draw in operation
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good idea. Don't forget other loads as well such as alarms, control boards, etc.
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04-04-2012, 09:47 PM
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#9
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Rivet Master
2020 30' Classic
Derwood
, Maryland
Join Date: Mar 2010
Posts: 1,515
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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?
__________________
John "JFScheck" Scheck
2020 30’ Airstream Classic
**I Love U.S.A.**
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04-04-2012, 10:52 PM
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#10
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Wise Elder
2010 30' Classic
Vintage Kin Owner
South of the river
, Minnesota
Join Date: Dec 2009
Posts: 4,169
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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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04-05-2012, 05:13 AM
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#11
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Rivet Master
1981 27' Excella II
mays landing
, South Jersey
Join Date: Nov 2009
Posts: 1,179
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Quote:
Originally Posted by Jammer
Nightwatch
You may find that it is more practical and cheaper to bring oxygen bottles than to increase battery capacity.
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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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Sal & Nora
Let us live so that when we die even the undertaker will be sorry. Mark Twain
AIR 42483
TAC N.J. 17
WBCCI 24740
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04-05-2012, 09:11 AM
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#12
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Vintage Kin
Fort Worth
, Texas
Join Date: Nov 2006
Posts: 8,014
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Increased batt capacity and increased O capacity sounds good from here.
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