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Old 07-23-2014, 08:13 AM   #1
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Battery usage

In anticipation of possible dry camping, I upgraded my fiver's Grp 24 to a Grp 27 battery and LED bulbs and successfully camped 4 days in Grand Teton with enough left to bring in slides, stabilizers and landing gear with no charging mechanism. The only issue then was the CPAP I use for sleep apnea. I used a large jumper battery with a 300w inverter, but it only lasted the first night. I left it at the ranger station for charging, or carried it in the truck using the inverter to charge it, but never got enough charge where it wouldn't quit by 4 or 5 am.
Now I have a 100w Renogy portable solar kit for charging the house battery, and have run my CPAP via the inverter on the house battery with a battery status in the morning of 12.54v with nothing else running. I discovered that 12v cables are available for my CPAP and have ordered one, but also saw on the CPAP that it draws 1 amp on 110V and 3 amps on 12V.
So my question is whether it is more efficient to use the 300w inverter to run the CPAP on 110, or to plug directly into 12v.
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Old 07-23-2014, 09:12 AM   #2
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Well 100 watts isn't going to cut it, U need 3-400 watts, I have 585 W, in the Tetons now about 12 days always have power have flipped on the genny.
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Old 07-23-2014, 11:00 AM   #3
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Baby steps. Will replace the single Grp 27 12v battery with 4 six-volt in the future. We did 4 days without charging and just running the fridge (propane) and light furnace (propane and 12 fan plus LED lights. It looks like the 100w solar will bring it up to full in a couple of sunny hours. I understand it will take longer with more batteries. Was just exploring the 300w inverter with CPAP as an additional load. I'm guessing you ran the generator for 110v appliances and I don't plan on using 110 when dry camping.
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Old 07-23-2014, 11:11 AM   #4
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CPAP will use less Battery AH's power when running directly on 12 VDC versus 12 VDC converted to 120 VAC via the Inverter.

Dr4Film ----- Richard
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Old 07-23-2014, 11:13 AM   #5
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Quote:
Originally Posted by Dr4Film View Post
CPAP will use less power when running directly on 12 VDC versus 12 VDC converted to 120 VAC via the Inverter.

Dr4Film ----- Richard
Thanks, that is what I was hoping for. The 1 amp vs 3 amp confused me, but the inverter has a cost too. My 12v cable has arrived so I will test it in a couple of days.
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Old 07-23-2014, 11:54 AM   #6
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"... 1 amp on 110V and 3 amps on 12V."

Using AC: 1 amp x 110v = 110 watts from the battery plus the inverter loses.

Using DC: 3 amps x 12v = 36 watts from the battery.
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Old 07-23-2014, 12:05 PM   #7
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Originally Posted by BendOR View Post
"... 1 amp on 110V and 3 amps on 12V."

Using AC: 1 amp x 110v = 110 watts from the battery plus the inverter loses.

Using DC: 3 amps x 12v = 36 watts from the battery.
Wonderful! Now I understand, thanks!
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Old 07-29-2014, 07:46 AM   #8
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I was disappointed that my battery charge was the same after a night of direct 12v running of my CPAP vs an inverter. Then I checked the status before going to bed and found that after a day on 110v to the house, the charge was about 12.6, but after a few hours playing with the new Renogy 100w portable solar, the charge was 12.68 before plugging in the inverter. So the starting charge was the difference in outcomes.
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Old 07-29-2014, 07:47 AM   #9
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At this point, I have unplugged the house umbilical to connect the solar charger. Are there issues with house and portable solar connected concurrently?
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Old 07-29-2014, 08:48 AM   #10
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At this point, I have unplugged the house umbilical to connect the solar charger. Are there issues with house and portable solar connected concurrently?

Yes, there could be. The chargers sense voltage to determine what "stage" the battery charging process is in. If the onboard charger senses a higher than normal voltage because of the solar panel, it will probably shut down to just a trickle charge.....or the roles could be reversed depending upon conditions prevailing at that moment. Bottom line is that if the batteries are low on charge you should disconnect the solar panel, allowing the on-board charger to charge at its fastest rate. This should bring the batteries back to full charge quickest. When the batteries are at full charge, you could connect your solar panel, and turn the on-board charger off, to get some "free" charging.

100 watts is really just enough for maintenance charging while the coach is parked and not being used, and not plugged into shore power. You need more solar panels to be able to charge by solar alone while using the coach at the same time. Anywhere between 500 watts and 1500 watts. (1500 watts is obviously for "serious" boondocking!)
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Old 07-29-2014, 09:09 AM   #11
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Thanks, I am just dipping my toes in at this point.
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Old 07-29-2014, 09:39 AM   #12
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Most converters and charge controllers play well together. The ONLY way to keep your battery(s) above the magical 50% SOC (state of charge) is with the use of a battery monitor which keeps track of what goes into and comes out. The two main ones are Trimetric and Vectron. Going below 50SOC shortens the life expectancy of you battery.
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Old 07-29-2014, 04:10 PM   #13
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Originally Posted by SkiSmuggs View Post
I was disappointed that my battery charge was the same after a night of direct 12v running of my CPAP vs an inverter. Then I checked the status before going to bed and found that after a day on 110v to the house, the charge was about 12.6, but after a few hours playing with the new Renogy 100w portable solar, the charge was 12.68 before plugging in the inverter. So the starting charge was the difference in outcomes.
There is no difference in 12.6 volts and 12.68 volts, both indicate a fully charged battery.

Quote:
saw on the CPAP that it draws 1 amp on 110V and 3 amps on 12V.
So my question is whether it is more efficient to use the 300w inverter to run the CPAP on 110, or to plug directly into 12v
This doesn't make sense either. If the CPAP takes 1 amp on 120 VAC, it should take close to 10 amps on 12 VDC. There could be a little lost with the 120 VAC conversion, but not much.
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Old 07-29-2014, 06:19 PM   #14
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Turning on our 4.0 kW PSW inverter uses around 50 to 60 watts. Then there is definite efficiency loss through the inverter.We agree with Dr4Film at post #4 that AC would use less energy since it is direct from battery.

We have 1420 W of solar, so we are close to what Pasdad1 calls "serious" boondocking.

How much power does a CPAP use?
Reed and Elaine
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