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Old 05-15-2020, 03:22 PM   #1
ObiRich
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Battery issues?

Ok...I've got a question regarding batteries. Let me preface it by saying that my wife uses a CPAP and oxygen concentrator at night. Since we just got the 5'er last year, we haven't been boon-docking with it, but plan to. We have a 3000W inverter so that we can get power for them. So I decided to run a little test today, and I'm kind of dumbfounded by the results. We have 2 6 volt GC2 batteries. Here's the results:

Start voltage: 13.28 (right after unplugging but before powering up the inverter).

At about 4 hours: 12.21

At about 6 hours: 11.98

At about 8 hours: 11.78

At about 9 hours: 11.51

I have all the usual power vampires, plus the TV, since it's plugged in on the same circuit, and just added in a digital thermostat powered by 12V. But nothing else...no fridge, no lights, etc. This seems to be a whole lot less capacity than I would have thought we would have. Our old camper had 2 12V 85 amphour batteries, and they lasted all night, albeit without the concentrator.

So, I've been thinking that I might need to add a couple more 6V batteries. Before I do, I think I'll pull the ones I have and get them tested. Since they were new last summer, would it be ok to use them if they are good, or should I get 4 brand new ones?
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Old 05-15-2020, 03:36 PM   #2
flybouy
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My first question would be what's the load of the cpap and oxygenator? Second would be have you ever run the batteries to a "dead state". Third is are your voltage measurements from the batteries being under load or just disconnected and standing?
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Old 05-15-2020, 04:45 PM   #3
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Originally Posted by flybouy View Post
My first question would be what's the load of the cpap and oxygenator? Second would be have you ever run the batteries to a "dead state". Third is are your voltage measurements from the batteries being under load or just disconnected and standing?
The CPAP and concentrator combined are about 400W, 110V.

No, batteries have never been run dead. The lowest they got was the 11.51 today in the test.

Yes, the voltages were read while the batteries were under a load. I read them through the solar charging port on the side, which is a direct access to the batteries.
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Old 05-15-2020, 05:03 PM   #4
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Cpap will use 3.6 AH 3k inverter idle could draw 1 ah with that alone the 230 ah batterys could be in flat dead in 19 hours. You should plug a couple 100 watt panels into your solar prep plug .
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Old 05-15-2020, 05:12 PM   #5
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We only run a CPAP but we run it on 12v. Look at the label on your equipment and see what the power requirements are. Most CPAP are either 12 or 24 vdc and the manufacturers have the necessary adapter.

I'm not familiar with O2 concentrators and don't know what they require.

Not trying to be perfectly accurate but am going to toss some numbers. That 400w at 110 is about 3.4 amps but at 12v is 33.4 amps. Add about 10% inefficiency of inverter and you have a draw of about 37 amps

Two GC2 is about 215ah so it isn't surprising the batteries discharged so low.

You need more batteries.
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Old 05-15-2020, 05:27 PM   #6
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There is an inherent inefficiency using an inverter to power a cpap. I have had several units since 1992 and they have all been 12-24v dc. When plugged into 110v ac the unit converts to 12-24v dc. So if you have dc power (camper batteries) run the cpap off a 12v outlet instead of going from dc to ac then back to dc. Look into the concentrator too, since I see a lot of portable units that run off of dc batteries.
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Old 05-15-2020, 06:34 PM   #7
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Thanks for the replies. I don't have any 12v outlets in the trailer (aren't newer ones nice that way).
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Old 05-16-2020, 03:15 AM   #8
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Even though ours is “newer”, I added the 12v outlets (as I have done on all of my campers over the years. On our current trailer the bedroom had USB charging ports on each side, but it didn’t have a 12v outlet so I installed a dual USB/12v in the same place. All easy to do and looks nice.
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Old 05-16-2020, 03:42 AM   #9
ctbruce
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Are you running the humidifier on the CPAP also? That is a huge draw.
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Old 05-16-2020, 03:21 PM   #10
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Don't be so quick to dismiss running your equipment directly off of the 12v power. As was mentioned, sockets can be added. I did this in my trailer. I then got the 12v adapters for both CPAP machines - now they run off of the batteries. Much more efficient this way.

Once we got our trailer converted, the pandemic hit and we haven't had a chance to go test it out yet. Hoping to do that pretty soon.
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Old 05-16-2020, 07:01 PM   #11
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If the battery voltage went to 11.51 it was 10% of a full charge.

Taking it down below 12.1 (50%) and it is not recommend and can cause damage.



A 3K watt inverter wastes a ton of power, 2 X GC2 batteries store a max of 235 AH (some are in the 200 to 215 range) and you should use less than 50% before recharging.

As others have suggested, try running things natively, on 12V power, without all the inefficient conversions.

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Old 05-16-2020, 07:35 PM   #12
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That chart is accurate only when the battery is at rest and has been so for an hour or so. When there is a load on the battery, it's going to read lower than it actually is.

This is why if you want to know the state of charge on your battery, you really need a shunt-based battery monitor.
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Old 05-17-2020, 05:58 PM   #13
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Thanks for the replies. I don't have any 12v outlets in the trailer (aren't newer ones nice that way).
I've added about a dozen 12vdc receptacles in our trailer since purchased last June. It's a very worthwhile project.
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Old 05-21-2020, 07:50 AM   #14
ObiRich
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Thanks for the advice all. I'm going to add in a couple more batteries, and may see about putting in a 12V outlet in the bedroom.
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Old 05-21-2020, 05:08 PM   #15
Harried Harry
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I use a CPAP running on 12v dc in my truck camper. I'm using the 12v outlet which is supposed to be for the Television. Works very well, but no humidifier. It works out okay, but I would like the humidifier since I live in a 10% humidity State. My point is the CPAP runs fine on the 12v port which is in the camper. My batteries are the marine type rechargeable ones which work very well.

Someday I may figure out how to power the humidifier but it does use a lot of juice, even on 110v ac. Good luck with your upgrade. Wiring in a 12dc outlet is fairly easy, but use a line gauge a bit larger than what you think you need since it will carry the load better, I think.
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