If you measure good voltage at the batteries and low voltage at the load point (the load point is where you are seeing the lower voltage alarm) than the problem is voltage drop across the cable or a connector in the path.
I am an electrical engineer and I explain to people that you can use you voltmeter to measure voltage drop between two points and you do not always have to go from positive to negative. You could put one probe of meter on the positive at the battery and the meter probe at the positive of the load point. If you see 1.4VDC for example, then you are dropping 1.4VDC of your voltage between these 2 points. move your probe from the load point to points closer to the battery and you will see where the weak point is in the path. If positive side is good, the do the same for the negative side. You have to do this will full load to see the voltage drops. E=I*R where "E" is the voltage drop in VDC, "I" is the current in amps and "R" is the wire/connection points resistance in ohms. So you can see that current must be at max to be able to measure voltage drops. If you have a high resistance point at high current, then you drop voltage across these points and this can cause local heating (P=I*E where "P" is power in Watts) which causes the resistance to go up, which causes the voltage drop to go up and if bad enough causes thermal runaway over time. I only mention this because if connection/wire heating is a factor, then this could take a long time to show up (hours...)
Lots of good suggestions above, but my guess is that you just aren't recharging you batteries enough for the load you're running. I'm assuming that the problem didn't start the first night? You can charge a battery to 80% or so pretty quickly, but that last bit of charge takes a while, and many converters don't have high enough voltage to get you there. A solar panel and good charge controller will give you both the time and voltage you need. If you want more info, read this blog.
Thanks to all for so much information. I will try to answer a few. I am boondocking right now but fortunatly have 3G connection.
I still have not had a chance to replace my faulty multimeter. I am going with the batteries being full by the idiot lights in the trailer (E, 1/3, 2/3, Full) and it shows full. When the alrm goes off it shows 2/3. Generally they are charged by either a long day of driving (6-10 hours) or we are sometimes in a camp with electric hookup (not boondocking). The problem did start pretty much the first night of using the CPAP and Furnace. It did not however start the first time the heater kicked on. It worked until 1-2 in the morning (3-4 hours and I'm not sure how often the furnace ran). Once it goes off it goes off everytime the furnace comes on. This went on for a while as it took me a while to figure out the connction of the 2 devices and the alarm going off. The clincher for me was the furnace ran fine after I got up (turned off the CPAP). Since sometimes my bettery is charged all day and night when hoked up to an electric campground and I still have the issue, it seems my CPAP is just drawing so much power that it lowers my voltage (drains my batteries) after just a few hours to the point of not being able to support the load. With the idiot lights it is possible that 2/3 battery charge is really 1/2. I was just surprised 240 amp hours would drain that fast from my CPAP.
I purchased a DC pwoer supply for my CPAP so it is operating off DC. This was my answer to avoid getting an inverter since that is really the only thing I needed AC for. I can live without the TV.
The charger is OEM and is 50 amp. It is unfortunatly far more than 3 feet away from the batteries. It is by the electic panel about 10 feet from the batteries.
I don't believe I can turn off the humidifier on my unit. I agree it is the power hog of the two. I did not bring the book with me and we are on the road for 6 months. I will go through the menus to see if that is possible.
I will try to turn off the humidifier and see if that works. I sure hate to invest in more battteries but that may be my best choice. Either way better cables will help some.
Thanks so much for the quick responses from people much smarter than I.
I liked the idea for checking the load drop. I am not sure how I would do that so additional info on how to would be appreciated.
I forgot to mention that I do have 2 Honda 2000W generators and run them about 2-4 hours when boondocking with the 30 amp plug connected to charge and to run the coffee post etc...
I just went throught the menus on my CPAP and found an option to set the humidity level to off. I will try it tonight. Not sue how much I'll need the furnace tonoight though. We pulled off in the Mojave around Barstow CA to sleep.
Thanks again. I really enjoy and learn so much from this community.
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I think the humidity switch should help a lot. But the problem remains unsolved at the core.
I would encourage you to regard charging by DRIVING as non existent. Your Honda's have the potential to drive your 50 amp charger but you will need to run them for at least 6 hours to put a full charge on deeply discharged batteries which you seem to have. Your best bet is to simply plug in somewhere overnight and let the converter do its' job. You have checked water levels right? I encourage you to get a $10 turkey baster hydrometer at the local Track Auto and check your batts cell by cell for charge as well as differences in measurement between cells which can indicate battery problems or the need for an equalization charge.
If you're gonna get a new multimeter anyway, you should get an AC/DC clamp meter from Amazon for about $50 bucks and this will let you easily measure the amp draw you are experiencing as each item is turned on an off. With a pair of 6V's you have roughly 110 amp hours before recharging is needed on a battery that is full to start.
That gives you 10 hours of use at 11 amps... 5 hours (less actually) at 22 amps so you can guestimate your furnace and CPAP and other items drain time till recharge needed and also preserve your battery life.
Of course... the meter will also let you test the voltage being put out during bulk charging by measuring across the output of your converter where you should see 14.4V being output to your batteries. Measuring voltage at the batteries by comparison can show you the loss in the wire and if you need to upgrade the wire size...OR if there is corrosion or bad connections.
You really can't use the idiot lights in the camper to indicate much. They measure voltage on the main DC panel and as soon as a converter puts out more than 14 volts they will indicated a full charge when you really aren't close to that. Voltage is not a good measure of battery charge, especially if you are actually drawing (or charging) with any current or have seen significant currents for the last 6 or 12 hours. The batteries need to settle without a load for a while before voltage means anything.
My CPAP will draw about 50 watts minimum and about 100 watts maximum (with humidifier). At 12 volts that is between 4 and 8 amps and is about the same draw as the furnace blower. That means that with both the CPAP and furnace you might be drawing between 10 and 15 amps of DC and 8 hours of that (the CPAP runs all the time, the furnace will cycle) could be close to 100 amp hours. I suspect that your batteries might be weak (hydrometer will tell all) if a 15 amp draw will drop the voltage too low for the CO detector and an 8 amp draw won't. If it was a really bad connection it would probably show right away, rather than wait for the batteries to get low.
Turning off the humidifier will definitely help, as will a long charge on 110V with your converter. Once your batteries are charged they should show a good specific gravity with the hydrometer, the question is how fast that SG drops when you draw significant current for a while.
Deep Cycle battery capacity to deliver VOLTAGE is demand dependent and will recover when the amperage demand decreases.
It sounds like for your particular demand situation you will need more capacity; (another pair of Trojans). Here is why.
As the amperage demanded goes up (above 5 amps), the stated capacity of the battery stack (AH) goes DOWN fast. See graph. Compare that effective capacity to its equivalent voltage in the chart to appreciate your problem.
Note the second graph that indicates your battery will "appear" to be fully charged quickly when running the generator (voltage wise), but its "capacity" will not recover completely for many hours of charging using the onboard converter.
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2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
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X2 on completely discounting charging your batteries while driving. You are most likely just maintaining the charge while driving due to the voltage drop in the charging circuit. It does not sound like there is anything wrong with your electrical system, you are just asking for more than it is capable of providing.
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