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Old 04-23-2016, 11:20 PM   #1
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DC Power Issue understanding

To start, I have a Rockwood 8289WS fifth wheel. I took out the single 12 volt battery and instlled 2 6 volt Trojans in series (240 AH).

Everything has been fine until this extended trip my CO/Propane alarm started sounding the alarm when my furnace was running. Very scary! I looked at the details however and the alarm signal of Red-Green-Red-Green... Indicates a low voltage default (not CO or Propane detection). I got to thinking and I have a new DC power supply for my CPAP. I realized the alarm is going off only when I am using the CPAP and the furnace at the same time. Sure enough I slept without my CPAP last night while boondocking, used the furnace and no problems.

I am an amateur electrician but I was surprised my voltage is dropping that low. Batteries are fully charged. I would love to post the voltage coming off but I am having problems with my multi-meter and on the road. Everything is working perfectly fine with the exception of this situation.

I am looking for your thoughts on how I can improve my voltage situation. The first thing that occurred to me was the cables. I noticed when I put in the dual 6 volts the cables installed by OEM were tiny. Not sure what guage but I used a 2 guage to connect the two batteries together and it is many times thicker than the OEM cables (which I did continue to use on the new batteries for ground and positive feed). Would lower guage cables reduce voltage loss likely correcting my problem?

Thanks for your thoughts.
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Old 04-23-2016, 11:26 PM   #2
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How do you know your batteries are fully charged?

Where is the powerconverter/charger located in relation to the battery? The two should be nearby no further than 3 feet apart.

Is the CPAP 12 volt? If so, what gauge wire and distance from battery are you running?

Do you have an on/off switch? If not, your propane detector is draining the battery and will kill it in 2-3 days.
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Old 04-23-2016, 11:33 PM   #3
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Are you running the CPAP off AC or DC? It's more efficient to run it off DC if you can. Second, are you running the humidifier on your CPAP, or just the air pump? Adding humidity to the CPAP air takes quite a bit more power than just running the air pump.

One more thing that will help your voltage situation is solar panels or a generator. You mentioned that it was an extended trip, but you didn't say whether you were daily putting juice back into the batteries after taking juice out overnight. You can only go so far on the charge you got from the drive in.

As far as the OEM cables, yes, those little cables they install will give you more voltage drop than thicker cables, but they are likely not the problem. The problem is your power budget -- How much you consume, versus how much you put back in.
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Old 04-24-2016, 07:19 AM   #4
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If your batteries were fully charged, the furnace should only draw something like 7 to 9 amps, not that much for what you have. CPAP humidifier is a good thing to turn off. If you are using an inverter, you should check all of your connections first for corrosion and tightness. Low voltage at the detector sounds like a bad connection or that your batteries were never actually at full charge, which is common unless you were connected to shore power for a couple of days before you left. It takes a long time to get to 100% SOC but I would be surprised if your batteries are really that low. Multimeter on the posts while the CO detector is complaining should tell you what's up.
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Old 04-24-2016, 12:35 PM   #5
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My wife uses home dialysis and I am running into a similar problem, Hers draws about 500 watts off and on through out the night from the AC convertor. The trailers inverter doesn't seem to put a full charge on batteries. I am looking at either using the trucks alternator system to charge the trailers batteries, or make a gas battery charger from a automobile alternator and a small motor and charge the batteries directly separate from the trailers system. This way I am only going from DC to DC instead of an ac to ac to dc.
If I go with the truck alternator as a charger I will use a low voltage battery isolator so I can use the 2 truck batteries as a dc source also.
Either one I will be able to have around a 139 amp system to charge the batteries.
You are correct on the cables, I made my cables out of 1/0 awg welding cable. Since dc needs the large cables to conduct efficiently. I may have to either put in a dual alternator system or a larger 240 amp alternator in the truck.
I still haven't decided which way to go at this time.
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Old 04-24-2016, 12:40 PM   #6
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I know there is simple answer, but I have been RVing for over twenty years and can't figure out what the acronym CRAP refers to, other than something my dog does.
Thanks.
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Old 04-24-2016, 01:16 PM   #7
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The most likely thing to me is that you don't really have fully charged batteries to begin with. You can't rely on "down the road" charging to put or keep a full battery bank. How do you know they are full?? If you are using the "idiot lights" OR if you are charging and measuring with a voltmeter immediately... you are getting false readings.

You need to use a charger (suggest minimum 20amps..maximum 50 amps...preferablly 3 stage or 4) overnight to put a full charge on. Then WAIT 24 hours and disconnect the negative wire when you do so. THEN measure with a voltmeter and you should see 12.6 or 12.7 volts. Alternatively you can do the reading with a "turkey baster" hydrometer.

Your battery voltage WILL drop under load and if the load is sufficient, it may drop to the trigger point for the alarm solenoid even with fully charged batts...but it is more likely with semi charged ones. This is NOT an indication that you have a dead battery. You can kill the loads and watch the voltage climb right back up. I've had completely full new batteries read under 11.5 under a big load and be back up to 12.5 a couple of hours later. ONLY the voltage of a battery that has been static for 24 hours with no load or charging can be accurately read. (BTW...my cables were 000! )
If you're gonna be boondocking a lot you really should get a true battery monitor that can tell you what is going on in REAL time like the Victron or Trimmetric. Alternatively, you could put in a dedicated CPAP battery & socket and charge that separately so as not to influence the voltage cutoff of the CO/propane monitor.
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Old 04-24-2016, 01:25 PM   #8
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Quote:
Originally Posted by Edmund View Post
I know there is simple answer, but I have been RVing for over twenty years and can't figure out what the acronym CRAP refers to, other than something my dog does.
Thanks.
CRAP = Dog dodo
CPAP = continuous positive airway pressure
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Old 04-24-2016, 01:28 PM   #9
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Originally Posted by Edmund View Post
I know there is simple answer, but I have been RVing for over twenty years and can't figure out what the acronym CRAP refers to, other than something my dog does.
Thanks.
What Is CPAP?

CPAP, or continuous positive airway pressure, is a treatment that uses mild air pressure to keep the airways open. CPAP typically is used by people who have breathing problems, such as sleep apnea.
CPAP also may be used to treat preterm infants whose lungs have not fully developed. For example, doctors may use CPAP to treat infants who have respiratory distress syndrome or bronchopulmonary dysplasia (brong-ko-PULL-mun-ary dis-PLA-ze-ah).
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Old 04-24-2016, 02:00 PM   #10
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Check connections. Nothing will cause voltage drop quite like a loose or dirty connection. I like the idea of trying it with the humidifier off also
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Old 04-24-2016, 03:05 PM   #11
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DC power issue

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...)
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Old 04-24-2016, 04:35 PM   #12
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Go Solar

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.

https://handybobsolar.wordpress.com/...ging-puzzle-2/
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Old 04-24-2016, 11:00 PM   #13
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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.

Thanks
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Old 04-24-2016, 11:07 PM   #14
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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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Old 04-24-2016, 11:54 PM   #15
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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.
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Old 04-25-2016, 07:32 AM   #16
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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.
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Old 04-25-2016, 08:06 AM   #17
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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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Old 04-25-2016, 08:14 AM   #18
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For example, if your demand on a 12 volt "single stack" of 2 6 volt batteries of 200 AH is 35 amps, your effective capacity to deliver voltage drops to 50%.

50% capacity voltage is 12.1 volts IF the batteries were fully charged.

If the voltage has decayed through the night to 50% "real" (no load) capacity, then you hit it with a 35 amp draw, you will have an effective voltage of 50% of that or 25%.

At 25% of effective capacity the voltage under that load will drop to 11.7 volts and may put the inverter or the CO monitor into its low voltage alarm.

Subsequent days if ineffective charging will only make the situation worse.
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Old 04-25-2016, 08:22 AM   #19
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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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Old 04-25-2016, 09:55 AM   #20
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X2 on all of this. If you are going to spend more than $50 on a new multimeter, consider a real monitor like the Victron or TriMetric. a good meter that monitors usage in invaluable for boondocking.
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