DC Power in an RV

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This may be a failry obscure question but here goes anyway...

I biought a travel trailer this summer. (24' Coachmen)

It is setup with the standard 120v electrical hookups but it also has a DC distribution center that most everything in the trailer runs off of. When 120v is available the distro center converts it to 12v and feeds everything as well as provides for charging the deep cycle batteries. When 120v is not avaliable the distro center automatically switches over and the lights, water pump, etc.. operate off of the 12v battery (I actually have 2 batteries wired up in parallel.)

The fridge is a 2-way 120v/propane system and the hot water heater and furnace also burn propane. All 3 of these have 12v electronic ignition for the propane system.

Anyway, I took the trailer out this past weekend and went to a state forest/park that had no electric hook-ups so we ran off of battery power. The lights and water pump worked fine but the fridge, hot water heater and furnance wouldn't kick on. The furnace blower worked but it didn't kick on any heat. The fridge and hot water heater were totally unresponsive.

Luckily, we were able to live without any of these for the weekend but when I got home I started digging and found that the fridge, hot water and furnace all need the battery voltage to remain above 10.8v for the electronic igintion systems to work. I checked the battery and by that time it was down to a whopping .47v. Razz

So my questions revolve around how I keep the battery volatge level up while still having lights and water. When I have 120v I have no issues but without it I can't charge the battery.

Thiusfar I've looked at several options:

a. The 7-pin pigtail from the tow vehicle has a battery line that is supposed to charge the battery but it appears that it does so at a very low rate.

b. I could drag my generator along but would prefer to not have to do that for short trips plus, it's noisey and I'm not real thrilled with the idea of disturbing other campers.

c. I could hook up jumper cables from the truck battery to the deep cycle batteries. This seems cumbersome and slightly dangerous to be doing around open propane tanks on a regular basis. (The battery box is an aluminum box located immediately behind the propane tanks.)

d. I can put a battery isolater in the truck and wire up another connector so that I could charge the RV batteries from the truck alternator without having to use jumper cables.

Option d. seems like the "best" option to me at the moment but I'm wondering if there are any major downsides to doing this. Are there other options I've overlooked? Farmerman? Anyone else?

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I like (d), but would have expected your setup to already include one, just to keep the appliances and lighting systems from draining your vehicle's battery. If it doesn't, it sounds like a good addition, unless you can't, or don't want an additional deep cycle battery connected in series to the existing one.

If you don't have an isolater, you should get one just to avoid using the jumper cables for their more conventional use of starting the vehicle.

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roger wrote:

I like (d), but would have expected your setup to already include one, just to keep the appliances and lighting systems from draining your vehicle's battery. If it doesn't, it sounds like a good addition, unless you can't, or don't want an additional deep cycle battery connected in series to the existing one.

If you don't have an isolater, you should get one just to avoid using the jumper cables for their more conventional use of starting the vehicle.

The "standard" 7-pin connector that connects the truck and trailer elelctrical systems has a line that does charge the batteries. Apparently it is isolated through the truck's electrical system but it charges at a very low rate - in the 5-7 amp range.

I did order a 3-way isolater last night so that I can keep the truck's battery and the 2 deep cycle batteries all isolated form each other while charging. That will give me a 120 amp surge/65 amp continous charging ability. I'll need to add some diodes so that the batteries don't feedback to each other on the drain side as well.

I haven't found anything that indocates that there are any drawbacks to this type of a setup but I've never done it before either so I'm proceeding cautiously.

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Ah - It just sunk in that this is a trailer, not a camper, so naturally there wouldn't have been an isolater installed. That must have been a confusing answer on my part.

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First of all, something isn't right. Test the output of the 12v converter. My guess is that you've blown fuses to the 12v side of those appliances, because if your lights are working but you're only getting .47v to the ignition source, something is preventing it.

Secondly, the charge wire in the 7-pin connector should be an 8-gauge wire directly from the tow vehicle battery. It should be able to charge at about 40 amps constant. A good 1-hour drive should charge your deep cycles from almost dead. That wire (at least on most later trucks) is usually wired through a solenoid that shuts off the power when the truck switch is off. That way you can stop, camp, and not drain the truck battery. It used to be you had to disconnect the wiring to prevent draining the truck, but you can test yours with your volt meter.

Thirdly, all of those appliances use relatively small amounts of propane. The fridge uses next to none, the water heater a little more, and the furnace more yet. What sometimes happens especially with fridges is that the propane lines purge over time. When you turn the gas on at the tank and then turn the fridge on, you have 12' of atmospheric air in the lines and a tiny little bleed orifice. The fridge opens the valve, tries to ignite air, then quits. It will try that three times before defaulting to "check" mode. What you have to do is turn it off, then back on so it does another three cycles. Each time the propane is creeping back the line and it will eventually get there.

Its possible that the circuits for each appliance (not sensing any propane or heat from a flame) shut the ignition off which is why you registered no voltage. Try starting and stoppping each appliance several times. If you don't get ignition, check the fuses. If the fuses are fine, check the voltage at the ignitor right away as someone turns it on. If you turn it on then go outside to check it may have already turned itself off.

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curtis73 wrote:

First of all, something isn't right. Test the output of the 12v converter. My guess is that you've blown fuses to the 12v side of those appliances, because if your lights are working but you're only getting .47v to the ignition source, something is preventing it.

Errr.. No. You've misread something (or I didn't make it clear enough.)

The batteries were fully charged a week before I headed out and the lights and water pump worked fine. As the weekend went on the lights did get dimmer and the water pump slowed which should be normal as the battery discharges. The .47v was read directly off of the battery posts after I got home Monday and pulled the batteries from the trailer. They weren't connected to anything at that point. I didn't see any point in reading voltage at the appliances at that point. Since recharging the batteries the voltage at the appliances is reading 12.6v.

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Secondly, the charge wire in the 7-pin connector should be an 8-gauge wire directly from the tow vehicle battery. It should be able to charge at about 40 amps constant. A good 1-hour drive should charge your deep cycles from almost dead. That wire (at least on most later trucks) is usually wired through a solenoid that shuts off the power when the truck switch is off. That way you can stop, camp, and not drain the truck battery. It used to be you had to disconnect the wiring to prevent draining the truck, but you can test yours with your volt meter.

Mine appears to be a 12 gauge wire (it's a 2002 F-250) and several other sites mention that they also only get about 5 amps at the pigtail. I found many sites that recommend the battery isolators just because of the same problem I had. For example: http://www.damouth.com/RVStuff/TrCharge.shtml

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Thirdly, all of those appliances use relatively small amounts of propane. The fridge uses next to none, the water heater a little more, and the furnace more yet. What sometimes happens especially with fridges is that the propane lines purge over time. When you turn the gas on at the tank and then turn the fridge on, you have 12' of atmospheric air in the lines and a tiny little bleed orifice. The fridge opens the valve, tries to ignite air, then quits. It will try that three times before defaulting to "check" mode. What you have to do is turn it off, then back on so it does another three cycles. Each time the propane is creeping back the line and it will eventually get there.

Its possible that the circuits for each appliance (not sensing any propane or heat from a flame) shut the ignition off which is why you registered no voltage. Try starting and stoppping each appliance several times. If you don't get ignition, check the fuses. If the fuses are fine, check the voltage at the ignitor right away as someone turns it on. If you turn it on then go outside to check it may have already turned itself off.

There is no problem with any of the fuses. All of the appliances work properly now that the batteries have been recharged here at home - just as they did a week before I headed out. (I checked them all this week and checked the fridge on both 120v and battery/gas.)

I did try cycling the fridge (several times) as you mention while I was out there but that didn't work. The only explaination I can find for it is that the battery voltage had dropped below the required 10.8v that is specified in the Fridge Installation/Owner's Manual. I suspect that using the battery to open/close the slide when I packed up and then again when I set up was enough to drop the battery's charge. That is the only high draw item that I had used in between there.

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I did misread... sorry. I thought you were saying that the lights were fine but only .47v at the appliance ignition.

The 2002 truck should also have a separate fuse for the coach charge circuit. Its possible that its blown, but a simple test will figure that out.

The amount of amperage you can successfully get from the wire is not maxed out by the wire, its maxed out by the fuse. If you put a 100-amp draw on that 12-gauge wire, it will draw 100 amps for a split second before it melts the wire or blows the fuse.

The wire won't limit the amperage except by melting it or blowing a fuse. A 12 gauge wire should carry at least 20 amps, and most likely the fuse for that circuit is 20A. I've blown that fuse many times after a deep discharge because the alternator has 140A on tap and the trailer batteries are eager to take it.

If you have a Dometic fridge (specifically the push-button models like the Americana) they are insanely sensitive to low voltage. They default to check mode ANY time you drop voltage, not just when they are trying to ignite. My 95 F250 (isolator bypassed) would drop enough voltage during engine starting that the fridge would instantly go to "check" mode.

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How old are your batteries? Unless you have a great deal of leakage in the 12 volt system on the trailer you should be able to open and close the slides and have plenty of power to run appliances and lights for a weekend without the battery voltage dropping anywhere near .47 volts.

Do you leave the batteries connected to the house charger all of the time when you are plugged into 120 volt? I made this mistake and boiled dry a pair of batteries which evidently warped the battery plates and shorted out the charging circuit from the tow vehicle blowing the fuse. The house charger is not well regulated on most trailers so it does not drop to a trickle charge.

Since I replaced the batteries I installed a battery disconnect switch so that the batteries are not connected when plugged into AC while storing the trailer. If you do this you should turn the batteries on once a month for 8 to 12 hours to maintain the batteries.

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1) It's an inverter not a "converter" there is a big difference between the two.

2) Any decent 120 VAC battery charger will not over-charge the batteries and can be left on all the time.

3) No offense intended, however there are so many errors / omissions in the responses to the OP, it's hard to know where to begin correcting them all!

4) Here is just one example that is so wrong "The amount of amperage you can successfully get from the wire is not maxed out by the wire, its maxed out by the fuse."

5) Here are the facts: the fuse protects the wire, the fuse does not set the wire's ampacity. In fact it's the insulation rating that's one of the main variables delineating the ampacity of the wire.

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what's wrong with candlelight, gazing up at the stars, and talking quietly?

just sayin.

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landiwiley wrote:

How old are your batteries? Unless you have a great deal of leakage in the 12 volt system on the trailer you should be able to open and close the slides and have plenty of power to run appliances and lights for a weekend without the battery voltage dropping anywhere near .47 volts.

Do you leave the batteries connected to the house charger all of the time when you are plugged into 120 volt? I made this mistake and boiled dry a pair of batteries which evidently warped the battery plates and shorted out the charging circuit from the tow vehicle blowing the fuse. The house charger is not well regulated on most trailers so it does not drop to a trickle charge.

Since I replaced the batteries I installed a battery disconnect switch so that the batteries are not connected when plugged into AC while storing the trailer. If you do this you should turn the batteries on once a month for 8 to 12 hours to maintain the batteries.

This thread is fairly old now and since originally posted I had both of the original batteries tested and replaced. (I also added two more batteries!).

Thusfar I'm manually disconnecting the batteris to prevent over-charging but looking of an easy-to-use single diconnect switch so I can remove all 4 from the charging circuit at the same time.

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Chumly wrote:

1) It's an inverter not a "converter" there is a big difference between the two.

In the RVing world the two terms have very different meanings. A standard RV configuration has a power distribution system that is comprised of 3 seperate boxes. You have a fuse/breaker box, a "converter" section which handles your AC-to-DC stage to provde 12v power when conmnected to an AC power source and an "inverter" for DC-to-AC power.

When you talk to any RVer a "converter" will be limited to just the AC-to-DC system. An "inverter" is limited to just referring to the DC-to-AC system. Both the converter and inverter sections together will make up the "Power Distribution System".

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2) Any decent 120 VAC battery charger will not over-charge the batteries and can be left on all the time.

Hence the problem with RVs and their battery systems. The majority of RVs don't come from the factory with an automatic float charger. The factory AC-to-DC converter and battery charging stage are of mediocre quality and they WILL boil your batteries dry. Since starting this thread I've purchased a seperate battery charger that has both a higher charging capacity and a float charger.

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dagmaraka wrote:

what's wrong with candlelight, gazing up at the stars, and talking quietly?

just sayin.

lol Well, lighting a candle every time you go into the RV just to pee seems a bit silly. Plus, they're a fire hazard! Flashlights do help though. If I could figure out a way to run the water pump off of a candle that would be great. :p

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@fishin,

Your "converter" section is not correctly called a "converter", that stage is correctly called a rectifier.

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