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Open discussion on improving the DC Electrical System.


"DC Electrical System"

DC Help

Measuring DC I started this thread as an off shoot of the refrigerator exhaust fan thread. Most of the comments in the other thread were related to the battery losing power. To restate, I ran the fridge on DC Friday night for 3 hours while driving. We stopped to find we had a dead trailer battery ( no lights or anything in the trailer). This was after rewiring the DC to go directly from the battery to the fridge. I did not run it on DC for the rest of the trip. I returned home yesterday. Last night I measured the DC with one of those meters from CW that you stick in the DC outlet. It measured 12.4v (25% discharged). This morning it measured 12.1v (over 50%) discharged. I measured it tonight and it was 13.2V. I borrowed a digital multimeter today. Measurements in the trailer were 12.38v.This was at the battery, at the fridge, at the converter and in the DC outlet. Since I did not measure last night with the multimeter I don't know if I had this variance last night. The trailer was not plugged in today so why would the reading goup? Are these plug-in voltage meters inaccurate? Should I buy a digital multimeter if I want to monitor the battery or should the plug-in model give correct readings? Mary - Per Interstate documentation "If the battery is operated in a high heat environment, (consistently above 90 degrees F) the typical number of cycles could be drastically reduced."

Yes, those plug-in meters aren't very accurate, but your battery voltage might also have gone up. It isn't unusual for a battery's voltage to rise after it has "rested" from a heavy current load. (Don't ask me why, something to do with the chemicals.) Also, the voltage at the DC outlet may drop when the fridge is running if it's on the same circuit. This is caused by voltage drop in the wires due to resistance. That's why you want heavier wire for higher-current loads, and why you want to measure battery voltage at the battery. I think it would be a good idea to get a multimeter to back up your plug-in meter.The plug-ins are good for spotting trends and changes in voltage, but not so good for accurate absolute voltage readings. And DVMs are pretty cheap these days. 1. Yes, get a volt meter. Doesn't have to be easier than a swing meter. 2. Yes, to some extent, batteries have a "resting recovery rate" due to the acid action on the plates but it usually is a false reading. Good example of this would be a dead car battery. You grind and grind till it won't go no more. You wait awhile, and then get another grind out of it. One way to evaluate battery health is to fully charge the battery and then disconnect it so that you know there is no way it can be discharged by sneak loads. After are sting period of 24-hours, measure the voltage across the terminals with a good digital voltmeter. If the batteries aren't holding 12.6 Volts, (12.8 for gel cells),then they are in poor health. Maybe I'm way out in left field, but it sounds to me that your car alternator is not charging your house battery. You should have around 13.x DC volts when you are driving and with the fridge on. And that is where the plug in meter is a great "indicator". if it is showing 13.x when you start up on your trip and 3 hours later it shows 12.x, you know you got a big difference. Since the fridge draws 15 amps when on and you ran it for 3 hours, that would be about 45 amp hours -- enough to take a battery down, especially if it is a type 24 (has around 80-90 amp hours). Bob Steffen

I checked the charge line today coming from the exact reading, but I think it was 13.6v. I'll check it again later.

I use one of those "plug-in" digital type,in my cig. lighter when on the road just to monitor for the fun of it. With the refrig. on 12v, and running down the highway with everything settled down, it stays a steady 13.6v. Where this means anything or not,just thought I'ed throw it in. If you ran the 12v line to charge the trailer battery thru the connector, you need to check the voltage IN THE TRAILER. A digital voltmeter will give you a better reading if you are looking for voltage drops thru connections, etc., and I have them for $10. Analog meters are show changes easier, but for exact measurements, digital is much better. (I have been looking at the Casita's for the last year, and plan on getting one probably when the factory does their end of season mark downs after Oct.) ... friz

* DF Electronics

Bill, Where did you check the voltage from the truck: at the connector socket?, at the trailer battery? If at the trailer battery, was the battery (+) wire disconnected from the battery? If your truck alternator is putting out 13 volts or more, I would suggest the wiring is at fault. Are you sure you have a good ground through out the wiring harness, or is the trailer being "grounded" through the ball? This in not good. John

Bill, Couldn't sleep thinking about your problem, thought I'd better send my thoughts on so I could get some sleep!! I'm not sure where you are on circuits and electricity, so if I sound rather "basic", please bare with me. The battery voltage you are measuring is the"no load voltage" or the "surface voltage". A more accurate reading is the "loaded voltage" (turn on something, the greater the "load" the more the voltage will drop). Another thing about batteries: temperature increases, voltage increases; temp. decreases, voltage decreases. Let them rest, the "surface voltage" increases, but there is no reserve. If the alternator charges your vehicle battery, it will charge your trailer battery, if the wiring is OK. Now to circuits. To charge the trailer battery and run the 12v appliances from the car, electrical current must travel from the car battery, to the trailer battery and 12v circuits, and back to the car battery. From what you describe, this is not happening. Trouble shooting: You will need a multi-meter for this, it doesn't matter if it is digital or analog (swing needle), just make sure you understand the function and range selections before using it. You will need a wiring diagram of the connector,also. Check for 12v (+) at the connector: Set the multi-meter to DCV with a range that is above 20 volts (DC) Red lead to the connector 12(+), black lead to something metal on the car. Is voltage present? (it doesn't matter how much) Y: move on. N: check the car wiring. Check for "Ground" at the connector. If 12(+) is present, touch the black lead to the"Ground" terminal. Is voltage present? Y: move on. N: The ground circuit is open (not connected). Rewire the ground. Check for 12v (+) at the trailer battery: Disconnect wires at the battery, check on top of the battery for (+) and (-). Plug in the trailer connector. Connect the multi-meter, red to the (+) wire, black to the (-) wire. Is voltage present? Y: reconnect the battery wires and move on N: there is a problem in the trailer wiring, possibly the converter. I have not determined if there is a fuse in the battery charge circuit on the Casita, if there is, it might be blown. Check for ground continuity at the connector: (Note: This is another method to check for a ground. If a "ground" was present using the voltage method, this step is not necessary.) Unplug the connector. Set the multi-meter for a resistance reading (Ohms), it doesn't matter which one. Touch the leads together and set for "0". Touch one lead to something metal on the car, the other to the "ground" terminal in the connector. Be sure to check which terminal is the "ground", have a helper hold the flap open, for if you should connect to a voltage while in a resistance function, you will be replacing a fuse in your meter or purchasing a new one. Reading (it doesn't matter what)?Y: this is good; N: the trailer is not being grounded through the wiring harness, only through the ball. Rewire the ground. I have said this in other threads, the ground must be 10ga. (Factory wiring is most likely 14ga.) The small wire will not carry the load. This is why the 12v(+) wire must be 10ga. also. 30 amp. protection (circuit breaker) is needed. I have suggested a relay in other threads, also. Give this a try, let me know the results. If you still have problems, I'll put on my "thinking cap" again. John Hamilton

Hi John! Boy, everyone should print out your last post and carrying it with them. Good explanation of how to trouble shoot electrical wiring in any trailer! According to an old factory wiring diagram of a 16 footer, the black (+) direct charge line is protected by a 20-amp fuse. But in the back of my mind, it's one of those auto reset able breakers, believe it's attached to the back (inside the trailer) of the battery box. Don't quote me on any of this ... and as Isaid, my diagram is an OLD one, so may not be current. You said it should be a 30 amp fuse.Wouldn't a 20 amp be better? Showing my vast ignorance here, but if there was a problem, a 20 amp would blow faster, correct? I still think he ought to have the battery tested. John,

Thanks for the detailed response. You can't be too basic for me. This will take a couple of days before I can look into all of this.In the meantime I think I will take the battery in to be checked. I plugged the trailer into house power last night. The reading this morning was 12.73 on the multimeter and 12.7 on the plug-in meter. I'll let you know what I find out. I am always amazed by the knowledge and helpfulness of the people on this forum. Bill

I took the battery to NTB today. The battery tested fine. Hello Charles, The amperage rating of circuit protection (either fuse or circuit breaker) is determined by the wire size. IE: 10ga. wire can safely carry 30 amperes = 30; 12ga = 20 amps; 14ga = 15 amps.;16ga = 10 amps. It is safe to put small protection on a large wire; 20 amp protection on a 10 ga. wire; but not, say,30 amp protection on a 12ga. wire. I say this, because some individuals will replace a 15 amp fuse with a 20 amp fuse, not safe.So, I am saying 30 amp circuit breaker on the 10ga. wire, but a 20 amp breaker will give an extra margin of safety on the 10ga wire, if it will carry the load. All wire has internal resistance, long wire runs and small wire ga. increases resistance. Resistance reduces voltage and will produce heat as electrical current flows. Long wires need to be large to have electrical energy at the end. Wire also needs to be large enough not to over heat as current flows. Bottom line- if in doubt, go big. This also applies to house wiring and extension cords. If your skill saw seems to have little power at the end of a long extension cord, it is due to the wire size of the extension cord being too small for the length and the load. This in another way to think of current flow: Electrons moving easily in a large wire, like a 4 lane highway. Then the electrons come to a smaller wire and must squeeze together to get through, like traffic merging from 4 lanes to 3 lanes or to 2 lanes. Traffic slows down, radiators and tempers overheat. In electricity, wires over heat or can not carry the load. Therefore, the recommendation to add a larger ground wire between the connector socket and the chasis. Most trailer wiring, including factory wiring, is for lighting only and uses 16ga. or 18ga. wire, which is not large for the added load of a refrigerator or battery charging. Case in point, I check the trailer lights by turning on the running lights and the 4-way flashers at the same time. When I did this, I noticed the clearance lights would dim when the turn signal/brake lights came on. The lights no longer dimmed after adding a larger ground wire. John

"Not all who wander are lost."


I tested the voltage coming from the truck (at the connector) and it was 13.8v. I then disconnected the wires at the trailer battery. The current from the wires was 13.4v. Hello Bill,

So far, everything sounds good. What is the voltage at the refrigerator? Is the polarity correct at the refrigerator? ( negative to negative, positive to positive) You said you ran new wires directly from the battery to the refrigerator, did you put a circuit breaker in the (+) wire? If I remember, it was 10 ga. wire, a 20 amp breaker should work. I love challenges, keep me posted, something has got to work! John Hamilton

I have 10g wire from the battery to the fridge. I put a 15 amp fuse on the line. I will check the power going to the fridge. Interesting note (maybe), I had the battery charged at NTB on Tuesday. After the charge it registered 12.73v. Yesterday it was 12.43. This morning it was 12.3. Last nightI turned on 5 lights and the Fantastic. The reading was 11.8v. All of this was done with no connection to shore power. NTB said the battery was good. I'm not so sure. I bought one of those Vector Start-It things that Target has on sale just in case.

Hi Bill ... as I posted before ... a 24 hour test is a better test of a battery's ability to hold a charge. One way to evaluate battery health is to fully charge the battery and then disconnect it so that you know there is no way it can be charged by sneak loads. After a resting period of 24-hours, measure the voltage across the terminals with a good digital voltmeter. If the batteries aren't holding 12.6 Volts, (12.8 for gel cells), then they are in poor health I will disconnect it and check it after 24 hours to see what happens.

Since I have moved to the heat of Texas, I have never had a battery last through the warranty period. I now replace vehicle batteries after 3 years rather than risk a dead battery late at night, etc. Our Explorer battery failed after 2 years. If I have doubts about this battery I will replace it before we head out to the Grand Canyon. I've had enough trailer related problems already. I don't want to risk pulling over and not having power. Especially if we decide to dry camp. Bill, not familiar with the vector ... does it have a 12-volt torpedo plug-in spot? If so, make yourself a "connector" with a torpedo plug and two alligator clips which you can use as an "emergency" power source for the trailer by disconnecting the leads to the house battery and clipping on the alligator clips. Another was to do it is to make a "double" torpedo plug (making sure the polarity is correct), and then disconnect the house battery, plug one torpedo into the vector and the other into the 12-volt-plug on the rear deck. Have a great time at the Grand Canyon! I agree, batteries are cheap and I, too, replace before they fail. Charles,

The Vector has the DC torpedo outlet. I read your post in another thread about making a connection cord. I don't remember the thread but someone mentioned the Vector and you mentioned using the Prestone Jump It. I plan to build the cord this weekend. Where did you buy your torpedo plug? At $45 the Vector was hard to pass up. I guess I can also buy an inverter and use the combination of the Vector and an inverter to run some small power tools in the field. Bill

Walmart sells them in the auto department The Walmart connectors used to have wires attached, but replaced with 12 gauge wire. Didn't fuse my wires since I was attaching the alligator clips to the Casita house battery leads which lead to grounds and fuses ... and the torpedo plugs have fuses in them also. Use this set-up all the time to extend dry-camping ... and also for winter camping when the house battery's removed for safe winter storage. When our 2000 model was new we put the that without the truck running the battery dies in about 20 minutes (verified timing at Casita). Also learned that when running with head lights on AND fridge on DC the battery was slowly dying -- fix: turn off the lights. Finally learned that the fridge seals so well that all food stays cold a/o frozen for at least 12 hours driving or until we go to either AC or propane (never drive with propane on). We don't use DC any more for fridge. Additionally, our solar panel controller keeps us apprised of the battery's status (Seimens). Good point, Doug. Bill did say he was went dead. This would indicate the alternator does not have enough capacity to keep up. Some manufactures offer alternators with more output, and are sometimes standard with trailer towing packages. If the refrigerator is operated on propane while on the road, the unit should be turned off, including the pilot flame, when in a gas station. John Hamilton

Bill, Besides the ability for a battery to hold a charge over time, as Charles mentioned, it should be tested under a load. It is normal for the voltage of a battery to drop as load is applied, the greater the load, the lower the voltage. The usable capacity of a battery is how long it will last under load. If a battery drops voltage in a short time or doesn't last very long under use, it needs to be replaced. "Doesn't last very long..." is subjective. The wiring to your refrigerator sounds A-OK.I was going to suggest a 15 amp. fuse, not the circuit breaker. Already done. Now I am of the option that Doug S. has identified your problem, the alternator could not keep up. I would still suggest a larger size ground wire, if the one in your installation is small. Good Luck,

John H.

I can't tell the ground wire gauge for sure but it is the largest of the wires going to the connector. I would guess at least 12 gauge. The truck is an F250 Superduty with all the heavy duty tow stuff. I would think that it has about as powerful an alternator as you are going to get without moving into the medium duty trucks (F450/F550 and greater). It came pre-wired including charge and brake wires. Re: The fridge holding temp. while driving, my fridge has never been able to hold the temp when driving. It's not so bad at losing temp in the winter but the Texas summers are very hard on the fridge. Last week I lost 20 degrees in 3 hours. And that was not a particularly hot day. In the past I have had trouble running the fridge on propane while driving. The flame kept going out. Casita must have worked on it the last time they had the trailer because it seems to stay lit now. Also, it is the fridge that comes with the 17 foot trailer. I saw a battery at Pep Boys. An Energizer made by Johnson Controls. It has 120 amp hours and costs $59 with trade. One year full replacement warranty/24 months prorated. Any comments. They also had an Optima. It had 55 amp hours. The price was not listed but I understand they are expensive. I took the battery out of the trailer and checked voltage - 12.49v. I will check again in 24 hours. While battery was out, I connected the trailer to my truck to check power going to the battery. I had the following readings. With radio on in truck 13.83v With radio and lights on in truck 13.61v With radio, lights, AC and fan on max 13.43v Lights and AC were on while towing the other day with fridge on DC. The battery was dead in 3 hours. Bill, Ground wire size is ok.

Connect the trailer and check the voltage at the truck battery with the full load; lights, A.C., frige, etc. It should be above 13 volts. with the engine running. John H.

Bill, if it is the house battery you are buying a "Deep Cycle" type. A regular auto battery can't handle all the discharges.

Bob Steffen

Bill: If you're buying a battery, DEEP CYCLE for sure, might consider whether you want a type 24, 27, or 29. I think they'll all fit in the Casita, length differs and would require drilling a hole for the angle bracket. I think the 27 gives you about 50% more amp/hrs than the 24 and the 29 doubles a 24. All are at walmart for under $70. Not to recommend walmart batteries, but to be aware of the capacities according to the type, this holds true regardless of the brand of battery. Don in OKC