12V Draw While Driving? Request Peer Review.

[StrongJava]

Hello!

In my 2008 Bighorn 3670RL, I've got a potential wiring issue with the 12V system. I seem to have a parasitic draw on my house battery even during short drives. I conclude this because the indicator panel shows F even after a few hours and the only system intentionally turned on while driving is the refrigerator, but I don't have indications that is the problem. Here is what I've tested and know:

- Pulled the battery and had it tested professionally. It passes good.
- The convertor has recently been replaced and all the systems seem to work fine when on shore power. I believe the parasitic draw predates the convertor failure in November.
- Following suggestion of repair guy at RV place, I disconnected shore power and turned off all systems in the RV. With the negative terminal connected the battery, I touched the positive terminal and received a spark, thus indicating there was still a load drawing on the battery.
- Pulled all the fuses and repeated the battery terminal test. Two of the fuse fault lights lit up (#6 and #8, labeled "Fans/Bathroom" and "Rear/AC" respectively). If I'm correct, this indicates some systems are still trying to draw 12V power on those circuits. For #8, I believe this is because the LP detector is hard wired (which I suppose makes sense). And I suspect the "AC" part of that label refers to the voltage draw from the thermostat which was turned off for the test.
- The two fans powered by circuit #6 are the bathroom fan and a fan above the kitchen controlled by a Honeywell thermostat. This fan has another fuse in it. When this fuse is removed, and the battery terminal test is repeated, the fuse fault light for this circuit DOES NOT light up.

Assuming my testing procedures are sound, my analysis leads me to believe that something wrong with the Honeywell thermostat or the fan it controls. Does this analysis make sense or am I misunderstanding something?

If the analysis makes sense, does someone know of a way to further test this system to determine where the fault truly lies?

Many thanks in advance!

Tim

 

[danemayer]

Hi Tim,

I'd suggest disconnecting the wires from the kitchen fan's thermostat and repeating the test. There should be no power to the fan and the fuse box LED should not light up. If so, the next step would be to check continuity on the thermostat terminals with the thermostat set OFF. Should be open. With the thermostat temp set higher than ambient, you should have continuity.

I'm also wondering if the kitchen fan works normally when you push the thermostat to a high temp setting.

 

[StrongJava]

Hi Dan,

Thanks for the quick reply. Removing the Honeywell thermostat did not result in the fault light turning off when the fuse is removed.

There is not continuity on the thermostat when removed from the circuit.
There appears to be continuity across the thermostat leads when in the circuit but with the thermostat selector set to off, but it might be intermittent. I need to test and research this more to make sure I'm reading the indications correctly.
The fan does generally seem to operate correctly when I turn it on.

With the thermostat connected and in the off position, -5.24V is read across the terminals. When the fan is turned on the voltage drops (back to ground?). Can I infer from this that the thermostat is fine, and that the problem is in the fan?

Tim

 

[StrongJava]

Perhaps a little more information regarding the continuity test when the thermostat is wired into the circuit. I am using an old Fluke 77/AN and, I understand the continuity tester works in a limited resistance range.

I'll also add the fan unit itself has a switch on it that controls the fan and not the cover. With that off, the fuse panel fault light still comes on when the fuse is removed. When the fuse (4A/250V) is removed from the fan unit, the fault light does not illuminate.

Does this further indicate the problem is with the circuitry for the automatic vent cover?

Tim

 

[JanAndBill]

FYI while recently trying to diagnose a furnace power problem I discovered that even with a blown fuse I was showing 11 volts on the back side of the panel with a digital meter, but when I put a load on it the voltage went away. You sure you're not reading phantom voltages?

 

[StrongJava]

Ok! I have, I believe, uncovered more of the story. The Fantastic fan (not sure of model #) has a manual or automatic mode. Even when the handle is pulled out for manual, and the thermostat is off, pulling the fuse lights the fault LED. Putting the fuse back in caused the cover motor to run even in manual mode. The voltage across those motor leads is 9V (shouldn't it be 12?).

Tim

 

[danemayer]

I'll also add the fan unit itself has a switch on it that controls the fan and not the cover. With that off, the fuse panel fault light still comes on when the fuse is removed. When the fuse (4A/250V) is removed from the fan unit, the fault light does not illuminate.

Does this further indicate the problem is with the circuitry for the automatic vent cover?

Tim

I'd be looking at the vent cover. Also, there's probably a control board in the fan that operates both the fan and the cover.

If the thermostat works well enough to turn the fan on, it's probably ok.

- - - Updated - - -

You might want to call Fantastic. I think their # is 800-521-0298. From past posts, they are supposed to have really good customer service. If you ask them what might account for a continuous current drain, even when the fan is turned off, they might know the answer and might have a fix for you.

 

[pegmikef]

Don't those thermostats draw a small amount of electricity or is that only when the unit is on?

 

[danemayer]

Don't those thermostats draw a small amount of electricity or is that only when the unit is on?

Mike, I would think the circuit board on a digital thermostat might draw a very small amount of current all the time it's on, and might use a little even when off, if only to monitor for button pushes.

On the other hand, the typical old-style analog thermostat is just a switch, usually operated by a temperature sensitive glass vial filled with mercury, or by a temperature sensitive coil spring. If ambient temperature is below the set point, no power flows and none is used.

I'd expect the older Fantastic Fan thermostats to be analog. But as control units get fancier, they may have put out some digital models.

 

[StrongJava]

Thanks again to those who have chimed in.

The thermostat I have is indeed analog.

As I've described previously, the only 12V circuits I was able to determine to include devices wanting juice were 6 & 8, and since the fan cover seems to be the culprit I'm focusing there. Admittedly, my logic may be flawed because the cover motor may be designed to always have a small draw. If this it the case, my problem is elsewhere and I'm clueless. I'm going to call Fantastic on Monday to see if they'll care to chime in. The tech at Camping World said these fans have a lifetime warranty, so that would be nice if it is in fact the problem.

One test I'll probably do today when we head out for a few hours is to kill shore power and turn off everything except the fridge and the furnace (we're up near Seattle right now and have a dog). While that won't be as good of a test as when driving, if I come home and the battery is low, I can be reasonable confident that something else is going on.

I realize the other part I have to be careful about is that I'm relying on the information panel in the coach, which may be flawed. Specifically, when I am on shore power, the battery indicator shows C/Green, and after driving for a few hours, the indicator usually shows F/Red, but sometimes G/Yellow. So, maybe I need to get a better battery meter before I tear the fan completely out

 

[danemayer]

You can read the voltage across the battery terminals to get a more precise readout of battery charge. When on shore power, it's usually going to be around 13.4 V. To get an accurate readout without shore power, I think you need to have shore power disconnected for 15-20 minutes.

 

[Bob&Patty]

Disconnect the negative battery cable. Put your Fluke connectors between the battery post and the cable. Set it the mil-amps and see what the reading is. Then pull fuses until the reading drops to near zero. Quick and easy. Sometimes it takes a few minutes for some things to power down to get a correct reading.

 

[StrongJava]

Hi Bob,

At risk of being dense, won't this simply tell me what I already know: that circuits 6 and 8 have equipment drawing power continuously? Or do you think my approach of using the fault lights at the fuse panel is flawed?

Thanks,

Tim

 

[Roller4tan]

I may be missing something, but, while travelling my truck has a 12v lead that maintains my battery on the trailer while driving. My battery voltage level stays up during those times. When I first got the truck with the traillering package I needed to insert a fuse under the hood to get voltage to that 12v lead. I run with the fridge running on 12v/propane.

 

[danemayer]

That brings to mind the possibility that something not going through the fuse box is draining the batteries. If there's a problem with the breakaway switch, you could be powering the trailer brakes - yes you'd think you'd notice if brakes were on, but if they're screwed up, maybe not. If you have a generator, the starter power doesn't go through the fuse box either. Same thing for hydraulic pump, rear stabilizers and other things getting powered from the buss bar near the batteries.

 

[StrongJava]

Excellent discussion.

Roller4tan, I will check that when I get my truck back from the body shop. However, I think the issue is more than that because the drain seems to be too fast even if the trailer wasn't getting a charge from the truck.

Dan, I was not aware of the buss bar reflecting circuits that bypass the fuse panel, but it makes sense for some of it. I'm confident the breakaway isn't messing with the brakes as I've recently redone those and they seem good. However, the switch for the landing gear sticks: I can center it so the motor turns off, but perhaps that is still pulling juice. Is that 12 volt or is there an inverter involved? It seems 6-8 hours is quite quick to drain a battery.

I'll be back at the rig tonight around 8p, at which point it will have been sitting with just the furnace and the fridge turned on for 9 hours. When I left the battery voltage was 12.91V.

 

[danemayer]

Landing gear is 12V DC, regardless of whether the motor is electric or hydraulic. All the high current 12V devices get battery power by way of the buss bar/circuit breakers.

 

[StrongJava]

After 9 1/2 hours, with only the fridge and the heat on, the reading across the terminals dropped to 12.04V. So, it appears I don't have the issue figured out.

But, I don't have a good baseline either. The battery installed is an Interstate SRM-27, which according to the specs should have about 17-18 hours of life at 5A. I'm not positive about the furnace's draw, but I believe the fridge draws ~2.7A. My research for the furnace fan makes me think ~11A when running. If I assume a 50% loading (we had it set at 60 degrees and it was about 40 outside), then perhaps I should have drawn an average of 8A per hour? That makes me think I should have had several more hours of battery life.

Any baseline thoughts?

TIA,

Tim

 

[danemayer]

After 9 1/2 hours, with only the fridge and the heat on, the reading across the terminals dropped to 12.04V. So, it appears I don't have the issue figured out.

But, I don't have a good baseline either. The battery installed is an Interstate SRM-27, which according to the specs should have about 17-18 hours of life at 5A. I'm not positive about the furnace's draw, but I believe the fridge draws ~2.7A. My research for the furnace fan makes me think ~11A when running. If I assume a 50% loading (we had it set at 60 degrees and it was about 40 outside), then perhaps I should have drawn an average of 8A per hour? That makes me think I should have had several more hours of battery life.

Any baseline thoughts?

TIA,

Tim

Without trying to do the math, I would say that running the refrigerator and the furnace for 9 1/2 hours would be enough to bring the battery down quite a bit. Some people get more. But unless your battery fairly new, this might not be unusual.

This is a bit different from the original problem description - battery draining while towing. As noted, the tow vehicle should be providing some power, and that should be enough to run the refrigerator. And no furnace reduces the drain significantly.

 

[JanAndBill]

A fully charged automobile battery "at rest" will only show around 12.7 volts. Apply a load without benefit of a charger hooked up and the voltage will drop. The amount of the drop depends on the load and the condition of the battery. Discharging a battery over a period of time will also drop the voltage dependent on the percentage of discharge. I guess I fail to see your problem.