Understanding 12 V - Montana Owners Club

whutfles · 11-06-2020, 07:53 PM

I thought this was pretty simple - My Montana has circuits powered by 120V and stuff powered by 12V. And the batteries can be charged by my truck or by 120V.

So I decided to install a battery disconnect switch on my negative battery cable. After it was installed, I turned the switch off. Then I turned on the 12V light in my garage and the light came on. Oh ****, something must be screwed up.

Now all this while, the RV is plugged into a 120V shore power. So with the negative battery cable turned off and the garage light on, I disconnect the 120V shore power. The garage light goes off. Then I turn on my battery negative cable and the garage light comes back on.

So, if I take the RV batteries completely out and plug into shore power, my 120V as-well-as my 12V circuits all work? My converter is plugged into the back of my fuse pannel for the source of it's power. The other 2 cables run from my converter to a connection on the wall behind my batteries. So the converter powers the 12V circuits as-well-as the batteries?

I've always read on here that turning off the battery disconnect switch in the convenience center didn't turn off everything 12V and if you wanted to turn off everything, you needed to disconnect the negative battery cable or install a disconnect switch on that cable. Well, that newly installed negative ground disconnect switch isn't doing anything if you are plugged into shore power other than maybe you are not charging the batteries. I guess I'm surprised as I thought the load for the 12V circuits was comming from the batteries when in fact it is comming from the converter if connected to 120V and if not, then from the batteries.

triplebvalp · 11-06-2020, 08:07 PM

Quote:

Originally Posted by whutfles

I've always read on here that turning off the battery disconnect switch in the convenience center didn't turn off everything 12V and if you wanted to turn off everything, you needed to disconnect the negative battery cable or install a disconnect switch on that cable. Well, that newly installed negative ground disconnect switch isn't doing anything if you are plugged into shore power other than maybe you are not charging the batteries.

The disconnect switch you installed disconnects the battery from the RV. The converter cannot charge the battery when the disconnect switch is off. The converter does indeed supply 12vdc power to the RV. The battery is needed to provide enough amperage for things like your slides and auto-levelers.

CarWin · 11-06-2020, 08:08 PM

As you stated....when connected to shore power, all 12v items will still work with the battery shut off switch "turned off(key removed)". When you disconnect from shore power and key is removed, you should have no 12v operation. The same is true for the shut off switch that you installed on the negative side of the battery cable.

triplebvalp · 11-06-2020, 08:13 PM

Quote:

Originally Posted by CarWin

When you disconnect from shore power and key is removed, you should have no 12v operation.

Not entirely accurate if you're referring to the on-board battery disconnect.

The OP is talking about a battery disconnect that he installed.

dieselguy · 11-06-2020, 08:45 PM

Everything seems to be working as designed as I see it. The converter will supply 12 VDC while you are hooked to shore power whether you have a battery connected or not. All the battery disconnect at the battery does is totally isolate the battery from any parasitic drain over and above what the 12 VDC disconnect does in your convenience center. You need the batteries connected under normal use as they act as a "reservoir" of power so to say. The converter charges the batteries as well as supply a low amperage voltage supply into your fiver. The hydraulics and electric slides draw such a high amperage amount that you need the battery(s) to take that enormous load off the converter. All is well as I see it.

whutfles · 11-06-2020, 10:57 PM

So, with shore power disconnected, if I turn off the negative ground disconnect switch I installed on the battery, everything should be off right? There should be no need to turn off the disconnect key in the convenience center is there?

jeffba · 11-07-2020, 05:48 AM

Quote:

Originally Posted by whutfles

So, with shore power disconnected, if I turn off the negative ground disconnect switch I installed on the battery, everything should be off right? There should be no need to turn off the disconnect key in the convenience center is there?

Correct. I refer to the "disconnect Key" as the "converter isolator" It isolates the battery from the converter.

I use a disconnect at the negative battery terminal and never turn the key

RMcNeal · 11-07-2020, 05:58 AM

That is correct. With shore power disconnected, your only source of power is the battery. If you disconnect it at the negative post (not the red key), the coach has no power source and nothing will run. Also, nothing can drain your battery which is the main reason for the disconnect on the negative terminal. Without this disconnect on the battery, a few things in the coach like the propane and CO detectors will still pull power and eventually drain your battery while in storage, even if the red key disconnect in the convenience center is disconnected and removed.
If you have shore power connected, keeping your battery connected (negative terminal NOT disconnected), the converter in the coach will keep your battery charged AND supply power to any 12 volt electrical items, such as lights, in the coach.
Since I installed a negative battery terminal disconnect switch on mine, I have never turned off or removed the red key. Really no reason to turn it off whether hooked to shore power or not. It has become unnecessary. I only disconnect the battery when in storage and NOT on shore power AND NOT connected to the truck now. With the negative terminal on the battery disconnected and not on shore power, nothing 12 volt will operate. Also, when towing and hooked to truck, the battery won't be charging either. So make sure and connect the battery when towing. If you have a residential fridge, your battery will supply power to the fridge when not hooked to shore power. This is limited to your battery capacity, however, and will not last indefinitely even with TV hooked up and running. The charging circuit cannot supply enough power to keep battery charge and run fridge indefinitely, at least not on my rig.

whutfles · 11-07-2020, 07:51 AM

Thank you all for your replies. Very clear. This little $16.00 switch gave me a whole lot more in understanding how my 12V system works than just how to install it. Well worth the exercise.

DebNJim B · 11-16-2020, 06:47 AM

Your initial statement was correct, should be pretty simple. And it is. Two systems, 12v, 120v, both pretty basic. The confusing part usually comes where the two systems interact together.
When I first got my Monty I was having trouble getting the res/fridg to work off the battery. It was fine on shore power but when I unplugged...no juice. Service tech made a house call to replace a cabinet door that had been damaged in shipment and I asked about the fridge. Turned out there is a second disconnect from the battery to the inverter (it's the left one on the firewall in the picture, the right one is the neg disconnect) and as it turns out also blocks the converter from charging the battery. That had been left off when I picked up the rig and the battery had dropped to 11v. At that point the inverter shuts down. So we had to turn that switch on and wait for the battery to recharge and then all worked just right.
Like I said it's pretty simple but if you follow the wiring around a bit it seems to become a never ending learning experience.
BTW, the negative disconnect switch is the only one I use now.

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