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Old 08-16-2017, 07:54 PM   #1
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Battery Disconnect - Not on the Negative Side

Motorhomes always can supply surprises.

When I upgraded my batteries on my ACE 30.1, I also installed a battery disconnect switch to provide positive power shut off... or so I thought.

I installed the switch on the battery negative, just like I did on my old travel trailer. The switch on a wall in the battery compartment, so the original negative battery cable was rerouted to the switch and I installed a short battery cable jumper.

This week I turned the disconnect switch off before putting the Use/Store switch in the Store position. But when went put it in Store, I realized there were some lights still on. Funny... these should have gone off when I turned off the battery disconnect. What wiring error did Thor make?

Then after I bit of thought - probably no wiring error. I presume that the coach 12V negative gets tied to the frame, and the negative cable for the battery is also connected to the frame (haven't tried to trace this yet). The chassis battery is also tied to the frame, and its positive can be paralleled to the house 12V positive through the emergency switch. But the important thing is that the house and chassis batteries share the same ground.

So with the disconnect switch on the negative, all I'm doing is routing return current to the chassis battery. Not sure what the impact might be if current ran from the house positive to chassis negative for a long time, other than discharge the house battery. For me its been a non-issue as I always go to the Store position when I leave the motorhome (except for when I forget)

So it appears in this case if I want positive shut off of power, I need to put the disconnect switch in the +12V line and not the negative return. Having the chassis battery sharing a ground with the coach batteries results in a negative disconnect not working.

Has anyone run into this "feature"? Or did Thor actually make a wiring error?
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Old 08-16-2017, 08:58 PM   #2
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No, you have an electrical logic error. Current has to flow through a battery just like it flows through a load. The chassis battery supplies your lights....period. By installing the disconnect on the house negative you have isolated at least one house battery from the circuit. But check to make sure there isn't another chassis ground connected to the other house battery. And you should put the USE/STORE switch in STORE before flipping the disconnect.
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Old 08-17-2017, 04:26 AM   #3
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I had a lawn chair bump the light in the storage compartment which made the coach battery go dead after sitting for 3 days with the unit in store mode. Had to jump the coach battery in order to start the generator. Truck battery worked fine.
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Old 08-17-2017, 11:49 AM   #4
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Quote:
Originally Posted by glamperjan View Post
I had a lawn chair bump the light in the storage compartment which made the coach battery go dead after sitting for 3 days with the unit in store mode. Had to jump the coach battery in order to start the generator. Truck battery worked fine.
????
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Old 08-17-2017, 12:40 PM   #5
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Hmm the storage lights should have been disconnected with the unit in "Store".
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Old 08-17-2017, 01:56 PM   #6
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Had the same problem as Jamie. Tried the disconnect switch, and still had current flowing. Switched the switch to the other battery, and had the same thing, lights stayed on. I did notice that both batteries have positive and negative leads coming from them, even though they're hooked up in parallel.

Does that sound right? If they're in parallel, shouldn't the positives come off one battery, and the negatives off the other?

I got another disconnect switch, and put one on each negative terminal. That worked, all power shut off.

Should really add one to the chassis battery, too, but that's a later project.
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Old 08-17-2017, 03:19 PM   #7
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I have two 6 volt batteries connected in series so there is only one 12v negative wire connected to the batteries. The disconnect switch is in this wiring. So when open, there should be no ground connection, and no current flow. As I think more about this, I need to do more troubleshooting. I haven't checked the emergency switch so I may have the chassis and house batteries unintentionally connected. I really hate chasing ground problems.

When I was in college, the Chemical Engineering dept had a plaque that said

A Chemical Engineer knows

Enough Chemistry to confuse a mathematician
Enough Mathematics to confuse his boss
And enough Electricity to confuse himself.

At the moment I'm confused. To quote an old machinery engineer - we had an electrical problem. Somebody let the smoke out.
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Old 08-17-2017, 04:15 PM   #8
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Quote:
Originally Posted by DenverTransplant View Post
I have two 6 volt batteries connected in series so there is only one 12v negative wire connected to the batteries. The disconnect switch is in this wiring. So when open, there should be no ground connection, and no current flow. As I think more about this, I need to do more troubleshooting. I haven't checked the emergency switch so I may have the chassis and house batteries unintentionally connected. I really hate chasing ground problems.

When I was in college, the Chemical Engineering dept had a plaque that said

A Chemical Engineer knows

Enough Chemistry to confuse a mathematician
Enough Mathematics to confuse his boss
And enough Electricity to confuse himself.

At the moment I'm confused. To quote an old machinery engineer - we had an electrical problem. Somebody let the smoke out.
A battery disconnect switch should always be placed between the battery positive post and the load(s). This requires lifting all positive wires from the battery post. For us there are 3. They are coach loads, jumper to second house battery, and wire to the cranking battery.
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Old 08-17-2017, 07:33 PM   #9
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Quote:
Originally Posted by Hikes in Rain View Post
I did notice that both batteries have positive and negative leads coming from them, even though they're hooked up in parallel.

Does that sound right? If they're in parallel, shouldn't the positives come off one battery, and the negatives off the other?
When wired in parallel all Positive terminals are at the same point electrically and all Negative terminals are at the same point electrically. It doesn't matter electrically if the leads come off one battery, the other battery, or both batteries. That's why I said in post #2 in this thread "But check to make sure there isn't another chassis ground connected to the other house battery." Since the OP only has two 6 volt batteries connected in series, that is not his case.
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Old 08-17-2017, 07:34 PM   #10
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Originally Posted by scrubjaysnest View Post
A battery disconnect switch should always be placed between the battery positive post and the load(s). This requires lifting all positive wires from the battery post. For us there are 3. They are coach loads, jumper to second house battery, and wire to the cranking battery.
Electrically, it makes no difference, but for safety reasons you are incorrect. The disconnect should always be on the negative side.
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Old 08-17-2017, 07:36 PM   #11
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Good quote!
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Old 08-17-2017, 08:17 PM   #12
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Quote:
Originally Posted by DenverTransplant View Post
Motorhomes always can supply surprises.

When I upgraded my batteries on my ACE 30.1, I also installed a battery disconnect switch to provide positive power shut off... or so I thought.

......cut.....
One possibility may be that your switch is not opening the circuit completely for some unknown reason, and is allowing enough current to pass to power the lights you saw.

I would rule that out first. I'd replicate the condition you described above in initial post and then I'd disconnect the battery ground cable right at the battery terminal. If lights go off, your switch isn't working as intended.

If interior lights stay on even with house battery "physically" disconnected at battery post, then I'd start to see how chassis battery is powering the lights.

And for initial simplicity, I'd make sure shore power is not connected and generator is not running. Assuming you don't have solar, that would leave the chassis battery as only source of electricity.
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Old 08-18-2017, 01:03 AM   #13
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Pulled the switch and tested it - it opens the circuit and shows open on the ohm meter. Haven't tested pulling the battery lead yet. Still need to check the emergency switch next time I visit it. Not on shore power or generator or engine running
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Old 08-19-2017, 09:21 PM   #14
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Went over to the coach today and got to the bottom of the problem. Although I've never touched the emergency switch (at least not intentionally until today), it looks like the house and chassis batteries have been paralleled for who knows how long.

First thing I did was pull the negative cable off the batteries with the coach ceiling lights on - they stayed on. (engine/generator off, no shore power)

Second step was to press the Emergency switch - I got a sound that sounded more like an electric actuator than a solenoid, but the ceiling lights went out.

Reconnected the battery negative and the ceiling lights came on.

I also noticed that the studs of my disconnect switch looked uncomfortably close to the metal battery compartment sidewall. I cut a square of some 1/16" rubber sheet I had laying around and put it between the battery compartment wall and the switch. So no longer a chance of shorting the switch by the battery compartment wall.

With the disconnect switch re-intalled in the negative path, it appears that everything now works. It bothers me a bit that the emergency switch can be activated without knowing it. From what I can tell its not a two position switch - its a set/reset button. Push it once, the batteries are paralleled. Push it again, and they are separated. All with no indication of the current state.

This isn't what I could call a quality mammal/machine interface. Its plenty easy to just hit the switch accidentally, which is what may have happened while leaning across the dash to get the windshield curtains open and closed.In my past life, I've run engineers off of projects for coming up with these kinds of designs.

I'm going to call Thor this week and see what they say about this behavior and see if I can get a drawing of the DC supply system
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Old 08-20-2017, 01:05 AM   #15
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I was under the impression the Emergency Start switch was as a switch and hold spring return off. Does it have an indicator (such as a LED) when it is engaged?

I've never had to use mine; guess I'll check that in the morning.
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Old 08-20-2017, 01:24 AM   #16
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Quote:
Originally Posted by Tfryman View Post
I was under the impression the Emergency Start switch was as a switch and hold spring return off. Does it have an indicator (such as a LED) when it is engaged?

I've never had to use mine; guess I'll check that in the morning.
What passes for a manual for the ACE says that the switch is momentary. You are supposed to hold it down to connect the starter to the house battery for no more than 30 seconds. That's not the behavior I saw. Now I'm wondering if I have a flakey BIRD. Thor has many questions to answer. Mine has no LED indication
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Old 08-20-2017, 04:19 PM   #17
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So I checked my 2016 ACE this morning; the emergency start switch is indeed spring return off with no indicator (which makes sense for a spring return off switch) but with the MH on shore power nothing happens when I engage the switch. This may be normal if the switch engages the same relay that links the chassis and house batteries together for charging. I'll have to do some more testing when I have more time. I don't want to discover it isn't working right when I need it most.
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Old 08-20-2017, 06:22 PM   #18
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Originally Posted by Tfryman View Post
So I checked my 2016 ACE this morning; the emergency start switch is indeed spring return off with no indicator (which makes sense for a spring return off switch) but with the MH on shore power nothing happens when I engage the switch. This may be normal if the switch engages the same relay that links the chassis and house batteries together for charging. I'll have to do some more testing when I have more time. I don't want to discover it isn't working right when I need it most.
That is what I would expect... It should be 'momentary on' - only on when pressed - and should be using the same relay - so if the batteries already tied together based on being on shore power, genny running, or engine running - it would do nothing.
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