Battery Disconnect Question

[FuzzWantsABus]

Hey all! I am installing a master battery disconnect on my bus, and I'm a little stumped. The bus has two batteries wired in parallel, but it's a weird configuration. It has one positive lead plus a positive-to-positive connection, but it has two negative leads (one going to the negative of each battery) and a negative to negative connection. How do I know which negative lead to disrupt with the battery disconnect switch? Does it matter?


Thank you!

[BarnYardCamp]

I believe that the negative battery connections are connected to the bus chassis. So, your disconnect or battery isolator switch actually only disconnects the positive side....however...I would certainly defer to one of the more knowledgeable people on this board....paging Cadillac Kid.....

[CHEESE_WAGON]

12V systems are typically negative-ground, and it honestly is safer IMO to put disconnect switches on the ground side. Reason being that if connections somehow come loose, no harm done, as a ground is very unlikely to contact a 12V lead if it should come loose. I would put such a switch at least 3-4 feet away from known 12V lead sources, inside the cabin, if possible.

[Phatman]

Am I mistaken ? Isn’t a lot of the automotive switching done on the ground side ?

[CHEESE_WAGON]

It's safer to do so, yes, and commonly done in newer vehicles due to the reduction in the wiring gauge required in some cases, not to mention safer and more reliable for the 'brain boxes' used to control multiple circuits and entire systems. But typically older vehicles are exactly the opposite, in my experience, anyway.

[Phatman]

Go figure. Most of my vehicles are older. A lot older !!

[Phatman]

Battery isolators though operate the hot side +, right ?

[CHEESE_WAGON]

Originally Posted by Phatman

Battery isolators though operate the hot side +, right ?

My electrical knowledge is pretty basis, so I couldn't answer that one for sure.

[BamaBus]

Originally Posted by Phatman

Battery isolators though operate the hot side +, right ?

I would think so. The schematics for the BB Vision has the disconnect switch on the positive(+) side.

[Danjo]

Sounds like it’s going to be easier to install it on that one positive lead.

[FuzzWantsABus]

My understanding is that it's always preferable to do the disconnect on the ground side, though it can be done on the positive side, too. If there was only one ground lead in my setup, I'd just put it there and be done with it. Because there are two, that gives me pause. I see the following options:


1. Connect both ground leads to the same post on the disconnect, then connect the other post to one of the battery negative terminals. Keep the negative-to-negative connection between both batteries.


2. Put in a disconnect on each ground cable. This seems needlessly complicated.


3. Crawl around under the bus in the dirt and try to figure out where both of the ground cables go, and whether or not they both need to be grounded. I.e. Maybe one is for the right side of the bus and one is for the left. Or one is for all the old light/control wiring that I took out already and isn't needed anymore. This may answer the question of which ground lead to put the disconnect into, or it may not.


4. Just put the disconnect into the positive lead and be done with it. The issue this raises is that the wire I have made to go from the switch to whichever terminal on the battery is a smaller gauge than the existing wire, and I'm concerned that may cause issues on the positive lead side.


And this is why I have come to all of you very smart people to help tease this out!

[Danjo]

#4, but make a new cable of the correct gauge.

[FuzzWantsABus]

Because I am stubborn, I went for option 3. It appears that one of the ground cables goes directly to the frame. The other ground cable, along with the positive cable, both go to what I assume is the starter (see attached photos). If that's the case, it would seem that if I put the disconnect switch in the cable that goes to the frame, the disconnect would function as intended. Anybody see a flaw with my reasoning?

[Danjo]

Originally Posted by FuzzWantsABus

Because I am stubborn, I went for option 3. It appears that one of the ground cables goes directly to the frame. The other ground cable, along with the positive cable, both go to what I assume is the starter (see attached photos). If that's the case, it would seem that if I put the disconnect switch in the cable that goes to the frame, the disconnect would function as intended. Anybody see a flaw with my reasoning?

I don’t think Your starter is isolated. It will still be grounded. Disconnect the one that goes to the frame And see if everything still works.

[Danjo]

WAIT, that picture of the starter is hard to see on my phone, but aren’t the cables both on the positive post?

If they are then that’s not a negative cable.

Double check

[FuzzWantsABus]

The pics aren't great cuz I was upside down under the bus. The positive lead goes into the solenoid on the starter, and the negative lead goes to a post on the "front" (towards the front of the bus) of the starter itself.

[BarnYardCamp]

could be that one pos cable is power for the starter motor and the other pos cable is power for the solenoid...starter is attached to the engine which is grounded to the chassis via a flat metal, braided cable or something...
again, I would certainly defer to the many on this board more knowledgeable than me.

[BarnYardCamp]

can you see the post near the front of the starter? If it has a insulator around it, then it is most likely "hot" or from the pos side of the battery.

[flattracker]

If you use the Cole-Hersee battery switch you can disconnect either or both batteries. Crown didn't put them on all of their buses, but My "old Crown" had one in the engine compartment on the left side of the bus (driver's left) on the bulkhead for the air box. The Cole-Hersee switch can be found on eBay right now for $50 - $65. I plan to install one on the "new Crown".

[FuzzWantsABus]

I appreciate the replies, but let's not get too off target. I'm not sure how much more clearly I can say this, but I have physically traced the three leads that go into my battery box: 2 negative leads (which are attached one to each of the negative terminals on the two batteries, along with a lead that connects the two negative terminals together) and 1 positive lead (which is attached to the positive terminal of one battery, with a lead that connects the two positive terminals together-a parallel connection). One negative lead goes directly to the frame and is bolted to it. The other negative lead goes to the front (nose?) of the starter-due to the position of the starter, I can't actually see this connection, I can only feel it. The positive lead goes to the solenoid on the side of the starter.


I am trying to get advice on whether or not it will be effective to put the battery disconnect switch in the negative lead that goes to the frame. Here is a link to a switch that is essentially the same as the one I have.


https://bit.ly/3qc0Smt


Thank you.