Battery Disconnect Question

[BeNimble]

It sounds like you have one battery connected directly to the starter, the other one is the 'house' battery that powers everything else, but they are connected together.
The starter draws a lot of power, so having wires go directly to it is a good thing.
The starter negative connects via the engine to the frame.
The positive to the starter would only connect to the starter, so if you disconnect the other positive, it should work.

[FuzzWantsABus]

Originally Posted by BeNimble

It sounds like you have one battery connected directly to the starter, the other one is the 'house' battery that powers everything else, but they are connected together.
The starter draws a lot of power, so having wires go directly to it is a good thing.
The starter negative connects via the engine to the frame.
The positive to the starter would only connect to the starter, so if you disconnect the other positive, it should work.

This is all well and good, except, as I have stated previously, it is always preferred to put the disconnect on the ground lead. I believe (though am not certain) that this is because if you put the disconnect on the positive lead, there will still always be current in the lead from the positive battery terminal to the disconnect, and in some weird unforeseen circumstance, you could short something out or electrocute yourself in that section of the lead. If the ground lead is disconnected, then there is no charge in the system at all.



So, instead of everyone telling me to disconnect the positive lead, I'd love it if someone could help me understand if, how, and/or why not I can't/shouldn't put the disconnect in the ground lead that goes from one of my two batteries to the chassis?


I will also clarify that these are engine batteries only. They do not connect to anything "house" related.

[roach711]

My Ford 7.3L Powerstroke (essentially a T444E) has a ground cable running directly from the batteries to one of the mounting bolts of the starter. It also has a battery disconnect which is wired into the positive battery cable. Both were apparently installed by the bus body manufacturer.

[JackE]

If you want the disconnect to the negative side of the electrical system, you will need to disconnect both negative leads from both batteries. If you disconnect only one, there will still be connection from the second one going to the starter housing. The factory electrical system will have several ground loops connecting the frame to the engine to the body. Each negative cable goes to a different place to offer redundancy to the system. Contrary to logic, the negative side is the more important connection in the electrical system, that is why they designed it with multiple connecting points. You can rewire both negative cables to the same disconnect switch, but that deletes the redundancy of the intended system. The easier disconnect would be the positive since there is only one. I really don't think electrically speaking, there is a difference in which line is interrupted by the shutoff switch, aside from the main wire coming off the positive to the shutoff switch always being live.

[john61ct]

I would get a handle on your wiring, know exactly what's going on, and correct any illogical faults ASAP.

In the boating / cruising world, the standard is for a single positive and single negative on the House bank ( separate circuit from engine / Starter batt)

And the Master shutoff to be right there off the positive, just like the CP within 7" of the post.

Only truly safety / emergency loads you never want disconnected would bypass that switch, e.g. bilge pumps, stop the boat from sinking.

[Danjo]

Originally Posted by 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.

It would, except I think you have redundant paths to ground. Disconnect the ground to the chassis and see if anything works. I think you’ll find everything still works as it does now.

[Booyah45828]

I'm not following your fear of having the disconnect on the positive lead. Every one that I've installed went on the positive lead without trouble. That goes from race cars to class 8 semi trucks.

If I'm remembering from college right, computers switch the ground because it's after the load and will therefore the switch will then have a smaller inrush current. Also, if there is a short to ground in the circuit, the high current won't be going through the computer.

But, if you're adamant about it being on the negative lead, I do know they make double pole single throw battery disconnects. So put one negative lead on one side, and the other negative lead on the other, and you have what you desire. I don't know if they make those style disconnects with a high enough amperage rating for use on a bus though.

I'm of the same opinion that one of those cables goes to the starter, and the other likely goes to a lug on the frame. IMO, that's more complicated then one cable from battery to starter and a 2nd cable from starter to frame, but what do I know.

[roach711]

In AC wiring the hot side is switched. With the switch on the ground/neutral side and multiple ground paths there is still electrical power to a device so if someone was working on, say, an alternator close to the battery, there is still the very real possibility of a wrench shorting from the alt to the frame and causing some fireworks. I became educated on this issue when working on a furnace which had it's shutoff switch wired to break the neutral. I was pulling wires off the control unit while holding onto a water pipe when I became shockingly aware that the shutoff wasn't working.
Admittedly, on a vehicle there are way fewer chances to get shocked in this way but it seems to me to be better to switch the hot lead.

[Rwnielsen]

I don't think that you'll want to interrupt the positive to the starter. You do want to isolate the chassis loads. In some applications the positive goes to the starter directly where it is split off back to a vehicle fuse panel. If you interrupt the chassis feed there will be no power in the bus, at all, coming from the battery. If you interrupt the starter lead you'll need a massive isolator switch to handle the startup load or it will overheat and meltdown. If this is how your bus is wired that would also be where the battery charge comes from, the chassis power wire.
Just an idea/opinion worth exactly what you payed for it

[Rucker]

Originally Posted by 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!

I re-read your original question and this post with some more detail. Sounds like the problem you are trying to solve is how to make use of the already-installed smaller-gauge wire used with the switch you are installing. The ground cable should be sized the same as the positive side, so I don't think you'll be able to solve the problem by putting the switch on the ground cable, if your switch cable is lighter.

I did a little research and learned there is marginally more value in putting the disconnect on the negative cable.

And by the way it is not recommended to use a disconnect as a kill switch when the engine is running. If you intend to have this switch double as a kill switch, there is a proper way to set it up. You can find that information by a google search.

[Samweld]

The switch that you linked to is rated for 1250 amps for 10sec. So as long as your starter isn't asking for any more than 1250 amps when starting than you can put the battery Isolator switch on the positive lead. The negative lead going to the starter is to create a shorter (read: low resistance) path to the starter. But there's a better than good chance that even if you put the switch on the ground lead going to the chassis that there is still going to be another path to the neg side of the battery. If your starter is asking for more than 1250 amps while starting, than your new switch will get very hot because it is the bottle neck. Make sure all your ring terminal connections are good and have plenty of surface area. As an electrician that's my 2 cents. Hope that helps.

[Elliot Naess]

Originally Posted by FuzzWantsABus

... ... 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. ... ....

My guess is... if you put a temporary eyeball on the tip of that finger, you will find that this cable connects to ground. Absent a rental eyeball, you could break a rear view mirror off somebody's Rolls Royce and use it as a dental mirror.

Running a separate ground to a starter mounting bolt (and another to the vehicle frame) is actually very sound practice.

The classic example of why... is The Famous Glowing Throttle Cable. This happens when a shade-tree mechanic replaces the engine and leaves off the ground strap between the engine and the frame. Then he cranks the engine and wonders why the throttle cable or linkage glows red hot.

[BusWanderer83]

You can use the frame as a bus bar, disconnect all the negative leads from your batteries and reconnect to the frame. Install a large enough diameter cable with the disco in the middle from the battery to the ground. Check the NEC on grounding and bonding. Should provide guidance on the "large enough diameter cable" portion.

[Bus ted1]

Wow, this has become complicated with all the 'suggestions'. Just put it on the Pos. side, do a good job and be done with it. Likely the 2 neg. cables go to the engine and one to the chassis. Switching just one will do nothing as the other will back feed. Most (diesel) trucks have a negative to the engine or starter because of the current draw. It is not wanted through the chassis which does not provide a good enough ground. The other goes to the chassis to prevent a back feed through some accessory connection on the engine which in many cases is isolated with rubber vibration mounts. Some have a jumper cable to the chassis acroos the mounts from engine to chassis. Just put it on the hot side and be done with it.

[Rwnielsen]

^^^^^this right here
Make sure its of adequate size, along with the cables, to handle the startup load. Move on to something else.

[john61ct]

Originally Posted by BusWanderer83

You can use the frame as a bus bar, disconnect all the negative leads from your batteries and reconnect to the frame. Install a large enough diameter cable with the disco in the middle from the battery to the ground. Check the NEC on grounding and bonding. Should provide guidance on the "large enough diameter cable" portion.

Terrible advice

[john61ct]

Originally Posted by Bus ted1

Just put it on the Pos. side, do a good job and be done with it.

Good advice

[rubydoooooo]

If you just put the disconnect on the negative lead going to the frame, it’ll not work as you’d want. Things will still be grounded from frame to motor to starter back to the batteries. If you’re set on doing the negative side might need to do two switches. :/
If it was me I’d buy the correct sized wire/switch and do it on the positive side. For simplicity
Just my thoughts.

[FuzzWantsABus]

Thanks for all the responses, everyone. Who knew (I should have) it could be so complicated?



At this point, I don't have access to the proper gauge wire to put the switch in the positive side. After reading all of this, at this point, the easiest thing is to leave the switch out for now. I don't have any parasitic drain currently-it was mostly to make any further changes to the bus (not house) wiring simpler.



I will tackle this again in the future, if I get my hands on the right stuff.


Again, thanks for all the response and insight.

[Squigley]

As others have said previously, the "extra" ground wire running from the battery to the starter is just a direct ground, rather than trying to use the chassis.

It's also going to be more reliable in case the engine somehow gets electrically isolated from the chassis.

I have seen this commonly done when installing large amplifiers/subwoofers in cars. While the amp only "needs" a positive heavy gauge wire, and can be hooked to the chassis for the ground return, it usually has a dedicated ground wire run in parallel with the positive wire, and is used instead of the chassis.

There are probably audiophile reasons for doing that as well, but I consider most of that to be woo.

And you can put your disconnect on either side of the battery, it really doesn't matter. disconnected is disconnected. Ideally you would have it as close to the battery(s) as practical, to remove the possibility of a short occurring between the battery(s) and the disconnect.