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Old 11-09-2023, 06:05 PM   #1
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Battery Disconnect Question.

So I think I answered my own question but would like some confirmation.

So my Bus I found has 2 ground cables coming off of the battery system from a single point. One goes out straight to the chassis. The other to the starter.

When I add the Cole Hersee 2000 AMP disconnect to the mix, I'm told we shouldn't add it to the positive line. But I have two Negative lines. Do I disconnect the two ground lines at the battery terminal point, and hook it to the exiting point of the disconnect, and then add a short spare line from battery to the disconnect so that both ground lines are disconnected? This sounds like the correct method to me.


See my current and final planned setup pics below and please confirm if this looks like the correct way to do this.
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batterysetupCurrent.png   batterysetupFinal.png  

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Old 11-09-2023, 06:18 PM   #2
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Originally Posted by nikitis View Post
So I think I answered my own question but would like some confirmation.

So my Bus I found has 2 ground cables coming off of the battery system from a single point. One goes out straight to the chassis. The other to the starter.

When I add the Cole Hersee 2000 AMP disconnect to the mix, I'm told we shouldn't add it to the positive line. But I have two Negative lines. Do I disconnect the two ground lines at the battery terminal point, and hook it to the exiting point of the disconnect, and then add a short spare line from battery to the disconnect so that both ground lines are disconnected? This sounds like the correct method to me.


See my current and final planned setup pics below and please confirm if this looks like the correct way to do this.
I donít know why you were told not to put the switch on the positive cable. Another member had a similar question. He had uploaded a wiring schematic for his bus and in the drawing the cutoff switch is on the positive.

Read this thread for more info.

https://www.skoolie.net/forums/f39/2...off-42575.html
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Old 11-09-2023, 06:31 PM   #3
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I don’t know why you were told not to put the switch on the positive cable. Another member had a similar question. He had uploaded a wiring schematic for his bus and in the drawing the cutoff switch is on the positive.

Read this thread for more info.

https://www.skoolie.net/forums/f39/2...off-42575.html

I've read this. My mechanic friend told me gravely do not add to positive line. He's seen some fires personally when on the positive side. I've also heard this from other mechanics as well. Never explained to me how it happened or why. Personally if it is true, I think the reason being is as you disconnect a live line, if current is going through it, being grounded to the chassis helps dissipate the short better as it's disconnecting, since the chassis is a larger surface area to absorb it, rather than the batteries taking the hit. I don't know if there's any truth to it, but those are my thoughts.


But anyway, is this an acceptable way to hook it up as in my "Final setup" picture? The other obstacle to using positive is that prior people have hooked up a bunch of additional wires to my positive cable which bypass the main positive cable, and they will still drain the batteries which defeats the purpose of why I'm adding the disconnect in the first place.
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Old 11-09-2023, 07:37 PM   #4
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My bus's Cole-Hersee M-705 (the predecessor to the current M-750) controls the start batteries' positive, not the negative, and that's the way Crown did it on all their buses. MCI's blade switch for their start batteries was wired similarly.

One thing that may be confusing you is the placement of catastrophe fuses or breakers for large battery banks such as the house batteries. These are typically placed on the negative from each bank, as explained by Sean Walsh on the BCM forum some years ago. I have a pair of 300A Class T catastrophe fuses, one for each of my house banks, and they are on the negatives. Simple load switches or breakers should however still be on the positives.

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Old 11-09-2023, 07:43 PM   #5
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Originally Posted by nikitis View Post
I've read this. My mechanic friend told me gravely do not add to positive line. He's seen some fires personally when on the positive side. I've also heard this from other mechanics as well. Never explained to me how it happened or why. Personally if it is true, I think the reason being is as you disconnect a live line, if current is going through it, being grounded to the chassis helps dissipate the short better as it's disconnecting, since the chassis is a larger surface area to absorb it, rather than the batteries taking the hit. I don't know if there's any truth to it, but those are my thoughts.


But anyway, is this an acceptable way to hook it up as in my "Final setup" picture? The other obstacle to using positive is that prior people have hooked up a bunch of additional wires to my positive cable which bypass the main positive cable, and they will still drain the batteries which defeats the purpose of why I'm adding the disconnect in the first place.
I can certainly agree with putting the switch on the negative side
Putting it on the negative reduces the chances of sparking, kinda like when you connect jumper cables.


But I also ask myself why is it OEM's put it on the positive side?

I was working on an Atlas Copco air compressor today and the cutoff switch was on the positive!

???
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Old 11-09-2023, 07:55 PM   #6
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The other obstacle to using positive is that prior people have hooked up a bunch of additional wires to my positive cable which bypass the main positive cable, and they will still drain the batteries which defeats the purpose of why I'm adding the disconnect in the first place.
Typically, the only Constant-On positive should be for the engine's computer/ECM. I inadvertently connected my ECM's power wire to the switched side of the start batteries' master switch, and the ECM produced spurious CELs and SELs and set various trouble codes. When I connected the ECM correctly, all that nonsense stopped. I still needed to clear all the codes, but no lasting harm was done.

If previous owner(s) have added mystery wires directly to the start batteries, I suggest that you A) find out what they all do, and B) if you decide to keep any of them you should install separate fuses or circuit breakers for each one. Adding wires directly to start batteries is a lazy, sloppy and dangerous way of doing things; additional circuits should only be added to the bus's main breaker panel, with appropriate notations on the bus's wiring schematics and circuit diagrams.

John
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Old 11-17-2023, 03:45 PM   #7
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Thanks for all of the advice. I added the wiring to to negative side because the ECM is the item draining the battery and I donít want it to.

I rewired the whole battery compartment to make it clean and to reroute the ground cables to the disconnect switch. I donít drive the bus enough like a school district would daily to keep the batteries charged so I donít have a choice really but to disconnect the ECM after every drive. It can get over it. Pictures attached.

Used a 2000A Hersee switch. Really solid switch IMO.

I also have a starter issue at the same time so I didnít know if my battery rewiring was the cause but didnít believe so because I was sure it was correctly done.

Today I confirmed it was not my wiring job because I could jump the starter manually. Which means my battery cable rewire worked.

We can end this thread unless someone else has questions about it, but the emergency is over now and job complete with disconnect upgrade working. Enjoy the pics.
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Old 11-17-2023, 05:10 PM   #8
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I still don't understand why anyone would put a battery disconnect on the negative side.
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Old 12-07-2023, 07:38 PM   #9
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I still don't understand why anyone would put a battery disconnect on the negative side.
It's not bioinformatics and genomics and I don't think the bus is going to be inspected by the NHRA, so it shouldn't be too hard to understand or at least try to understand. I suspect a person's preference for a negative terminal disconnect is likely due to their personal safety concerns over accidents and the possibility of exposed terminals or a broken positive cable making contact with the vehicle's sheet metal or frame... thus creating a short to ground and potential safety hazard.

I think you might have a habit of frequently saying "I don't understand why..." with anything you disagree with, dislike, or just think you know better without explanation. It's a talent, being able to know so much yet miss the simple art of letting people be, grammatical error or proprietary eponym aside... But hey, what do I know? I'm just someone who occasionally calls a spade a spade, or should I say a manual earth-restructuring implement?
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Old 12-08-2023, 12:17 AM   #10
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I've hooked up a few thousand batteries in my lifetime. If there is any kind of load on, and there usually is, it doesn't matter which cable you hook up first there is going to be a spark. One way the spark jumps from cable to terminal the other way it jumps from terminal to cable. The spark may be a bit smaller when hooking up the negative last. I've use disconnects in pos and neg. I don't think it really matters. Because it's more likely that there is multiple grounds it's usually easier to put the disconnect on the positive.
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Old 12-08-2023, 12:42 PM   #11
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I feel like the components will take less of a hit when reconnecting with a switch on the negative.



Think about it. A battery itself is a resistor to some degree In order to push a spark from the negative side to the positive it would have to travel through the 2 batteries. I look at the batteries as an absorber of the initial shock and help protect components more. The more stable you can make that initial connection the better off any components like an ECM or whatever with a positive power line going into it will be shocked far less on reconnect.


On the other side of the disconnect, you have a large ground plane which is more metal and surface area to absorb any shock.


So ultimately you have batteries on one end of the disconnect to help absorb the initial shock/spark/reconnect, and a large ground plane on the other side of the disconnect. Logically this seems safest place to put it.

If you put the disconnect on the positive side and it sparks, you have batteries on one end, which is good, but components on the other. If the components are the path of least resistance, (lesser than the batteries), guess which is more likely to pop? The components. Like water, electricity flows in the path of least resistance.


So with that logic it seem safer to put it on the negative side simply for better absorption.
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Old 12-08-2023, 03:17 PM   #12
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Originally Posted by nocoasty View Post
It's not bioinformatics and genomics and I don't think the bus is going to be inspected by the NHRA, so it shouldn't be too hard to understand or at least try to understand. I suspect a person's preference for a negative terminal disconnect is likely due to their personal safety concerns over accidents and the possibility of exposed terminals or a broken positive cable making contact with the vehicle's sheet metal or frame... thus creating a short to ground and potential safety hazard.

I think you might have a habit of frequently saying "I don't understand why..." with anything you disagree with, dislike, or just think you know better without explanation. It's a talent, being able to know so much yet miss the simple art of letting people be, grammatical error or proprietary eponym aside... But hey, what do I know? I'm just someone who occasionally calls a spade a spade, or should I say a manual earth-restructuring implement?



yea i would agree and considering all these so called experts. most here have NO clue of what they are talking about. we still do not really know which direction the electrons are moving. we use 2 theorys when designing systems. 1 conventional theory pos to neg. we use this mostly so average people can understand complete circuits such as in cars and busses. 2 electron theory is neg to pos such as joe lucas designed for cars and busses that incorporated a positive ground system. common in britan but also used a lot in the us. so if you had a positive ground you would put it on the negative if you wanted but all this just proves there are advantages to either system. put it where you want
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Old 12-08-2023, 03:19 PM   #13
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Originally Posted by nikitis View Post
I feel like the components will take less of a hit when reconnecting with a switch on the negative.



Think about it. A battery itself is a resistor to some degree In order to push a spark from the negative side to the positive it would have to travel through the 2 batteries. I look at the batteries as an absorber of the initial shock and help protect components more. The more stable you can make that initial connection the better off any components like an ECM or whatever with a positive power line going into it will be shocked far less on reconnect.


On the other side of the disconnect, you have a large ground plane which is more metal and surface area to absorb any shock.


So ultimately you have batteries on one end of the disconnect to help absorb the initial shock/spark/reconnect, and a large ground plane on the other side of the disconnect. Logically this seems safest place to put it.

If you put the disconnect on the positive side and it sparks, you have batteries on one end, which is good, but components on the other. If the components are the path of least resistance, (lesser than the batteries), guess which is more likely to pop? The components. Like water, electricity flows in the path of least resistance.


So with that logic it seem safer to put it on the negative side simply for better absorption.

more food for thought when we weld on these we only remove the ground cable to protect the system
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Old 12-08-2023, 06:32 PM   #14
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yea i would agree and considering all these so called experts. most here have NO clue of what they are talking about. we still do not really know which direction the electrons are moving. we use 2 theorys when designing systems. 1 conventional theory pos to neg. we use this mostly so average people can understand complete circuits such as in cars and busses. 2 electron theory is neg to pos such as joe lucas designed for cars and busses that incorporated a positive ground system. common in britan but also used a lot in the us. so if you had a positive ground you would put it on the negative if you wanted but all this just proves there are advantages to either system. put it where you want

Yes I mean we know electrons flow the path of least resistance. Also take battery sources. To charge a 12V battery you have to send 14V from an alternator or battery tender which reverses the direction of the charge towards the battery, When the alternator dies, the battery is being drained so more electrons are leaving vs coming in. So the end with a higher voltage output is going to flow towards the least resistance direction which is the lowest voltage area.



Electricity flows a lot like water. If you had two tanks of water, and one tank was full and the other was half full, with a tube connecting them at the same height but at the bottom, the water will equalize once you open up the tube to flow. Except electricity isn't affected by gravity like water, so that's one difference but otherwise a similar concept.
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