Military Cutoff Disconnect Switch HMMWV Battery

[SmartDrug]

I appreciate your help with this. I have a much better sense of the electrical system now.
Regarding the switch, I’ve seen a handful with the same specs but different model numbers. I think I’m going to call Hella tomorrow to clarify.

I’m also going to take this opportunity to remove the slave port and the rear shelter power plug to clean up the box a bit.

 

[nikojo]

In your diagram you can wire it so you cut off the 'green' and leave the "blue" alone or as Mogman pointed out do it the opposite way. That effectively does the same thing assuming you cut off both switches. I like doing it my way by cutting off the interconnect between the batteries in case I added other 12V circuits in the future. That way I don't have to add any more to the switch.

The switch I used is 2 switches in one so it does it all together.

 

[Chadwickadam]

Hey, I'm new here and this thread has been extremely helpful so thank you. I was wondering if anyone has tried this or knew the answer:

If you do the same setup as nikojo with a single dual pole switch but instead of interrupting the ground and battery interconnect, can you interrupt the ground and the opposite batteries hot?

If so, does this achieve the goal of total power shut off so nothing comes on?

Are there concerns with having the interconnect joining the two 12v batteries and then battery 1 hot and battery 2 ground going to the separate circuits of the cutoff switch?

Would I then attach the "out" pole from the switch to the hot and ground of the starter?

Thanks for the help

 

[Mogman]

Hey, I'm new here and this thread has been extremely helpful so thank you. I was wondering if anyone has tried this or knew the answer:

If you do the same setup as nikojo with a single dual pole switch but instead of interrupting the ground and battery interconnect, can you interrupt the ground and the opposite batteries hot?

If so, does this achieve the goal of total power shut off so nothing comes on?

Are there concerns with having the interconnect joining the two 12v batteries and then battery 1 hot and battery 2 ground going to the separate circuits of the cutoff switch?

Would I then attach the "out" pole from the switch to the hot and ground of the starter?

Thanks for the help

Yes interrupting the ground and 24V hot would do the same thing, no current could flow in any circuit.

 

[nikojo]

Hey, I'm new here and this thread has been extremely helpful so thank you. I was wondering if anyone has tried this or knew the answer:

If you do the same setup as nikojo with a single dual pole switch but instead of interrupting the ground and battery interconnect, can you interrupt the ground and the opposite batteries hot?

If so, does this achieve the goal of total power shut off so nothing comes on?

Are there concerns with having the interconnect joining the two 12v batteries and then battery 1 hot and battery 2 ground going to the separate circuits of the cutoff switch?

Would I then attach the "out" pole from the switch to the hot and ground of the starter?

Thanks for the help

The problem with this is that the 'high' side battery (running 12V) can still find ground through the voltage regulator.

Steven (RWH) pointed this out earlier in the thread.........."connection to the 14v tap off the regulator can and will also act as a ground when the ground is disconnected allowing the truck to be cranked over but on 12v only".

I would recommend cutting off the negative of both batteries, ie interconnect on low side battery and ground on high side battery.

If you do that there is no way for anything to get power.

But if you cut off the positive side on the first battery it will also accomplish same thing and isolate the battery bank.

 

[Mogman]

The problem with this is that the 'high' side battery (running 12V) can still find ground through the voltage regulator.

Steven (RWH) pointed this out earlier in the thread.........."connection to the 14v tap off the regulator can and will also act as a ground when the ground is disconnected allowing the truck to be cranked over but on 12v only".

I would recommend cutting off the negative of both batteries, ie interconnect on low side battery and ground on high side battery.

If you do that there is no way for anything to get power.

But if you cut off the positive side on the first battery it will also accomplish same thing and isolate the battery bank.

I would like an explanation, if the 14V reg tap and trans tap cannot flow any current to the low side negative terminal because it is disconnected and it cannot flow any current to the 24v pos terminal because it is disconnected then just where would the current flow occur ?

 

[Mogman]

The problem with this is that the 'high' side battery (running 12V) can still find ground through the voltage regulator.

Steven (RWH) pointed this out earlier in the thread.........."connection to the 14v tap off the regulator can and will also act as a ground when the ground is disconnected allowing the truck to be cranked over but on 12v only".

I would recommend cutting off the negative of both batteries, ie interconnect on low side battery and ground on high side battery.

If you do that there is no way for anything to get power.

But if you cut off the positive side on the first battery it will also accomplish same thing and isolate the battery bank.

One of the first things a student in electronics/electrical 101 learns is that in order to have current flow one must have a path to and from the source.
Because the vehicle is not running the only source is the batteries.
A 12V battery in itself is a simple series circuit, 6 cells connected in series, removing any of the cells OR removing ether the positive or negative terminal will prevent any current flow through the battery.
The circuit below represents what the OP was talking about, since he has effectively removed the negative terminal from the low side battery and the positive terminal from the high side battery no current can flow through ether battery, effectively isolating all energy from the vehicle circuits.


50 years experience tells me the following is the preferred switch setup, I use the little weeny battery switches that some think are adequate for a main switch for the 12V and a 1500A Hella switch for the negative (24V) disconnect.
This way I can first isolate the 12V equipment then the 24V volt equipment with zero chance of reverse polarity of the 12V equipmewnt IE
12V OFF first and 12V ON last

There is no need to disconnect the large lead between the batteries, depending on if the 12V equipment is connected to the positive terminal of the low side battery or the negative terminal of the high side battery (works ether way) you can leave yourself unprotected.