Reproduction Parts for 1916-1964 Chevrolet Passenger Cars & 1918-1987 Chevrolet & GMC Trucks


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#470474 06/04/22 09:30 PM
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Hello All,
I am looking for a knife style battery disconnect for my 51, 6 volt battery. I now have a green knob type disconnect but it is unreliable on this car. Works great in the Corvette but here, not so much. All the switches I see on Amazon say 12-24 volts but I don't see why they wouldn't work on a 6 volt system- it's just a switch, right. What do you all think?
Thanks!
Jeff

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Most of the shut-offs you can buy today cannot handle the high amperage needed to start an engine with 6 volts.

I have a “green knob” one on a ‘57 car. One day I took it apart enough the see the contact area at the disconnect point. I could see that it would be marginal at best in a 6 volt system.

It is the similar to the issue many first time owners of vehicles with 6 volt systems encounter. They buy “starter” cables at their FLAPS. Except they are for 12 volt systems.

Before you buy a knife switch try to determine the cross sectional area of the knife “blade”. It should be about the same as a 2/0 cable in order to handle the load.


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I use the Cole Hersee 75908. It can handle the amps.

Dave

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Thanks for the replies. I really was hoping for a knife style switch but I will look into the Cole Hersee above. Anyone out there have a good experience with a particular knife style switch?
Jeff

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Rusty 37 Master- Thanks for the help. I just need a little additional help here. Does the "cross sectional area of the knife blade" mean the area that is in the contact area when closed? I am assuming that the comparable area on a 2/0 cable is the area of the cable? I found one that specifies that it carries max 750 amps, normal 250 amps. Is this adequate? Here is the one i found:

https://www.ebay.com/itm/124892376378?hash=item1d142a6d3a:g:FskAAOSwE-tfMoiQ

Jeff

Last edited by Jeff1951; 06/05/22 10:28 AM.
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This is just me being me. I have become more leery of eBay Sellers over the last few years. While I agree you can find good products at good pricing, I am very cautious about their claims.

This catches my attention because the maximum current is 3 times the rated. On the other hand if it will handle 500 amps for a starting cycle it should be ok.

The area I am thinking about is the cross sectional area of the rectangle that is the knife blade itself. All the current has to pass through that.

On the other hand, I really like Cole-Hersee switches for applications like this. All the electrical connections and contacts are enclosed. If you drop a tool on a knife switch things can get interesting really quickly.

Hope this helps.


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Rusty 37 Master,
I helps a great deal. While I think that the Cole-Hersee is probably far superior, I am not sure how I would install the switch. The nice thing about the blade type is that it installs right on the battery- no modifications, drilling, etc. is necessary. I get the idea that the other has to be mounted remotely from the battery and screwed into something to hold it. Thanks again!
Jeff

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I bought a blade type for my 53 when it was still 6v. I didn't notice much drop in starter performance but the disconnect is pretty flimsy especially when you consider that the only thing supporting it is the battery terminal. I didn't trust it so it's living somewhere in my stash of bad idea parts/accessories.


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Had this on my '48 for over 10 years. Still works fine.

For the price of the one you are looking at, not much to loose if you are not satisfied.

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There are many ways to mount the Cole switch. I was concerned about that as you are since I did not want to drill any extra holes. I mounted mine on a short but heavy brass bar right on the starter switch to see if that would work. So far it has been okay but have to open the hood to use the switch. A friend of mine mounted his similarly but made a lever for the switch handle and uses a Bowden cable to operate it from inside the vehicle.

On most of my cars it is just as easy and effective to pull the battery cable.

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I put in a Battery Disconnect on my 1929, went with the rotary switch with removable key :)
mounted mine on the cross board right at the front edge of the seat, works great and out of sight.

Building Battery & Ground Cables w/ Disconnect Switch


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So here's the update. I purchased a blade style disconnect from Auto Zone today that states on the package..."250 AMPS CONTINUOUS * 750 SURGE". If that is true that's more than enough to start the car. It is made in China for Duralast part # DL20138. We'll see.
Jeff

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Let us know how the switch works for you. I am interested in how hot the knife gets when starting the engine. If the engine starts quickly I agree there will be no issues. If you have to crank a while it might get pretty warm.

“Surge” is a pretty nebulous term. Duration is a key variable.


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I will let you know. I don't plan on installing until next week. The green knob switch never got hot and the car cranked well even on start-up after sitting a while.
Jeff

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Jeff,

This is an excellent topic. Thanks for bringing it up.

I would be interested in knowing how your Duralast switch works as well.

One of those that had the green knob that mounted right on the battery and disconnected by using a male/female connection was on the 41 Cabriolet when I got it. It didn't want to release worth a darn so I eventually took it off. greenman

After almost burning up a 41 town sedan and the current 41 coupe up a few years ago when I couldn't get the "loose fit" battery cable off fast enough, I would like to have some sort of quick disconnect for them both now. Yes, I've been running the loose fit battery cables for some time but I find that having the fit loose presents problems in starting the cars unless the fit is just barely below tightness for wrenching it off with bare hands and with all the muscle power I can exert at my age, which is not much. Thus my interest in a lever switch that I can access from under the hood or on the inside firewall.

The last fire fire was owing to some old ragged bare wires under the dash that lit off after I replaced the hot wire from the ignition switch to the light switch. My fiddling around under the dash disturbed a wire and left me with a direct short which was was not manifest until I activated the park lights while driving down the road at dusk back in 1990 or so . During the fire I was not able to get the battery disconnected. Fortunately I had a fire extinguisher on board.

The oldest fire was owing to an after market clamp-on switch for the heater that shorted out against the lower lip of the dash where the switch was mounted. This one happened on a 41 Town Sedan and I burnt my hands so badly trying to put out the fire ball of wires that the flight surgeon put me on "no-flying" status for a couple weeks. I installed a Wirey Joe after that. That one happened back in 1977-78. You'd think I would have learned that old frayed wiring is not in my best interest.

Well, I have recently installed both a full wiring set from YnZ for the cabriolet and a good condition Wirey Joe for the coupe. The coupe doesn't warrant the YnZ. Too costly. Whatever.

Anyway, you can see that having to fight two fires that occurred under the dash that I need a quick release of juice switch. Accordingly, you can see my interest in the outcome of how well the Duralast switch works.

Maybe the best thing to do is snug up the terminals on the battery and make sure there are no potential causes for fire issues that calls for quick release of juice and make sure I have a good fire extinguisher on board. We'll see.

Best,

Charlie computer

BTW: Even with a fire extinguisher you still need to disconnect the battery because it will still be active in relighting the fire owing to the wires still being "hot". Having no need for the disconnect and the fire extinguisher are key here but still needed to lessen the likelihood of going into a state of pure panic. Believe me you will. Scrambling around trying to get the battery disconnected and while all the time the fire is burning is no picnic. I wonder if a good bolt cutter carried in the trunk to be used for snipping the battery cable(s) would suffice. Hmmm.

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Charlie,
I will let everyone know how the switch works when I get it installed next week. The problem with the green knob switch on this car is that when I turn the knob off the end of the assembly with the ground connected is wobbly and may touch the frame that secures the battery. This re-connects the system so it is live. I tried insulating but I couldn't seem to get it right and I can't depend on it to stay off. The blade type switch is solidly attached to the battery post and is definitely off when raised. Underneath is all plastic so it cannot ground unintentionally. When it's off, it's off.
Jeff

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Why i went with the key switch type, i can remove the key and not worry. been doing great. i flip the switch and then plug in the battery, as i have the hardwire kit with a plug for my Tender :) flip up battery lid grab cable, plug it to tender, and let it do its thing.


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Jeff and BF, et al,

I looked at what switches are available on Amazon. There a lot of them. Having been somewhat schooled here on Chat about the 6 volt needing a more "stout" or heavy duty connection to the battery than a 12 or 24 volt system, I noted that almost all of the switches were rated at the latter voltage. There were only one or two that were rated for 6 volt. Best I recall, most all the "blade" ones were rated at 12 volts or higher. Those rated at 6 volts seemed cheaper and give me pause as to whether they will do the job or not.

This information tells me that I should forget installing a cutoff owing to the heat that may be generated by one that is rated at 12 or more.

Is my take on the lack sufficient juice flow of the 12 volt ones correct? Does the installation of this gadget cost points off from judges or do they get a pass owing to it being a safety feature?

The bolt cutter in the trunk, at first meant as a joke, looks more and more like a good solution,

Observation and question: If you have Huck brakes and must, accordingly, pump them often to avoid the wheel cylinders pistons from sticking, why not just crank the car for a few minutes and by so doing keep the battery charged up? Would such action eliminate the need of a battery tender unless one has a relatively cheap battery? Agrin dance

Best,

Charlie computer

BTW: I think that most of us who grew up in the thirties thru fifties and ever parked in the woods with a "Miss Right," luv2and having to worry about playing the radio and/or also concerned as to whether the ole car would start when time to go and maybe having walk for help, nowadays go for the very best and highest rated battery that money can buy, every time. I know I do.

BTW2: If you didn't marry "Miss Right" then don't show the above BTW to the missus. luv2 It may not turn out to be in your best interest. Just saying driving

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It is not the voltage rating that needs attention, it is the current (Amps). Anything will be fine for a voltage rating. To do the same work at 6 volts you need double the current that you need at 12 volts, and four times the current you need at 24 volts. Thanks to Ohms Law, an ohm of resistance is a bigger problem at 6 volts than it is at 12 or 24. These are the reasons 6V battery cables are so big.

The extra current needed at 6 volts drastically increases the likelihood that the cutoff switch will interfere with starting, and the possibility that the switch will melt. What you need for 6 volts is a switch that is huge and well built. Look for one for a fire truck or a big bus. Those will likely be 12v or 24v vehicles. When old car people who work on electrical systems a lot say you need a 6v switch, what they REALLY mean is that you need a really big switch that can handle double the current (or more) than a 12v car needs without overheating, melting, or adding enough resistance to cause slow cranking. If some vendor is offering a switch that is only rated for 6 volts, and it is smaller or cheaper than the others, that is absolutely not the one you need.

A good switch may be shocking for price.

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Bad Pun!!


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I installed the switch this afternoon. I started the car (it cranked really strong) and let it run for a while. Shut it off and felt the blade. It was cold to the touch. I want to see what it does 1. When it cranks for a while after sitting for a couple of weeks, and 2. How warm it feels after a long ride. The only thing that was not exactly right was that the terminal on the end of the original ground was slightly small for the post on the switch but I got it opened enough to get it on.
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The specs on the Cole Hersee I posted earlier, it can handle the amps;


Cole Hersee 75908 Master Disconnect Switch

Single Pole. Off-On. Disconnects battery circuit only. Electrical Ratings: 2,000 amps intermittent. 30 seconds On, 3 minutes Off. 300 amps continuous. Plated steel case. Sealed between shaft and mounting stem, and case and terminal insulator. Mounting stem 3/4"-16 thread. Two 1/2"-20 copper stud terminals. Fits panels up through 1/4" thick. Indexing pin aligns switch and prevents rotation.

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The switch will only get hot during the starting cycle. The rest of the time the current that will pass through it is either what the generator is producing or what the car is demanding if the generator is not working. Neither of those loads are anywhere near the starter draw.


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The one I purchased was made for old tractors that were 6v. It is 2 large pieces of brass connected by a bolt. In this one there is a black knob on the bolt that you turn to make the brass bars make contact. loosening it holds them apart. I have one on the 41 and another on my Monte Carlo. What is great about these is that when working one wiring turning a dial allows us to check a circuit and then it disconnects easily without constantly dis connecting the battery terminals.

Funny story on this. Make sure you connect the battery tender wires to the correct side of the disconnect. My son followed the instructions perfectly on the pigtales for the tender except he connected it to the wrong side of the disconnect.


I have found that having an old car is a constant project that is never done. I think that is a good thing. Keeps me learning new things. Having two from different eras is just a form of higher education.

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