battery disconnect switch

[Gary G.]

When installing the battery disconnect switch (knife type) does one install it on the positive, or negative post? Does it make a difference?

Thanks for your help.

[Terry M.]

Re: battery disconnect switch

There may be physical differences -- clearance to parts with the opposite polarity for example, but electrically there is no difference. The switch makes an open circuit. An open circuit is just that, regardless of where the open is. In your case it may be easier to operate on one pole or the other, so try each to see which provides better access and better clearance to the other terminal of the battery or better allows you to close the lid of the battery compartment.

[Oliver B.]

Re: battery disconnect switch

Isn't it recommended to put the switch to the neg terminal in order to prevent short-cuts? I think so...

Yes, I guess you MAY run into clearance trouble with your blade-type switch, as the pos cable goes right over the neg terminal on the late 70s-cars... I have the screw-type on my '77 and still have to wiggle my fingers under the cable every time.......

[Stuart F.]

Re: battery disconnect switch

Believe it is called a "Ground Interrupt Switch" by the Mfg. I got mine from Lectric Limited many years ago, and just cleaned up a bit the other day as I am replacing my wet battery with a dry. C-1's and C-2's seem better situated for use of this type than C-3's with the battery in the rear compartment where you're limited almost exclusively to the screw type for clearances - same with most any side post batteries.

Stu Fox

[Stuart F.]

Re: battery disconnect switch

Believe it is called a "Ground Interrupt Switch" by the Mfg. I got mine from Lectric Limited many years ago, and just cleaned up a bit the other day as I am replacing my wet battery with a dry. C-1's and C-2's seem better situated for use of this type than C-3's and C-4's with the battery in the rear compartment where you're limited almost exclusively to the screw type for clearances - same with most any side post batteries.

Stu Fox

[Terry M.]

Re: battery disconnect switch

Oliver: That use of the negative terminal might be an issue for a tin car, or a C1 or C2. On a C3 in the fiberglass rear compartment the only physical contact issue is between the posts of the battery -- and that can be an issue regardless of which post you put it on. The switch can be assembled so that the knife blade itself is not energized once it is opened, and that is the safest method. If it is assembled that way it can be installed on either post of the battery without concern. I can't speak to a C4, since I have no personal experience with them.

Stu: I have had the knife type switch on my 1970 for a real long time -- since before NCRS gave points for it. Physical clearance is not an issue for me. The 1969 side terminal batteries -- where the terminals face the front of the car -- can be a problem, and I doubt either style switch will comfortably work on those.

[John H.]

Re: battery disconnect switch

Both the "green knob"-type and knife-type battery disconnects are designed to be installed on the ground (negative) battery post (on top-terminal batteries); the positive post is larger in diameter than the ground post, and the clamp on the disconnects is sized for the smaller of the two terminals.

[Jack H.]

Re: battery disconnect switch

An EXCELLENT comment, John! Few realize that there's a very subtle difference in the OD of post style battery terminals...

Plus, for the other comments in this post, I echo as Terry said: "Open circuit = open circuit". Even if the battery positive post is left connected, there can be NO CURRENT FLOW through any/all circuit branches if the physical path to the battery's negative post is left totally disconnected.

This discussion seems to come up frequently with folks beliving there's some kind of preferred connection for a battery disconnect switch with respect to the physical poles of a given battery/car. The ONLY way this is true is if the switch is intended to install on a post style battery and its physical ID has been design optimized for the typical post style battery's post OD geometry.

[Stuart F.]

Re: battery disconnect switch

Just a couple of adders to this thread as I am in the process of replacing my battery as we post (so to speak). I made some additions to my installation in an attempt to monitor and control my battery during extended shutdowns.

First, I had purchased with my last battery from Restoration Battery, a Battery Condition Indicator/meter. It is a little square plastic meter box with a multicolor LED readout. You connect it to the battery, then press a small pushbuton to get "light" reading. It came with pigtail wires with standard color coded round lug connectors. I snipped them off and replaced them with color coded faston connectors, and mounted the little meter box with the provided Velcro on the firewall above the battery.

Second, I had also bought a "Battery Minder Plus" from them and had been using it religiously since installing their battery 7 years ago. It came with the usual "clamp" post connectors and another wire pigtail with color coded round lugs. My usual routine was to connect and disconnect the post clamps, then plug the inline wires together with their plug which has one male and one female terminal on each end. Both operations can, over time, be a little hasstle. So, I took their wire pigtail with the round lugs, snipped them off and replaced them again with color coded faston connectors.

Third, to bring this all togther, I found a unique item at Auto Zone which consists of a pair of brass discs, each having 3 faston "spade" connections. A disc slips over each post and has a piece that fits against the post and is clamped there with the spring connector from the battery cable to make a good contact. You have to position the disc in such a fashion that the faston spades do not interfere with the cable or battery hold down - of course. I bent the spade contacts up for easy access and to avoid interference.

Bottom line; I made positive connections for both the meter and the Battery Minder so, with the latter, all I need do is make just one connection/disconnection with their inline connector. I secured the pigtail with the inline connector (which has a protective cap, by the way) to my wiring loom on the fire wall. Now, as part of my restart routine, I will check my battery condition meter at the same time I disconnect the Battery Minder and perform my other fluid level checks, etc.

Sorry to take so long explaining this, but it made a neat and easy arrangement that I thought I'd share with ya all. Merry Christmas and Happy New Year.

BTW: you can see all of the material (except the discs) in the latest Driveline, Page 30.

Stu Fox