Dear Dave,
My trailer has two deep cycle 6-volt lead acid batteries. I want to add a switch to disconnect them but I am not sure the proper way to do it. Should it be on the negative side? What is the best type and size of switch? Are there any other considerations? —Ted, 2015 Starcraft
Dear Ted,
Since you have lead acid batteries and they are on a Starcraft, they should be mounted in a tray on the front tongue of the unit, right? I am surprised there are two 6-volt batteries as that is not typical of the smaller trailers. However, most come with no batteries standard and the dealer puts in what is best for the buyer.
Knife disconnect switch
There are several different types of battery disconnect switches on the market, from the simple “knife” type to an elaborate solenoid that has a remote switch placed in the doorway. Here is a simple knife switch available at Amazon here.
I am not a fan of the knife switch as there are too many connections that can corrode or get loose. This switch is installed on the negative side of one of your batteries by removing the negative cable at the battery, connecting it to the rounded male post, and connecting the female side back to the battery. Pulling the lever or “knife” disconnects the battery. Also, it requires connecting to a post connection, which is more likely to corrode or get loose as compared to the threaded bolt connection found on most deep cycle batteries. And you can see how the thin metal bar might not be the best connection to the receiving metal piece when you are using 30 amps.
In my opinion, it would be better to just disconnect the negative cable from the battery by loosening the negative cable connection, which is a little more work. The only advantage of this switch, in my opinion, is that it would be connected to the battery in the protective box—IF that is how your system is set up. If your batteries are on a tray with no protective box, you are already experiencing corrosion at the posts, I would assume, and this will just add to the mess.
Another battery disconnect switch
A better option, in my opinion, is the rotating switch that is also connected to the negative cable but is remote from the terminals and can be placed just about anywhere.
It has a molded plastic body and an easy-to-use on/off switch. Although it is not weatherproof, it does not sit exposed to the elements and can be protected. It can be found on Amazon here.
Using a “jumper cable” to connect negative post
Since you have two 6-volt batteries, they are connected in “series,” which means positive to negative on one battery to the other with a “jumper cable” that creates a 12-volt bank. The other positive terminal has a cable that goes to the distribution center and the negative has a cable that goes to a ground source. This is the cable that you will disconnect and attach to the battery disconnect to the outgoing side. You will need a “jumper cable” here to connect the negative post of the battery terminal to the outgoing stud. To attach the cables you will need to remove one of the side panels to access the studs. This leaves an open area for moisture so you need to access the location of this switch.
Where to mount the disconnect switch
Locating it in the battery box would be ideal as it would be protected. However, typically there is not enough room or a good secure mounting point as the box is thin plastic. It can be mounted to the outside of the box with a few modifications and some weatherproofing. Instead of removing the side panels, drill a small hole to run the cable in and use electrical grease at the connections and the appropriate silicone sealant on the housing. The switch could be mounted on the side of the box, directly on the tongue, or even inside a compartment.
Another option would be to mount the disconnect switch inside the unit close to the distribution center. The negative cable from the battery will be routed to the distribution center and what is commonly called the negative buss bar. It might be easier to place the disconnect switch closer to that inside.
And I always recommend connecting the disconnect to the negative side of the system.
You might also enjoy this from Dave
“But it worked yesterday!” RV “Gremlins”, Part 1: Lead-acid batteries
By Dave Solberg
How many times have you had something in your RV not work out on the road only to take it to the dealership or service center and pay them to tell you it’s working fine?! It happens all the time. I call it the RV “Gremlins” of the industry!
Dave Solberg is a leading expert in the RV industry and the author of the “RV Handbook.”
Read more from Dave here.
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All of the “add on” or aftermarket switches I’ve installed recommended using the negative cable. However I did go on the Winnebago site and looked up a typical Class A gas and they do use the positive cable. Everything I’ve learned from way back working on cars said disconnect the negative cable, as Snoopy stated, so there is no chance of sparking. Winnebago does use a switch activated disconnect rather than the manual style and I have not gotten a reply back from anyone at W.
My Winnebago travel trailer came with a factory installed battery switch mounted inside the front pass through. It is connected to the positive side. You never explained why you recommend the negative side.
Good question, if you disconnect the POSITIVE side first, you have a chance of touching the positive post to a grounded area & sparks fly! When you disconnect the negative side first, there is a lesser chance of any issues, however & it happens is when you accidentally touch positive post to the negative post with the wrench or whatever metal object you’re using to remove a terminal! Can be disastrous , even can blow up a battery!
Snoopy
This advice can create a dangerous and possibly illegal towing situation. The key is that this is for a trailer. The disconnect should always be on the positive and the emergency breakaway is wired prior to the disconnect. This insures that the breakaway is powered regardless of the switch position.
For a motorhome put it in the negative if you wish.
Wouldn’t that blade switch also arc when it is opened/closed? Sparks around lead acid batteries are usually not advisable.
Could a disconnect be installed inside near the input of the converter?
I use a dielectric grease on all battery cable connections, it helps to control the corrosion. CRC also has a battery terminal protection spray. A yearly maintenance item for me is to check the tightness of all connections, just make sure you are careful swinging a wrench around when on the positive connection, wrapping electrical tape around your wrench is helpful to avoid a big oops.
I’ve read that dielectric grease is a big NO NO as it wont conduct electricity. Since this grease is not an electric conductor, it may affect the flow, resulting in shorter battery life and poor connection. I use to coat the terminals & the post with dielectric grease & then assemble. I now have a clean & bright terminals & posts & then assemble & spray on a battery terminal sealer that will get into all the nooks & crannies. If you have a desire to, now after assembly of terminal & battery post you can apply dielectric grease! I’m sure there are other ways of doing this!
Snoopy
I installed the rotating switch last year on my bank of 4 x AGM batteries to stop the parasitic draw when we not not using the RV. Works perfectly, and super easy to install. Well worth the effort and minimal cost unless you are a full timer. I may put one on our car that we do not use for 6 months when we are South for the winter. Last spring we came back to a dead battery in the car, which just cuts down the lifespan of it.