This is actually a dinghy question but it needs tech answers, not towing answers.It seems like a stupid question. But I am a master at overlooking obvious facts, so I want to ask to be sure.On various Ford cars, to get them ready to tow behind a MH, one of the steps is to disconnect the negative terminal at the battery. This is not to prevent battery run-down, it is to prevent the electrically clutched "automatic gearbox" transmission from shifting out of neutral. So it is important to completely disconnect the battery, and not try to pull fuses instead. The negative post is difficult to access. Very difficult. The chassis ground end of the cable is easy to access, but unfortunately there are other negative wires coming off the battery, so disconnecting the main negative ground is not enough. So to disconnect the negative at the post, you have to work in a tight space. But the positive post is very easy to access.Now then, from working on cars, I know that the reason you disconnect the negative before the positive is for safety. If your wrench touches any metal, nothing happens. OTOH if you remove the positive first, and your wrench touches metal, you have sparks and a short circuit.But if I was to install a disconnect switch on the positive side, properly isolated, it will be good, won't it? I mean, the battery is just as surely disconnected by a switch at the positive as at the negative. ??? Like I said, I feel stupid asking this, but it's cheaper than a transmission. :)

As long as either the - or + is disonnected, it should make no difference.

The reason this is important on a car installation as there is usually alot of metal objects around the battery terminals. By disconnecting the NEGATIVE TERMINAL first then you may have less of a chance or sparking a wrench or something touching the POSITIVE TERMINAL.The RV Trailer battery terminals are most often all out in the clear where this is not a problem accidentally shorting something out. In a tight spot Battery cable installation I always remove the NEGATIVE battery terminal first.There is no electrical reason why the NEGATIVE or POSITIVE terminal is to be removed first. Just a shade tree mechanic's tip... Roy Ken

An on-off switch on the NEGATIVE battery lead would be wonderful for you. Easy to install and almost-fool-proof. Keep yanking that lead and you are going to damage the battery cable clamp. If it should subsequently fail when the alternator is working hard, say "adios" to a rather thick stack of hundred dollar bills

MEXICOWANDERER wrote:An on-off switch on the NEGATIVE battery lead would be wonderful for you. Easy to install and almost-fool-proof. Keep yanking that lead and you are going to damage the battery cable clamp. If it should subsequently fail when the alternator is working hard, say "adios" to a rather thick stack of hundred dollar billsWell, the negative side of the top of the battery is shoved under something. I'm not saying I can't work out some sort of switch there, but the positive side would be 100X easier.

My BATTERY SWITCHES in my RV TRAILER setup is all in the POSITIVE SIDE of the 12VDC runs. This was the easiest way out for my BATTERY selection and disconnect plans. This idea is great for multiple battery banks allowing you to use none, one, or all of the them for your RV Trailer battery supply.However if this was the TRUCK START BATTERY/ALTERNATOR circuit which you may be describing then I will go with MEX and only switch out the NEGATIVE terminal. I know in my FORD TRUCK alternator setup the few time I had to disconnect the start battery battery from the alternator I always end-up resetting all of the truck computers and have to go thru a new system learning time for all computer controlled things to get back to normal. My truck seemed to never re-learn the idle speed when stopping at a stop sign and my truck would die every time. Then I have to go to the Ford place and put it on their machine to set the values and that of course would be a minimum $90 charge. I am very hesitant of removing my truck start battery terminals at all for this reason...The RV TRAILER side batteries is a totally different story for me...Roy Ken

Disconnecting positive instead of negative at battery

22 Replies

MEXICOWANDERER
Explorer
Sep 09, 2013
The pictured "disconnect" utter garbage. First of all, if you have a moment, GOOGLE metal electrical conductivity and note the difference between "brass" and copper. Brass and lead antimony hate each other. It's the zinc that causes discord in metallurgy for the conductivity issue. Lost count but a lot of my "BOOM THUD" audio car, alternator repairs were caused by exactly this kind of terminal. The car audio gizmos drew a horrendous amount of quiescent power draining the battery. The owner installed a "handy" disconnect. The only problem was the "disconnect" happened when it shouldn't have and blew a hundred eighty dollar alternator to smithereens. I refused to work on the car until the "disconnect" went to the recycler and a proper disconnect switch was installed.
Bird_Freak
Explorer II
Sep 09, 2013
Snowman9000 wrote:
smkettner wrote:
Snowman9000 wrote:
But if I was to install a disconnect switch on the positive side, properly isolated, it will be good, won't it? I mean, the battery is just as surely disconnected by a switch at the positive as at the negative. ??? Like I said, I feel stupid asking this, but it's cheaper than a transmission. :)

Switch on the positive is fine. Since it is on the positive I recommend an insulated switch vs an exposed knife switch. Terminal switch with the knob is perfect.

That would fit nicely contained within the plastic battery box with removable plastic lid. However on Amazon I read reviews of various offerings of that disconnect and it sounds pretty sketchy. Even to the point that it can fail and reestablish a connection. Which would be disastrous.

So my options are either a knife switch or re-doing the cabling to accommodate a marine type switch. The cabling is funky and I would prefer to leave it alone. On the positive cable, there is a pair of ordinary primary wires, like 14-18 gauge, running along the cable and then crimped with the cable inside the post clamp. They spiral taped over the cable and 2 wires with electrical tape. If I didn't know better I would think I'm looking at an RV, lol.

The negative cable has a big plate on the post end, mounting some sort of little fuse or something, with wires coming off of it. It's so buried that I haven't looked too closely.

But if I have to take out one of the cables and rework it, I will. In the overall scheme of things it won't be a big deal. OTOH if I can safely use a knife switch, I will do that.
It doesn't matter which side you disconnect. We refuse to use this kind of switch on the cars we build. Too much arching with them. use a lever type swith as used for marine and racing app's. Positive on and off.
noe-place
Explorer
Sep 09, 2013
Argosy24 wrote:
2oldman wrote:
Is anyone actually reading the question?

Apparently not.

Put your switch in the positive lead. An open is open, it doesn't matter where it's at.

I wonder how many times I've disconnected/reconnected these things over the years and how many different vehicles with no trouble doing it either way. :h
Snowman9000
Explorer
Sep 08, 2013
smkettner wrote:
Snowman9000 wrote:
But if I was to install a disconnect switch on the positive side, properly isolated, it will be good, won't it? I mean, the battery is just as surely disconnected by a switch at the positive as at the negative. ??? Like I said, I feel stupid asking this, but it's cheaper than a transmission. :)

Switch on the positive is fine. Since it is on the positive I recommend an insulated switch vs an exposed knife switch. Terminal switch with the knob is perfect.

That would fit nicely contained within the plastic battery box with removable plastic lid. However on Amazon I read reviews of various offerings of that disconnect and it sounds pretty sketchy. Even to the point that it can fail and reestablish a connection. Which would be disastrous.

So my options are either a knife switch or re-doing the cabling to accommodate a marine type switch. The cabling is funky and I would prefer to leave it alone. On the positive cable, there is a pair of ordinary primary wires, like 14-18 gauge, running along the cable and then crimped with the cable inside the post clamp. They spiral taped over the cable and 2 wires with electrical tape. If I didn't know better I would think I'm looking at an RV, lol.

The negative cable has a big plate on the post end, mounting some sort of little fuse or something, with wires coming off of it. It's so buried that I haven't looked too closely.

But if I have to take out one of the cables and rework it, I will. In the overall scheme of things it won't be a big deal. OTOH if I can safely use a knife switch, I will do that.
time2roll
Nomad
Sep 08, 2013
Snowman9000 wrote:
But if I was to install a disconnect switch on the positive side, properly isolated, it will be good, won't it? I mean, the battery is just as surely disconnected by a switch at the positive as at the negative. ??? Like I said, I feel stupid asking this, but it's cheaper than a transmission. :)

Switch on the positive is fine. Since it is on the positive I recommend an insulated switch vs an exposed knife switch. Terminal switch with the knob is perfect.
YC_1
Nomad
Sep 08, 2013
I own a service center dealing with 12 volts every day in every type of vehicle known to exist. Go ahead and put the switch in the positive lead. You already understand any hazards. There are some excellent automotive/marine switches available that will easily handle the current.
mena661
Explorer
Sep 07, 2013
I 100X recommend a disconnect switch. If you want to be fancy, get a remote on/off one from Blue Sea.
Bird_Freak
Explorer II
Sep 07, 2013
Get a master disconnect from Summit racing, mount it anywhere you want and be done with it. No ware and tear on clamps or cables.
Argosy24
Explorer
Sep 07, 2013
2oldman wrote:
Is anyone actually reading the question?

Apparently not.

Put your switch in the positive lead. An open is open, it doesn't matter where it's at.
Snowman9000
Explorer
Sep 07, 2013
I found a good writeup, with photos, by a poster at the FMCA forum. He put a disconnect at the positive battery post. It is fully contained within the plastic battery box and fits under the plastic lid. I'm going to do the same thing.