|Dear Ole Mechanic:
Having had problems in the past with batteries discharging, I've gotten into the habit of installing battery disconnects on my tractors. I've always installed a disconnect on a battery's positive pole. Recently, a long-time tractor mechanic who writes regularly for one of the antique tractor magazines stated that he puts a disconnect on a battery's negative (ground) pole. So, which is correct? Or doesn't it make any difference? Should there be disconnects on both poles?
All service manuals say to disconnect the ground side of the battery. The reason for that is so that if the wrench slips and falls to some grounded metal, there won't be sparks. If you remove the hot side and the wrench contacts the hot side of the battery to grounded metal, there are going to be a lot of sparks--much like arc welding. There is enough amperage in a vehicle's battery to weld things together, as it can produce several hundred amps. Ninety Amps with 12 volts is about all you use weld with a one-eighth-inch welding rod.
Most all 12 volt systems have a negative ground, so the disconnect will normally go on the negative side. Six volt systems can be either positive or negative ground. However, if the ground is hard to reach, go ahead and put it on the hot side. Just be verrrrrry careful while installing it. Just to be on the safe side, remove the gorund connection before you install the disconnect on the hot side.