Battery disconnects go on the positive side as close to the battery as possible.
With some switches you find may have a "field disconnect" That is for use in a automotive style charging system. The basic switch is the same and can be used.
Higher quality switches will have a make before break connection. This will keep your circuit live while you switch between 1-2 or both. Meaning that if battery #1 is connected and you switch to battery #2, #1 will not be disconnected until the connection is made with #2
If just disconnecting the battery for maintenance or repairs etc, the negative comes off first. Prevents you from causing a direct short with a wrench while removing the positive. Negative is last on for same reason.
Remove The Negative Cable First,
Re-connect The Negative Cable Last:
Why? Because the wrench or socket is touching the live part of the electrical connector. There is a good chance that the wrench or socket handle will accidentally touch something. The entire car is connected to the negative terminal.
If your wrench is on the positive terminal and it accidentally touches anything metal, you will short circuit the battery. The voltage isn't harmful, but the sudden unexpected sparks will startle the **** out of you, and could even burn you. There is so much current (amperage) available that your wrench literally becomes an arc welder.
If you disconnect the negative cable first, and reconnect it last, then the car is not electrically connected to the negative battery terminal. After that you can disconnect the positive battery terminal with minimal risk, because if your wrench touches any metal parts of the car there is no complete circuit, and nothing happens. The only risk comes from touching the other (i.e. negative) battery terminal.
While connecting or disconnecting the negative cable, you don't need to worry about the wrench touching metal parts of the car, because everything is at the same electrical potential. You only need to keep the wrench from touching the positive battery terminal. That's easy.
When you boost a car with jumpers or booster cables the ground is what is the most dangerous connection.
The batteries will blow up with ground problem as well.
Either connection can be dangerous. It is best to not connect to a batteries post when jumping if possible.
The ground lead should not be attached to the battery.
Jump start:
positive to dead battery positive post
positive to booster positive post
Negative to booster negative post
Last connection and the one that can cause your problems;
negative to an unpainted frame or bolt (dead battery). This keeps any potential sparks away from the battery.
Some vehicle (trucks) may have separate posts just for this.
After jump - reverse the process.
Batteries "blowing up" are rare.
I have seen at least a half dozen blow the top off in my career. Including a 36 volt forklift battery.
I have seen 2 blow up in the face of the mechanic - not a pretty sight.
BTW - batteries going dead because they are left on a concrete floor is an old wives tale.