Why Disconnect Negative Terminal First. The reason they say to remove the negative first is because it is safer for you. So if you disconnect the positive terminal and then for example accidentally touch a spanner between the positive terminal and the chassis you will create a short-circuit.
Connect positive first negative having less potential wont arc. Connect the new battery in the reverse order positive then negative. Connect negative fisrt positive can arc and fuse.
The negative battery terminal is your ground which keeps you from getting shocked when working on the engines the electrical and the fuel parts of your car.
Which terminal to disconnect first the grounded one or the ungrounded one is primarily a safety concern of the service technician. The main reason for disconnecting the negative terminal on a car battery first is that the whole of the car body is linked to that negative terminal. When you remove a battery from a car you should ALWAYS disconnect the negative terminal first and then the positive. When disconnecting the cables from the old battery disconnect the negative first then the positive.
