There is equal risk with either connection, it depends on which one you connect last. As the last connection is made, there will likely be some sparks which could ignite the gasses. Since the negative terminal in a car is normally also connected to the frame of the car and all metal parts that connect to the frame, I usually recommend that the second negative connection be made to the frame of the car somewhere away from the battery. That way sparks are not likely reach the area around the battery. By the way, it is good to be aware of this and to be careful, but it is extremely uncommon for an actual explosion to occurr.
Source(s):
Personal Experience


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