To understand what happens when battery cables touch, you need to know about the flow of electric current. In any circuit external to a battery, direct current flows from the positive post of the battery, through the circuit, and back to the negative post. The current -- not the movement of electrons, then moves back to the battery's positive post, completing the circuit.
Jumping someone else's battery in the middle of a lazy river depends on the cables touching. You connect the positive posts of both batteries with the jumper cable, then you connect your battery's negative post to the negative post of your fellow boater's dead battery. The current doesn't just flow to the other battery, it flows from the positive post of your battery to the positive post of the other battery. It then completes the internal circuit through the other battery and back to your battery's negative post. Then, your battery's internal current moves back to the positive post of your your battery. The circuit is complete and the charging cycle begins again.
The boat battery's negative cable is connected to the boat's common ground. This is the cable to which, directly or indirectly, the negative wires of all the boat's electrical and electronic gear, including the engine's electric starter, are connected. By disconnecting the negative cable from the battery, the boat's electrical circuit is broken. Boat engine repair manuals routinely caution their readers to "remove the negative cable" from the battery before performing engine maintenance "for safety." Some tell their readers to remove the disconnected cable from the battery box and close the battery box, to prevent an accidental reconnection.
With One Cable Disconnected
If the terminal of the negative cable you disconnected to perform maintenance on your boat were to come in accidental contact with the battery's negative post, the boat's circuits would remain charged as if you had never disconnected the battery. If the terminal of one cable that's disconnected from the battery comes in contact with the terminal of the cable that's still attached to the battery post of the of the opposite polarity, a spark will jump between the terminal and the post, just like the spark of a spark plug jumps between the plug's electrodes. If the terminal remains in firm contact with the post of the opposite polarity, nothing will happen. The charge will flow from the positive post of the battery to the negative post of the battery. You will have created a short circuit that isolates the boat's circuitry, because electricity chooses the shortest path between the positive and the negative.
With Both Cables Disconnected
If both battery cables are disconnected from the battery and the terminals touch, nothing electrical will happen because the cables are mechanically isolated from an electrical source. Bang the battery cables together as you will, with the understanding that if you damage the terminals, they may require replacement.
- Electricity and Basic Electronics; Stephen R. Matt
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