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The connection of the negative pole to the metal or body is required when you are charging the car battery from another car
In that car if the Charging car is OFF and you connect the battery terminals from charging car to your car there will be huge flow of current and the charging car battery will be drained and there might be chances of explosion
Safest is to connect the terminal to metal body of the car but charging in this way requires a huge amount of time
If the caharging car is ON then you can connect the negatives and positives of the battery directly