Observing these precautions will help avoid damage to the vehicle's electrical system and ensure safe handling of the system components:
Be absolutely sure of the polarity of a booster battery before making connections. Connect the cables positive-to-positive, and negative-to-a good ground. Connect positive cables first and then make the last connection to an engine ground on the vehicle with the dead battery so that arcing cannot ignite hydrogen gas that may have accumulated near the battery. Even momentary connection of a booster battery with the polarity reversed will damage alternator diodes.
Disconnect both vehicle battery cables before attempting to charge a battery.
Never ground the alternator output or battery terminal. Be cautious when using metal tools around a battery to avoid creating a short circuit between the terminals.
Never run an alternator without load unless the field circuit is disconnected.
Never attempt to polarize an alternator.