What is the best method for fully charging a completely discharged 12-volt battery?

The best method for fully charging a completely discharged 12-volt battery is through a slow charge for 12 hours. Slow charging is a more gentle process that helps to safely replenish the battery's voltage and capacity without overheating it or causing damage. This method allows for proper chemical reactions within the battery, promoting better overall health and longevity.

When charging a battery, especially one that is completely discharged, it is crucial to maintain a controlled and steady charge to avoid the risk of overcharging, which can lead to increased heat and gas production. A slow charger provides a low current to the battery, which is ideal for a deep cycle of charging without causing stress to the battery’s internal components.

In contrast, faster methods of charging, such as those described with idling the engine or racing the vehicle's engine, may not provide a sufficient charge to fully restore a completely drained battery, and they possess the risk of overheating or damaging the battery. Fast charging for a short duration also does not allow enough time for the battery to reach full capacity. Thus, the slow charge offers a much more reliable and effective approach to bringing a fully discharged 12-volt battery back to full operational status.