Charge time for a dead battery can take up to 12 hours or longer. There are many factors that determine how long the charge will take, including the size and age of the battery.
The two common ways to charge a battery are fast and slow charges. The preferred method is a slow charge as it allows the battery to safely regain all its power.
It is generally safer to slow charge a battery. Slow charging requires the charger to be set at a consistent 10 amps. Slow charges are recommended for batteries that are to be used long-term.
A fast charge is when the charger is set to 20 amps. Although the batteries are charged in significantly less time, fast charging can cause them to overcharge.