A cell phone charger typically draws a current of around 1 ampere (A). A car battery has a capacity of around 40 ampere-hours (Ah), which means that it can deliver 40 A for 1 hour, or 20 A for 2 hours, and so on.
Therefore, a cell phone charger would take around 40 hours to discharge a car battery completely. However, most cell phone chargers will automatically turn off when the battery is full, so it is unlikely that the charger would ever actually discharge the battery completely.
In addition, most cars have a voltage regulator that prevents the battery from being overcharged. This means that even if the cell phone charger were to somehow continue to charge the battery after it is full, the voltage regulator would prevent the battery from being damaged.
Therefore, it is very unlikely that a cell phone charger would kill a car battery.
