What can overcharging a battery cause?

Overcharging a battery can lead to overheating and gassing, which is the correct outcome to consider. When a battery is overcharged, it receives more voltage than it can handle, causing excessive current to flow. This excess current generates heat, which can raise the battery temperature significantly. As the temperature increases, the electrolyte within the battery may start to break down, leading to a process called gassing where hydrogen and oxygen are released. This not only reduces the efficiency of the battery but can also pose safety hazards, as hydrogen is flammable and can lead to explosive situations if not properly vented.

The other options focus on effects that are less relevant to the immediate consequences of overcharging. Lower efficiency occurs as a result of the damage caused by gassing and overheating, while water retention and healing of battery plates are not direct consequences of overcharging and may contribute to an overall misunderstanding of the chemical and physical reactions occurring in a lead-acid battery under stress. Therefore, overheating and gassing stand out as the most direct and critical consequences of overcharging a battery.