1. Significant Capacity Decline
① During overcharging, excessive chemical reactions occur inside the battery. The active material, lead dioxide, on the positive plate gradually becomes loose and loses its original structural stability, reducing its ability to participate in electrochemical reactions.
② The active material, spongy lead, on the negative plate also experiences a coarsening of particles due to overcharging, reducing the contact area with the electrolyte and significantly decreasing the battery's capacity to store and release electrical energy, thereby shortening its effective service life.

2. Severe Shortening of Service Life

① Under normal circumstances, lead-acid batteries have a certain number of charge-discharge cycles (typically several hundred). Overcharging accelerates the corrosion of the plates and the shedding of active materials. Each overcharging event causes irreversible damage to the plates.
② Long-term overcharging will gradually thin the plates and even cause them to break, resulting in the battery being unable to function properly long before it reaches its normal cycle count, significantly reducing its service life.

3. Battery casing deformation and leakage

① During overcharging, a large amount of hydrogen and oxygen is produced inside the battery (the electrolysis of water intensifies), and the gases accumulate inside, causing a sharp increase in internal pressure.
② When the pressure exceeds the limit that the battery casing can withstand, it will cause the casing to expand and deform. In severe cases, it may even lead to the casing cracking, resulting in the leakage of the electrolyte (sulfuric acid solution). The leakage of the electrolyte not only damages the equipment around the battery but also has corrosive properties, which may cause harm to the human body.

4. Trigger safety hazards, even explosions

① Hydrogen is a flammable and explosive gas. When the hydrogen concentration inside the battery reaches a certain proportion and encounters a spark (such as an electric spark generated by an internal short circuit of the battery), an explosion is highly likely to occur.
② Overcharging may also cause the battery temperature to rise sharply, triggering a thermal runaway phenomenon. High temperatures will further accelerate the chemical reactions and gas production inside the battery, creating a vicious cycle. Eventually, it may lead to serious safety accidents such as battery combustion and explosion, endangering personal and property safety.