The most common failure modes are:

- Softening or shedding of the active material.

 During discharge the lead oxide (PbO2) of the positive plate is transformed into lead

sulfate (PbSO4), and back to lead oxide during charging. Frequent cycling will reduce cohesion of the positive plate material due to the higher volume of lead sulfate compared to lead oxide.

 Corrosion of the grid of the positive plate.

 This corrosion reaction accelerates at the end of the charge process due to the, necessary, presence of sulfuric acid.

Sulfation of the active material of the negative plate.

During discharge the lead (Pb) of the negative plate is also transformed into lead sulfate (PbSO4). When left in a low state-of-charge, the lead sulfate crystals on the negative plate grow and harden and form and impenetrable layer that cannot be reconverted into active material. The result is decreasing capacity, until the battery becomes useless.

Water loss:

lead-acid batteries in the charging and discharging process, due to the presence of overpotential, gas in the charging and discharging process, it is characterized by electrolyte movement of the electrolytic use of gas formation caused by; 2V monobloc batteries, for example, the battery charging to reach monobloc batteries 2.35v (25 ℃), it will enter the positive plate a large number of precipitation of oxygen state, on the sealing of the battery, the negative plate has the ability of the oxygen composite. If the charging current is relatively large, the negative plate oxygen composite reaction can not keep up with the rate of oxygen precipitation, the gas will top open the exhaust valve and cause water loss. If the charging voltage reaches 2.42v (25℃), the negative plate of the battery will precipitate hydrogen, which cannot be absorbed by the positive plate like the oxygen cycle, but can only increase the air pressure in the gas chamber of the battery, and will eventually be discharged from the gas chamber, resulting in water loss. Battery water loss is usually particularly serious in the case of overcharging.

Thermal runaway

There are two important cases of thermal runaway of batteries, one is lead-acid batteries in constant voltage charging battery heating. Under the condition of constant voltage charging, the oxygen cycle current is also involved in the charging current, so the charging current drop rate slows down. And the lead-acid battery heating will cause the charging current decline rate more slowly, or even the current counter rise. And charging current in the use of battery heating, once the current counter rise, and added heating. In this way, the charging current will keep rising to the current limit value. The battery generates high heat, and accumulates heat, until the battery case is softened and deformed by heat. And when the battery is thermally deformed, the internal air pressure is high, so the battery is presented as a bulging state. This is the battery thermal runaway and damage to the battery. Another reason is sulfation, sulfation directly lead to new battery internal resistance, which further causes lead-acid battery charging heat, heat and oxygen cycle current rise, so the sulfation of serious battery, thermal runaway occurs a high probability. The internal temperature of the battery rises, self-discharge is also large, the emergence of heat is higher. Therefore, under the condition of high ambient temperature in summer, the temperature rise is also high at the same time due to the decrease of dialysis gas level. This makes the probability of a colloidal lead-acid battery going into thermal runaway much higher.