The reasons for the failure of VRLA batteries are many, and the marine environment is complex and changeable. The main forms of failure of VRLA batteries on ships include thermal runaway, water loss, grid corrosion, short circuit, sulfate formation, and plate corrosion.
The environment temperature has the most obvious influence on VRLA battery capacity. Because the temperature difference is large in different navigational areas, and the temperature difference between the engine room and the deck is also large, the working environment of VRLA batteries on ships is harsh, which is the main reason for the shortened service life.
The relationship between battery service life and temperature is related to temperature, activation energy, Boltzmann constant, and constant factors. It can be seen from this that the battery has the longest service life and the most stable performance when used in an environment of 22℃-25℃. Either too high or too low temperature is not suitable for the use of VRLA batteries. In a low-temperature environment, the activity of the electrolyte decreases, the viscosity increases, the internal resistance of the battery increases, and the capacity decreases. When the environmental temperature is too high or the charging current is too large, the temperature of the electrolyte will increase, and the internal resistance will decrease. At the same time, the decrease in internal resistance will increase the charging current, and so on. In this way, the life of the battery will quickly decrease.
On ships, VRLA batteries used for small emergency lighting are generally stored in special battery compartments with poor ventilation, while VRLA batteries used for starting are generally stored in the engine room. Due to changes in the main engine operation and navigation areas, the environmental temperature can sometimes reach 50℃. During the charging process, the violent movement of molecules in the electrolyte will cause the battery to heat up, and the temperature will be even higher, causing serious shedding of active substances on the grid plate and affecting the service life.
The reasons for the water loss of batteries on ships are mainly due to inadequate sealing of the battery resulting in oxygen not reaching the negative electrode to react with the lead but escaping through the safety valve; improper design of the exhaust valve pressure resulting in frequent action and water loss. When the ship is sailing, the impact caused by the shaking of the hull and the vibration of the shaft system etc. will cause the electrolyte to overflow. In severe cases, it will cause grid pole corrosion and short circuit; corrosion and water loss of the positive grid plate; overcharging during maintenance causes the electrolyte H2O to undergo electrolysis and generate gas that escapes; water loss caused by self-discharge.
From the chemical reaction formula of batteries, it can be seen that during discharge, both the positive and negative poles generate PbSO4. During normal operation, the negative electrode will produce small particles of PbSO4, which will immediately transform into sponge-like lead during charging. However, when the battery is stored for a long time, the depth of discharge is frequently too high, and the charging is insufficient for a long time. Coarse and hard PbSO4 particles will be formed and attached to the negative electrode, reducing the amount of active material and hindering the contact between the electrolyte and the deep active material, resulting in a decrease in battery capacity.
The positive and negative plates of VRLA batteries are made of ultra-fine fiberglass, which has the function of preventing the shedding of active material and short circuit. However, if the assembly is improper, or if the crystals penetrate, the positive and negative pole plates will be connected, causing a short circuit. In addition, glass fibers cannot completely prevent the shedding of active substances, causing free active substances to float inside the battery and gradually settle between the plate lugs and sides of the positive and negative pole plates during the charging and discharging process.
Most of the grid used in VRLA marine batteries are made of lead alloy, which has strong corrosion resistance. However, it still corrodes in the H2SO4 environment for a long time. Water loss directly leads to an increase in the specific gravity of the electrolyte, an increase in acidity, and a deterioration of the grid plate environment. The corrosion resistance of the positive grid plate is closely related to the composition and properties of the grid alloy. The use of low antimony silver series corrosion-resistant positive grid plates can greatly reduce the corrosion impact on the battery.