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Why Lead Acid Batteries Fail or Suffer Reduced Performance

Electrolyte Loss

High temperatures, high charging rates, and over charging can cause a loss of electrolyte in non sealed batteries. In sealed batteries, the same factors will cause an increase in temperature and pressure which can eventually result in the release of gas (and possibly electrolyte) from valves.
Any loss of electrolyte resulting in part of the plates being above the electrolyte surface will result in reduced battery performance.

Sulphation

As a lead acid battery discharges, lead sulphate crystals are deposited on the plates as part of the normal chemical reaction that results in the flow of electrons (at the same time, the sulphuric acid electrolyte is being converted to water). During charging, the chemical reaction is reversed and the lead sulphate crystals are converted back to lead on the negative electrode and lead oxide on the positive electrode.
If however, the battery is left for sometime not fully charged, or is in use but not reaching a fully charged state, the lead sulphate crystals will harden and will not convert back to lead or lead oxide during charging. This effect will occur more quickly at higher temperatures.
Once this happens, the capacity of the battery will be reduced.

Electrolyte Stratification

The electrolyte in a lead acid battery consists of water and sulphuric acid which ideally would be thoroughly mixed. However, as sulphuric acid is more dense than water, a situation can arise where the acid concentration is higher at the bottom of the battery than at the top. This situation will reduce the performance of the battery.
Stratification can develop if a battery is not being fully charged (when a battery is fully charged some gassing will cause a mixing of the electrolyte) and if the battery is static (no movement to help mixing).
Stratification itself will not have a permanent effect on a battery's performance. A single full charge should sufficiently mix the electrolyte, although a battery left in a stratified state may result in sulphation where the acid concentration is high.

Separator Failure

If two battery plates come into contact, that cell will suffer a short and a 12 volt battery will effectively become a 10 volt battery.
Plate contact can happen if the separator material breaks down or plates become distorted due to excessive heat. Overcharging can cause plate distortion.

Full or Near Full Discharge

As seen above, allowing a battery to stay in a state of not being fully (or nearly fully) charged will lead to sulphation problems. What's more, fully discharging (or nearly fully discharging) can cause such changes to the battery's chemistry that it will not readily recharge.