What happens between normal charge and discharge cycles of a lead acid battery?

It is easy to charge Sealed Lead Acid (SLA) batteries, but how to maximize the battery life through chargers? It is said that checking and maintaining the battery on a regular basis may prolong its life. Therefore more solves are considered to maximize its life and let the investment get the value. Simple constant current or constant voltage chargers will do the job for a while, but the battery life expectancy quoted by the manufacturer will be greatly reduced by using non-intelligent chargers like this. Before looking at the different charging techniques it is important to understand the lead acid battery chemistry and what happens between normal charge and discharge cycles.
Typically the positive plates in an SLA battery are made from lead dioxide and the negative plates from a sponge lead. The electrolyte is usually sulphuric acid mixed with a gelling agent and is largely absorbed and held by insulating separators between the plates.
When an SLA battery is being discharged; the lead (Pb) on the negative plate and the lead dioxide (PbO2) on the positive plate are converted to lead sulphate (PbSO4). At the same time the sulphuric acid (H2SO4) is converted to water (H2O).
In a normal charge, the chemical reaction is reversed. The lead sulphate and water are electro-chemically converted to lead, lead dioxide and sulphuric acid. During a full charge cycle any gasses produced need to be re-combined in a so called ‘oxygen cycle’. Oxygen is generated at the positive plates during the latter stages of the charge cycle, this reacts with and partially discharges in the sponge lead of the negative plates. As charging continues, the oxygen produced also re-combines with the hydrogen being produced on the negative plate forming water. With correct and accurate cell voltage control all gasses produced during the charge cycle will be re-combined completely into the negative plates and returned to water in the electrolyte.
If an SLA battery is over-charged, the excess cell voltage will result in the conversion of electrolyte into large amounts of hydrogen and oxygen gasses which cannot be recombined by the normal processes. A pressure-release valve will open and vent the excess gas, resulting in the loss of electrolyte and a loss of capacity.
If the battery is undercharged; the low cell voltage will cause the charge current to diminish to zero well before full capacity is reached. This will allow some of the lead sulphate produced during discharge to remain on the plates, where it will crystallise, which also causes a permanent loss of capacity.
It is also important to remember that SLA batteries have a self discharge rate of approximately 5% per month. This is less than most other forms of rechargeable batteries, but has to be considered. Manufacturers recommend recharging when the battery reaches about 70% of its capacity (approximately 2.1 volts per cell). They use this to calculate the maximum life of the battery, but this is very difficult to implement in a real world application.
If you know this chemical principle, you are able to choose the best charger for your lead acid battery though there are many batteries available in the market.
Source: www.leoch.com