What happens when a battery is discharged and recharged?

In charged state, the battery consists of the lead oxide and sulphuric acid mixed with water at a density of approx. 1.28.
At discharge, the lead is converted into lead sulphate (a white powder in the open air) while the sulphuric acid content decreases in the acid solution (i.e., the density drops to 1.0 = only water).

How should a lead acid battery be charged?

Different recommendations apply to the different types of lead acid batteries. As a general rule of thumb, at +25°C ambient temperature the battery can be charged with a cell voltage of 2.3V/cell (13.8V for a 12V battery). Charging voltages below 2.2V/cell (13.20V for a 12V battery) will never fully charged the battery.

In order to speed up the charging process and to create a stirring of the acid in free acid batteries, the charging voltage in the topping charge can be increased to 2.4V / cell (14.40V for a 12V battery). However, this topping charge must be carefully checked by the charging automatics and halted before overcharging and degassing occur.

At degassing a highly explosive gas forms of a mixture of hydrogen and oxygen.
The limit for the gasification threshold drops at a higher ambient temperature and increases at a lower temperature. A 12V battery can be charged with max 14.04V at +45°C and max 14.85V at +/- 0°C

For tubular cells, a more complex charging process applies where charging voltages up to 2.7V/cell is required for proper stirring. The tubular design , often with tall plastic containers, may otherwise produce a layer formation in the container. Too high density in the bottom and too low in the top (where you usually also measure). Usually, cellular cell batteries are used in fork lifts where 2- or 3-shift use causes the battery to be exposed to short intermediate charges during breaks. Therefore, charging automation must ensure that equalization is performed as often as possible ( usually during weekends ).