Electrical Engineering ⇒ Topic : Indications of a Fully Charged Lead-acid Cell

Indications of a Fully Charged Lead-acid Cell

During the charging process, it is very essential that the battery is taken out from the charging

circuit as soon as it is fully charged. Overcharging as well as undercharging are undesirable and

should always be avoided. The indications of a fully charged cell (or battery) are

(i) Voltage

(ii) Specific gravity of electrolyte

(iii) Gassing 

(iv) Colour of plates

(i) Voltage. During charging, the terminal potential of a cell increases and provides an indication to the state of charge. A fully charged lead-acid cell has a terminal voltage of about 2.1 volts.

(ii) Specific gravity. During the charging process, the specific gravity of the electrolyte (H₂SO₄) increases and provides an important indication to the state of charge of the cell. The specific gravity of the electrolyte of a fully charged lead-acid cell is about 1.28. This can be measured by means of a hydrometer See Fig. (a). It consists of a long glass tube fitted with a thin rubber hose at its lower end and a rubber bulb at the upper end. Inside the tube is sealed glass float weighted at one end to make it float upright. By inserting the hose into the cell and then operating the bulb, a quantity of electrolyte (H₂S0₄) may be drawn into the glass tube. The depth to which the float sinks in the drawn liquid indicates the specific gravity of the electrolyte. If the float sinks low, the specific gravity of the liquid (i.e. electrolyte H₂SO₄), is low. If it floats high, the specific gravity is high. Hydrometer float is marked with a scale starting with 1 and extending to 1.3. When the cell is fully charged, the hydrometer reading approaches 1.28 mark

                                                                     Figure (a)

In practice, the state of charge of a lead-acid cell (or battery) is determined from the specific gravity of the electrolyte H₂SO₄ .The following table m av he useful in this regard

(Ill) Gassing. When the cell is fully charged, the charging current starts electrolysis of water.The result is that hydrogen is given off at the cathode and oxygen at the anode; the process being known as *gassing. Gassing at both the plates indicates that the charging current is doing no useful work and hence should be stopped.

(iv) Colour of plates. The visual examination of colour of the plates of a lead-acid cell provides yet another important indication to its state of charge. When the cell is fully charged, the positive plate gets converted into Pb0₂ (chocolate brown) and the negative plate to spongy lead (grey).