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7.1 LEAD ACID BATTERY – BASIC DESCRIPTION AND ELECTRO-
CHEMICAL REACTIONS
7.1.1 A Lead Acid battery consists of a number of 2 V nominal cells (actual voltage of
the cell is around 2.105 V) that are connected in series e.g. a 12 V nominal battery will
have six, 2 V nominal cells in series (actual approximate voltage of the 6 cells will be 2.105
x 6 = 12.63 V). Each 2 V nominal cell in this battery consists of an independent enclosed
compartment that has Positive and Negative Plates (also called Electrodes) dipped in
electrolyte that is composed of diluted Sulphuric Acid.
7.1.2 A fully charged Lead Acid Battery comprises of (i) Positive Plates: Lead Dioxide,
(ii) Negative Plates: Sponge Lead and (iii) Electrolyte: Mixture of 65% water and 35%
Sulfuric Acid with Specic Gravity = 1.265 at Standard Room Temperature of 77°F / 25°C
(Fully charged condition). During discharging, electro-chemical reactions lead to: (i) At
Positive Plates: Conversion of Lead Dioxide to soft Lead Sulfate crystals,
(ii) At Negative Plates: Conversion of Sponge Lead to soft Lead Sulfate crystals and (iii)
In Electrolyte: Conversion of portion of Sulfuric Acid to water leading to reduction in
Specic Gravity (1.120 for fully discharged condition).
7.2 TYPES OF LEAD ACID BATTERIES
7.2.1 Sealed Lead Acid (SLA) Or Valve Regulated Lead Acid (VRLA) Batteries: These can
either be Gel Cell or AGM (Absorbed Glass Mat). In a Gel Cell battery, the electrolyte is in
the form of a gel. In AGM (Absorbed Glass Mat) battery, the electrolyte is soaked in Glass
Mat. In both these types, the electrolyte is immobile. There are no rell caps and the
battery is totally sealed. Hydrogen and Oxygen released during the charging process is
not allowed to escape and is recombined inside the battery through use of Recombinant
Catalyst (s). Hence, there is no water loss and the batteries are maintenance free. These
batteries have safety valves on each cell to release excessive pressure that may be built
up inside the cell. The Gel Cell is the least affected by temperature extremes, storage at
low state of charge and has a low rate of self-discharge. An AGM battery will handle
overcharging slightly better than the Gel Cell.
7.2.2 Non Sealed (Vented / Flooded / Wet Cell) Lead acid Batteries: In these batteries,
each individual cell compartment has a rell cap that is used to top up the cell with
distilled water and to measure the specic gravity of the electrolyte using a hydrometer.
When fully charged, each individual cell has a voltage of approximately 2.105 V and
electrolyte specic gravity of 1.265. As the cell discharges, its voltage and specic gravity
drop. Thus, a healthy, fully charged, 12 V nominal battery with each of the 6 cells
SECTION 7 | General Information - Battery Related
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