Cells and Batteries
Overview
Cells and Batteries is a core topic in ITI Electrician Semester 1 (NCVT). It covers the conversion of chemical energy into electrical energy, types of cells (primary and secondary), their construction, EMF, internal resistance, capacity, and maintenance. Understanding Leclanche cells, lead-acid batteries, and Ni-Cd cells is essential. Questions on terminal voltage, capacity in Ah, and charging methods are frequently asked in NCVT theory and practical papers.
Typed Notes
Primary Cells – Types and Working
Primary cells produce electricity through irreversible chemical reactions and cannot be recharged. The most common is the Leclanche cell (dry cell): zinc (−) anode, carbon rod (+) cathode, electrolyte is ammonium chloride paste, depolariser is manganese dioxide (MnO₂). EMF ≈ 1.5 V. Shelf life is limited. Mercury cells give stable 1.35 V and are used in hearing aids. Button cells are used in watches and calculators. Key exam point: polarisation (hydrogen gas formation reducing EMF) is prevented by the depolariser MnO₂.
Secondary Cells – Lead-Acid Battery
Lead-acid batteries are the most widely used secondary cells. Each cell gives 2 V; a 12 V battery has 6 cells in series. Positive plate: lead peroxide (PbO₂), brown colour. Negative plate: spongy lead (Pb), grey colour. Electrolyte: dilute sulphuric acid (H₂SO₄). During discharge, both plates convert to lead sulphate (PbSO₄). During charging, plates revert to original state. Specific gravity of electrolyte is checked with a hydrometer: fully charged = 1.28, fully discharged = 1.18. Never discharge below 1.75 V per cell (over-discharge damages plates).
Nickel-Cadmium (Ni-Cd) and Alkaline Cells
Ni-Cd cells are secondary cells with positive plate of nickel hydroxide, negative plate of cadmium, and electrolyte of potassium hydroxide (KOH – alkaline). EMF per cell ≈ 1.2 V. Advantages: longer life, can withstand overcharging, works in extreme temperatures, low maintenance. Used in emergency lighting, portable tools. Alkaline cells (primary) use MnO₂ as cathode, zinc as anode, KOH as electrolyte; EMF 1.5 V with longer life than Leclanche. Key exam tip: Ni-Cd electrolyte does NOT change specific gravity during charge/discharge, unlike lead-acid.
Battery Maintenance, Charging and Connections
Regular maintenance of lead-acid batteries includes checking electrolyte level (top up with distilled water only, never tap water or acid), checking specific gravity with hydrometer, cleaning terminals with baking soda solution to remove sulphation. Two charging methods: (1) Constant current method – charging current kept constant throughout; (2) Constant voltage method – voltage kept constant, current decreases as battery charges. Gassing (bubbling) indicates full charge. Cells in series increase voltage; cells in parallel increase current capacity. Always connect positive to positive and negative to negative when connecting batteries in parallel.
Topics & Sub-topics
Har topic ke saath sub-topics aur example — chapter ka complete coverage.
01Cell Fundamentals
- Primary (non-rechargeable) vs secondary (rechargeable)
- EMF and internal resistance
- Capacity in ampere-hour (Ah)
100 Ah battery 5 A par theoretically 20 ghante chalegi (Peukert effect se thoda kam).
02Lead-Acid Battery
- Plates: PbO₂ (+), Pb (−), electrolyte dilute H₂SO₄
- Cell voltage ≈ 2 V, SG 1.21-1.28
- Charging: constant current/voltage
Inverter battery ke 6 cells × 2 V = 12 V; SG girti hai to discharge hua hai.
03Other Battery Types
- Ni-Cd, Ni-MH
- Lithium-ion — high energy density
- Maintenance-free batteries
EV aur inverter ab Li-ion par — 3.7 V per cell, BMS zaroori.
04Battery Banks and Maintenance
- Series = voltage add, parallel = Ah add
- Topping with distilled water
- Terminals clean + petroleum jelly
12 V chahiye to do 6 V battery series mein; terminals par petroleum jelly se corrosion rukta hai.
Key Concepts
Revision ke liye ek nazar — exam se pehle yahi padhna hai.
- 01A PRIMARY cell cannot be recharged (e.g., Leclanche/dry cell, EMF ≈ 1.5 V); a SECONDARY cell can be recharged (e.g., lead-acid, EMF ≈ 2 V per cell).
- 02Lead-acid battery: positive plate = PbO₂ (lead peroxide), negative plate = Pb (spongy lead), electrolyte = dilute H₂SO₄ (specific gravity 1.28 when fully charged, 1.18 when discharged).
- 03Battery capacity is measured in Ampere-hours (Ah); Capacity = Discharge current (A) × Time (hours). A 100 Ah battery can supply 5 A for 20 hours.
- 04Terminal voltage (V) = EMF (E) − I × r, where I = current and r = internal resistance. During charging, terminal voltage = E + I × r.
- 05Cells in SERIES: total EMF adds up, same current. Cells in PARALLEL: same EMF, current capacity adds up, internal resistance reduces.
Questions & Solutions
Pehle khud try karo — phir solution kholo. Yahi tariqa topper banata hai.
What is the specific gravity of a fully charged lead-acid battery electrolyte, and how is it measured?
Differentiate between primary and secondary cells with one example each.
A battery has an EMF of 12 V and internal resistance of 0.5 Ω. If it supplies a current of 4 A, what is the terminal voltage?
What is the capacity of a battery and what is its unit? If a battery is rated 120 Ah, how long can it supply 6 A?
State the function of MnO₂ in a Leclanche (dry) cell. What is polarisation?
Primary cells (Leclanche/dry cell, 1.5 V) are non-rechargeable; secondary cells (lead-acid 2 V/cell, Ni-Cd 1.2 V/cell) are rechargeable. Lead-acid battery uses PbO₂, Pb plates and H₂SO₄ electrolyte; specific gravity 1.28 (full) to 1.18 (discharged). Capacity is in Ah. Terminal voltage = EMF − I×r. Series connection increases voltage; parallel connection increases current capacity.
Chapter-end quiz
4 MCQsOption tap karo — turant pata chalega sahi ya galat, explanation ke saath.
Q1.What is the EMF of a single lead-acid cell?
Q2.Which electrolyte is used in a Nickel-Cadmium (Ni-Cd) cell?
Q3.When cells are connected in PARALLEL, which quantity increases?
Q4.What liquid should be added to a lead-acid battery when the electrolyte level is low?
Mark karo — dashboard par progress dikhega.