Batteries as electricity sources

Every torch, remote, phone and car keeps a tiny chemical reaction sealed inside a case, ready to push electric current through a circuit the moment it is connected. That sealed reaction is a cell; wire several together and you get a battery. This section compares the three electricity sources studied in CBSE Class 8 — the voltaic (lemon) cell that demonstrates the idea directly, the everyday dry cell that trades a liquid electrolyte for a moist paste, and the rechargeable lithium-ion battery that powers devices used every day. The same chemistry underlies all three; what differs is construction, how many times a cell can be reused, and which devices each one suits. Along the way we work out how many identical cells, connected in series, are needed to reach a given voltage — the arithmetic behind lighting an LED with lemons.

The core insight

A cell converts a chemical reaction between two different electrode materials in an electrolyte into a steady voltage, and connecting several identical cells in series simply adds their voltages, $V_{\text{series}} = nV_{\text{cell}}$ , which is why devices needing more voltage than one cell can supply use several cells together as a battery.

The core idea

An electric cell produces current from a chemical reaction between two different electrodes dipped in (or packed with) an electrolyte. A battery, strictly, is two or more cells joined together — though everyday speech often calls a single dry cell a "battery" too (a "torch battery" is usually one cell). Three sources built on this idea appear in the syllabus.

The voltaic (galvanic) cell is the most direct demonstration: two different metal electrodes — commonly copper and zinc or iron — dipped straight into a liquid electrolyte. The classroom lemon cell uses citric acid inside the fruit as the electrolyte; several lemon cells wired in series can light an LED.
The dry cell is the practical, single-use, everyday version. A zinc container is the negative terminal, a carbon rod capped with a metal button is the positive terminal, and the electrolyte is a moist paste rather than a free liquid, so the cell can be carried and used in any orientation. Once its chemicals are consumed it is dead and must be replaced.
Rechargeable batteries, most often lithium-ion (Li-ion) today, reverse their chemical reaction when charged, so they can be used hundreds of times before finally wearing out.

All three rely on the same underlying trick — a chemical reaction between different materials drives current — but a single cell of a given kind delivers a fixed, fairly small voltage. When a device needs more voltage than one cell gives, or when one cell's current is too weak, several identical cells are connected in series, positive terminal to negative terminal, forming a battery. The voltages of series-connected cells simply add.

V_{\text{series}} = V_1 + V_2 + \cdots + V_n = nV_{\text{cell}} \qquad (\text{for } n \text{ identical cells}).

where: $V_{\text{series}}$ is the total voltage available across the whole chain of cells, $V_{\text{cell}}$ is the voltage delivered by one identical cell, and $n$ is the number of cells connected in series. This is exactly why a torch that needs more push than one dry cell can supply is built with two or three cells stacked end to end, and why a science-fair demonstration needs several lemons in a row to light a single LED.

Figure

Three lemon cells connected in series with copper and zinc electrodes feeding a glowing LED, each lemon cell labelled zero point nine volts and the total labelled two point seven volts

Derivation AWhy cells add up in series

We want to show, from the basic idea of a cell as a device that does work on charge passing through it, why connecting $n$ identical cells positive-to-negative in a single chain gives a total available voltage equal to $n$ times the voltage of one cell, and why this simple rule is exactly what tells a student how many lemon cells are needed to reach a target voltage such as the roughly two volts an LED needs to glow.

Step 1. A single cell's voltage, $V_{\text{cell}}$ , is defined as the work done by the cell on each unit of charge that passes through it, driven by the chemical reaction between its electrodes. A lemon cell built from copper and zinc electrodes typically delivers about $V_{\text{cell}} = 0.9\ \text{V}$ .

Step 2. When cells are connected in series — the positive terminal of one joined to the negative terminal of the next — the same charge that leaves one cell must pass straight through the next cell, and the next, before returning around the circuit. Each cell in turn does its own $V_{\text{cell}}$ worth of work on that same unit of charge.

Step 3. Because work done adds when it happens in successive stages, the total work done per unit charge by the whole chain is the sum of the individual contributions: $V_{\text{series}} = V_1 + V_2 + \cdots + V_n$ . For $n$ identical cells this sum becomes simple repeated addition, $V_{\text{series}} = nV_{\text{cell}}$ .

Step 4. Check the lemon-cell case: three identical lemon cells at $0.9\ \text{V}$ each give $V_{\text{series}} = 3 \times 0.9 = 2.7\ \text{V}$ , comfortably above the roughly $2\ \text{V}$ an LED needs to glow, while a single lemon cell at $0.9\ \text{V}$ alone would fall well short.

Worked example

Question. A student has a supply of lemon cells, each delivering $0.9\ \text{V}$ , and wants to light an LED that needs at least $2.0\ \text{V}$ to glow. How many lemon cells must be connected in series, and what is the actual voltage supplied once that minimum is reached?

Step 1. Write the series-voltage rule from above: $V_{\text{series}} = nV_{\text{cell}}$ , with $V_{\text{cell}} = 0.9\ \text{V}$ and the requirement $V_{\text{series}} \geq 2.0\ \text{V}$ .

Step 2. Try $n = 2$ : $V_{\text{series}} = 2 \times 0.9 = 1.8\ \text{V}$ , which is less than $2.0\ \text{V}$ , so two lemon cells are not enough to light the LED.

Step 3. Try $n = 3$ : $V_{\text{series}} = 3 \times 0.9 = 2.7\ \text{V}$ , which is greater than $2.0\ \text{V}$ , so three lemon cells in series do supply enough voltage.

The answer: at least 3 lemon cells are needed in series, giving a total of 2.7 V, which exceeds the LED's 2.0 V requirement and lets it glow.

Competitive stretch

If the same student instead used dry cells rated at $1.5\ \text{V}$ each for the same $2.0\ \text{V}$ LED, a single dry cell at $1.5\ \text{V}$ would still fall short, but two dry cells in series give $2 \times 1.5 = 3.0\ \text{V}$ — more than enough, and fewer cells than the lemon-cell chain needed, since each dry cell already supplies a higher voltage than a lemon cell.

Common traps

Treating "cell" and "battery" as identical words in a strict sense — a battery is properly two or more cells joined together, though casual speech often calls one dry cell a "battery."
Assuming any chemical-reaction-based source is rechargeable — only specific types, such as lithium-ion batteries, are built to be recharged; dry cells and simple voltaic cells are not.
Forgetting that series voltages only add when the cells are connected the same way round (positive to negative all along the chain) — reversing one cell in the chain works against the others and lowers the total.
Thinking "more cells always means more current" — connecting identical cells in series raises the total voltage, not automatically the current, which also depends on the resistance of the circuit.

Ask the tutor: Want to work through how many dry cells or lemon cells would be needed to light a brighter bulb instead of an LED?

Why does a single lemon cell not light an LED, but three lemon cells do?

An LED needs a minimum voltage across it, roughly $2\ \text{V}$ , before it conducts and glows. One lemon cell only supplies about $0.9\ \text{V}$ , far short of that threshold, so nothing lights up. Connecting cells in series adds their voltages: three lemon cells in a chain give about $2.7\ \text{V}$ , which clears the $2\ \text{V}$ threshold and lets the LED glow. The rule is simply $V_{\text{series}} = nV_{\text{cell}}$ — you keep adding cells until the total exceeds what the device needs.

If a battery is technically several cells joined together, why do we call a single AA cell a battery too?

Strictly, a "cell" is one unit producing current from one chemical reaction, and a "battery" is two or more cells connected together — four lemon cells in series would correctly be called a battery of lemon cells. In everyday language, though, people call almost any single cell a "battery," as in "torch battery," even when it is really just one dry cell. Both usages exist side by side: the strict scientific meaning distinguishes them, while common speech blurs the difference.

Why do phones and laptops use rechargeable batteries instead of ordinary dry cells?

Phones and laptops are used constantly, often for hours every day, and need to be powered again and again without being thrown away and replaced each time their charge runs out. A rechargeable lithium-ion battery can have its chemical reaction reversed by charging, so the same battery can be used hundreds of times. Buying and discarding fresh dry cells for a device used this heavily would be expensive and wasteful, which is why constantly-used devices favour rechargeable batteries while occasionally-used devices, like a wall clock, still often use inexpensive dry cells.

Batteries as electricity sources

The core idea

Worked example

Competitive stretch

Common traps

Before this

Where this leads