What it shows
A cell that reads 1.4 V with nothing connected can still be nearly flat, because its internal resistance has grown. A voltage reading while it lights a small bulb shows the true condition.
The science
Every real cell acts like a perfect cell of voltage E with a small internal resistance r in series. With the bulb connected, current I flows and the terminal voltage falls to V = E minus I r. A new cell has r of a fraction of an ohm, so V barely drops; an old cell has a large r and V falls a lot. So r = (E minus V) / I, where I = V divided by the bulb resistance.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Four AA cells of different ages (new, half used, old, a dead one)
- A 2.5 V torch bulb with holder
- A multimeter
- Two crocodile clip wires
- A single AA cell holder
- A notebook
Safety
For this project: Use only small cells and a torch bulb. Never connect a cell directly across itself with a wire: it heats the wire and drains the cell quickly.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Number each cell with a marker and note its age or condition.
Set the multimeter to DC volts and measure each cell with no load. Record E for each.
Fit one cell in the holder with the bulb connected and watch how bright the bulb is.
While the bulb is lit, measure the voltage across the cell terminals. This is V.
Work out the bulb current: I = V divided by the bulb resistance, about 8 Ω for a 2.5 V, 0.3 A bulb (measure it with the meter switched off the circuit).
Calculate the internal resistance r = (E minus V) / I for each cell.
Rank the cells from best to worst and compare with the bulb brightness.
Put the cells away: keep the dead ones for a battery recycling collection point.
What you should see
The old cells show little drop at no load but a large drop under load, so their internal resistance is much bigger, and the bulb is dimmer.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why does the voltage fall when the bulb is connected?
Some voltage is lost across the internal resistance of the cell.
2Why is a no-load reading misleading?
A multimeter draws almost no current, so the internal resistance has no effect.
3What is the formula for terminal voltage?
V = E minus I r.
4Why should dead cells not be thrown into household waste?
They can leak chemicals; they should go to a battery collection point.
Take it further
Repeat with two cells in series and see whether the internal resistance simply adds.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
Hardware & repair
Using a Digital Multimeter
Measure voltage, current, resistance and continuity safely
Science projects
Science Projects for Class 9 and 10
Models and experiments for high-school physics, chemistry and biology