What it shows
Rechargeable and cheap zinc cells carry a printed capacity that is often much higher than the truth. A constant-current load with an Arduino logger finds the real milliamp-hours of a cell by recording voltage until the cell reaches its cut-off.
The science
The op-amp compares the voltage across the sense resistor with a set voltage from the potentiometer and drives the MOSFET gate so that the current stays constant, whatever the cell voltage. For 300 mA through 1 Ω, the sense voltage is 0.3 V. The Arduino reads the cell voltage with its ADC, adds up current multiplied by time, and stops the test when the cell falls to 0.9 V (alkaline) or 1.0 V (Ni-MH). Capacity in mAh is current in mA times hours.
Materials
Cost: under Rs 1,500. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- An Arduino Uno or Nano
- An LM358 op-amp, a logic-level MOSFET such as IRLZ44N
- A 1 Ω 1 W resistor as current sense
- A 10 kΩ potentiometer and a 1 kΩ resistor
- A single AA cell holder
- AA cells to test: two brands of zinc-carbon, alkaline and Ni-MH
- A laptop with the serial plotter
Safety
For this project: Test only single AA or AAA cells at a current of 100 to 500 mA. Never test lithium cells this way, never short a cell and never leave the rig unattended near paper. An adult must be present.
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
10 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.
Draw the schematic: cell positive to the drain-side load path, the MOSFET source to the sense resistor, the resistor to the cell negative, and the op-amp watching the resistor.
Build it on a breadboard and set the potentiometer to give 0.3 V at the non-inverting input.
Check with a multimeter that about 300 mA flows with a fresh cell, and that the current stays steady as the cell voltage changes a little.
Connect the cell voltage to analog pin A0 through a divider only if it can exceed 5 V; for one cell, connect it directly.
Write the sketch: every 10 seconds read A0, convert to volts, add current x 10 s to a running total and print time, volts and mAh.
Add a cut-off: when volts fall below the limit, switch the MOSFET off by dropping the control and print FINAL mAh.
Run one brand of alkaline cell and one Ni-MH cell to the end. Save the serial output to a CSV file.
Repeat with at least three cells of each brand and compute the average and the difference from the label value.
Plot voltage against time for each type and mark the cut-off.
Write the report with the test method, the plots, the comparison table and a note on how current changes the capacity (Peukert effect).
What you should see
Each cell gives a discharge curve and a real capacity. Typical branded alkaline cells give 1,200 to 2,000 mAh at 300 mA and Ni-MH cells 1,500 to 2,000 mAh, but cheap cells often give far less than printed. Your numbers may differ, and that is the finding.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why use constant current rather than a fixed resistor?
With a resistor the current falls as voltage drops, so the capacity is harder to compare between cells.
2Why stop at 0.9 V for alkaline?
Below that, most devices stop working and deeper discharge can make the cell leak.
3What does mAh mean?
The charge delivered: current in milliamps multiplied by time in hours.
4Why does a higher test current give a lower capacity?
Internal resistance wastes more energy and chemical reactions cannot keep up, so the cut-off is reached sooner.
Take it further
Test the same cell at 100, 300 and 500 mA and plot capacity against current to show the Peukert effect.
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.
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