How to Calculate the Capacity of Batteries

Hello! My name is Nicholas Anand! I am a 13-year-old electrical engineer, along with some other professions.

My science fair project for this year involves calculating the capacity of batteries, so I thought I would educate y'all on Instructables on how to do it, too!

I hope you learn something from my tutorial and enjoy it,

Unfortunately, calculating the capacity of a battery isn't as easy as you'd think it would be. You will need a lot of different items, which I have listed below.

• Multimeter with current test
• Multimeter test leads
• Resistor with low resistance but high wattage (I find 1 Ohm per 1.5 volts on battery)
• Battery holder(s)
• Battery(s)
• Smartphone with stopwatch app or other form of stopwatch
• Fan
• Material to protect surface from burn marks
• Switch (light switches work very well)
• Fire extinguisher for putting out fires caused by exploding batteries or flames caused by heat from resistors
• Notebook/Paper
• Pen/Pencil
• Computer with Microsoft Excel and Calculator (not required, but makes it MUCH easier)

The picture above is a diagram of my circuit. I have a series circuit with a multimeter on the current setting, a switch, and a resistor.

The circuit is relatively simple to build. Make sure your resistor can handle the wattage you are going to be putting on it! Also, be sure to put a fan blowing on the resistors, and put a material like hardwood under the resistors to ensure that the surface below won't be scorched.

WAIT! DON'T SWITCH ON YOUR CIRCUIT YET!

Before you start, make sure you're prepared and know what you're supposed to be doing.

Once your circuit is built, get out your paper/notebook and write down what batteries you are testing and what load you are putting on them. Trust me - it helps.

Next, draw this table:

At the same time you turn on your circuit, turn on your stopwatch. At certain intervals (e.g. 15 minutes), write down (1) the time displayed on the stopwatch (2) the current that the batteries are supplying.

Once the batteries have reached about 0.1 amps, they are dead. Disconnect them and let your circuit cool down. Copy the data from the notebook into Excel. I find it is often helpful to create a graph of your data, like the one pictured above.

Now, use this formula to turn your data into a number in ampere-hours.

Ah = (for every pair of numbers - add them together) T x C

Ah would be the capacity, T would be the time (in hours) since the last check, and C would be the current in amps.

If you want, multiply your number by 1000 to get milliamp-hours.

I hope that you learned something from this tutorial. Now, you should know how to calculate the capacity of batteries for many different purposes.