How to Make a Rapid AA Battery Tester Sorter Simple and Accurate

Watch the video above to see the tester in action! Have you ever been frustrated when trying to search for a good batteries from a mixed pile of good and dead batteries? Have you tried ineffective battery testing methods from the internet, for example:

1. The notorious battery drop test (limited effectiveness, watch till the end of https://www.youtube.com/watch?v=GVWD6YN0w0s&ab_channel=SciShow)
2. Test battery directly with a multi-meter (inaccurate, see 0:36 timestamp https://www.youtube.com/watch?v=JjElWj0fFX4&ab_channel=TheEngineeringMindset)

This project offers an efficient and accurate solution to test and sort AA batteries. The testing circuitry of this project is very simple and explained in Step 6. The basic concept is that the battery voltage when connected to a load can indicate the remaining capacity in the battery. Other steps mostly focus on the 3D printed structures and assembly. The circuit is the part that actually does the testing. The 3D printed structure makes this testing and sorting process more ergonomic and faster.

Optional reading: Personal anecdote and project motivation

If you think finding good batteries for household appliances is annoying, well it is actually even more annoying if you have to find batteries for circuits and robots you have built. Dead batteries have causes countless problems in my robotics and circuitry projects. They have caused me to think that I messed up the circuit design, leading me on countless wild goose chase trying to find the issue in the design, only to later realize it was caused by a dead battery. This problem gets amplified when using multiple batteries, increasing the probability of having a dead batteries; and mixing good and dead batteries together can lead to some very puzzling circuit phenomena and even more wild goose chases. I decided to build this battery tester, because I realized I desperately need an accurate and efficient way to test batteries for my future projects.

1. Multimeter
2. Veroboard (I just cut out a small piece from a larger board)
3. 10Ω resistor (5 to 20Ω should work fine, see Step 6 for resistor selection)
4. 4X M4 nuts and bolts 35mm long (25mm is also long enough)
5. 2X plug to socket jumper wires
6. 4X plug to plug jumper wires
7. 2X alligator clips
8. duct tape
9. aluminium foil

Tools:

1. 3D printer
2. Soldering iron
3. Scissors
4. Wire cutter
5. Double-sided tape
6. Wire stripper (optional)

The photos above show how the 3D printed parts look. I have attached the .stl files below and also the Fusion 360 archive file. Do NOT print 'batteryTester v10' stl file, which is only meant to show you how the 4 components fit together. Thus print box, cartridge, sliderV2 and support.stl files.

If the components are orientated correctly in the slicing software, very little support material and clean up will be needed for the 3D printing.

Bolt together the support and box component. The bolt holes do NOT have threads, so tighten the nut and bolt to establish the strong joint. The slider will move independently from other components and thus will not be bolted and the cartridge will be bolted at a later step.

Strip one end of the plug-to-plug jumper wire to expose the copper. Solder the copper onto the alligator clips. Do this to two jumper wires. The soldering might be tricky. The solder joint will NOT be mechanically strong, so wrap duct tape to reinforce it (see photo above), ensure the tape is secured to both wire and clip.

Like with the alligator clips, solder two jumper wires each to an aluminium foil. See Step 5 below to see how big you will need for cut the foil. I recommend folding the the aluminium foil once to create a double layer, increasing the thickness and structural strength of foil.

Being an electrical conductor, the aluminium foil will act as the electrical contact with the battery being tested. One aluminium pad will be at the positive terminal of the battery and other at the negative terminal.

Tape what you made in Step 4 to the structure as shown in the photos above. Remember the duct tape is needed to reinforce the weak solder joint.

The resistor, pair of alligator clips and pair of aluminum pads are connected in parallel on the veroboard (see diagram above). For the aluminium pads, the plug-to-socket jumper wire is soldered onto the veroboard, so you can plug this wire onto the wire you soldered onto the aluminium pads in Step 4. The alligator clips will be used in Step 9 to connect the circuit to multi-meter.

This is a voltage measuring circuit, since this voltage can indicate the remaining the capacity in the battery (see the PDF files attached below). In the PDFs, the service hour to voltage charts show that the battery voltage only drops below 1.2 volts towards the end of the service hours. Thus, I defined the threshold for a good battery as one that output 1.2 volts or more. Please note, 1.2 volts is a good threshold for rechargeable batteries, not necessarily for non-rechargeable ones; you can define the threshold voltage to your needs.

The 10Ω resistor is necessary for accuracy. Without the resistor, the total resistance of the circuit is that of the multi-meter, which is about 10MΩ since it is measuring voltage. Remember the threshold voltage is only 1.2 volts. Thus, you only need 1.2 volts / 10 million ohms = 120nA of current, which is not enough the power any typical household appliances. Thus, without the 10Ω resistor, the circuit is testing if the battery can send nano-ampere of current instead of can realistically power appliances. The 10Ω resistor serves to reduce the total resistance of the circuit, allowing a more representative current flow during the test.

Resistor selection:

Since the resistor has significantly less resistance than the multi-meter, the total resistance is approximately equal that of the resistor. At 1.2 volts, the 10Ω resistor allows a current of 1.2 / 10 = 120mA, which is representative of the currents used in household appliances. A resistor greater than 20Ω allows too little current. Also, a resistor less than 5Ω is too sensitive the resistance in the aluminium pads, unintentionally creating a voltage divider and thus inaccurate readings, since the aluminium's resistance is becoming comparable (so not negligible) to that of the resistor.

Use double sided tape for attachment.

Two bolts are used.

Use the alligator clips and set the multi-meter to read voltage.