There are all sorts of reasons why you may want to test battery cells. The simplest reason is to see if they still have a charge, but it’s also very important if you want to build your own battery pack out of rechargeable battery cells that need to be balanced. Usually that’s done by discharging and charging the cells equally, but if you’re working with salvaged batteries you’ll need to know if they’re even holding a charge in order to avoid damaging the other cells. You can test the voltage with a multimeter, but Danie Conradie has built a more elegant “Floppy Tester” using a flexible PCB.
Conradie’s motivation was simple: he doesn’t have three hands. Holding the battery in place while also holding both probes of a multimeter is difficult. Most multimeters also can’t provide a load, which means that battery voltage readings may not be accurate. The Floppy Tester solves both problems, and makes it easy to quickly test the voltage of battery cells. It’s designed to be built on a single flexible PCB, which keeps both the cost and complexity of the device low.
The PCB design has two long arms arms that act as the probes. Because the PCB is flexible, those can be bent around to accommodate different sized battery cells. At the end of each probe there is a small permanent magnet that snaps onto the battery cell for secure attachment. The circuit for measuring the voltage is based on a small, efficient, and inexpensive Microchip ATtiny85 microcontroller. It can measure either with or without a small load, and display the reading on a 0.96″ OLED display. If you’re looking for a convenient way to test batteries, the Floppy Tester is a great choice.