Going back to electronic after a 20 years break

So after solving the resistor ballast issue, let's have a look at the battery tester v2. What are the requirements ? Big color screen. Because it is prettier. Constant current discharge up to 1.x Amp. We mostly target 500 mA, voltage between 3 and 5v Rotary encoder to do the setup The screen will be a 320x240 ILI9341, you can get them on ebay for ~ 4$ We'll use a STM32F103 micro controller to drive it. Why ? Because it is much better than a cheap Atmel based board : 64 or 128 kB Flash, 20 kB Ram MUCH faster Cheap (~ 2$) You can actually debug with it, using a STLINK V2 clone or a blackmagic clone. The ATMEL chip is too slow to drive such a screen. Run at 3.3V, which is perfect for the ILI9341 which also runs at 3.3v on its IOs. Let's have a look a the constant current circuitry : The QUCS  simulation file is available here  on github . So we just have to provide VREF and the current will be adjusted to follow it. The simulator output...

While playing with the 2nd battery tester idea, i ran into the need for custom ballast resistors. They are resistors with a low value that will take the heat, so that the FET will not have all the power to dissipate. I have a few of them bought on ebay, but of course i needed a custom value, much lower than usual. Why not do them ourself ? What  we need  : Kanthal wires, good i'm a vaper. They heat a lot when current is passed through them, and they can stand the heat. We'll twist two 0.3 mm brand together Plaster (paris plaster actually) The wire is wrapped to be a coil  (this example is without a twisted wire, much better to twist it) You will have problem soldering the kanthal, wrap  it around the red/black wire and put a generous amount of solder around it. Let's 3D print a custom mold, you can use whatever you have available (cardboard...) Put the coild inside it, pour plaster in the mold, wait a couple of hours and : ...