I have been studying different batteries and cells. I catalog each cell, pack in my book - every time I charge them, use, discharge. But one tool I do not have, is to measure the current output of a cell, or pack.
I know some cells put out less current than others. They all seem to have some type of drop off, where if I draw too much energy - the battery kind takes a break, and usually in a few seconds I can continue to draw power. And I know the cells recovered from laptop batteries will output much less current, than LIPO battery packs used for quad-copters. And there are also 18650's that claim to have high-drain capacity, that I haven't tested yet.
I also discovered that SOME batteries can output much more current, but seem need a WARM-UP period. If they sit on the shelf idle for a week, then I just demand 20 amps out of the pack all at once, it doesn't seem to do that at first, but within the minute - it will actually do this. But if I take the same idle pack, and only draw a little at first, then rise up the draw - the battery pack will output good high current.
I want to build a tool to measure HOW MUCH current a cell can put out and log it's power curve. I will use a Nano board, with an Allegro current sensor and a sd-card logger or serial output to my Visual Studio application that can capture the data and store into a database to analyze in a chart.
I need help with the BUSINESS end of the test, which would be a constant known LOAD, that I can apply - which I can VARY. I would like to have a dial, or pwm output or whatever I need - to vary a constant LOAD on these cells, I guess range from 250 ma up to 50 amps.
So I was wondering if anyone had advice, ideas - I am a little confused myself. How do I design a load, that I can use to test with - which I can vary the amount of load from 250ma up to 50 amps (more or less)?