How to fully discharge a battery to determine capacity
The reason I decided to use diodes was because...

Regular diodes do not regulate current. It is possible you are confusing regular diodes with what were once called current regulation diodes. They are not actually diodes but FETs, usually with an added resistor, in a single two terminal package. They could regulate small amounts of current, but not very accurately. I have not seen these lately, as there are other, far more accurate ways to regulate current.