First, I was curious about the thing he mentioned where when the cells are full they form oxygen and hydrogen on the electrodes, but then it recombines into water. So the cell neither vents nor dehydrates. I don't understand it all, but apparently, NiMH cells can be maintained by a very small current without damage.

Second, I was interested in how to choose the various components to build the charger. I don't know if Clive did a bunch of math off camera, or just intuited it, but rather than simply copy what he did, I want to see how it all works. I've been trying to learn LTspice, and this seems a good project to simulate.

Third, I thought it would be neat to finally get one of those custom PCBs that You Tube DIYrs are always advertising. So I figure this would be a good project for using EasyEDA too. I came across this video of a tour of JLCPCB, and thought it was rather fascinating:

Anyway, starting with the cells, I've read that a safe trickle charge current is 0.03-0.05C. My cells have 2,600mAh (I tested some) so to be on the safe side, I went with 0.03C which works out to 78mA. I've also read that NiMH cells have around 50mΩ internal resistance.

I drew up the schematic (with only 1 cell for now) in LTspice, and this is what I got:

I used the modified Ohm's Law (R=(Vt-Vf)/If) to find the resistor value for the power indicator LED, and LTspice agreed with my math. But I used the same equation to find the resistor value for the charge indicator LED, and LTspice says I'm way off. Either I'm using it wrong, or my math is wrong. Maybe both.

I've got some parts on the way, so I can build this circuit temporarily to see what the real values are. I wonder if it has something to do with there being a cell in the circuit that isn't the voltage source. Like maybe the voltage is causing some sort of "push back" against the forward current, and that's why the resistors have to be a smaller value?

It's late, but maybe tomorrow I'll try something like R=(Vt-Vf-Vc)/If where Vc is the cell voltage. I wonder if that would mean that putting a discharged cell in would allow too much current through the LED because of the lower cell voltage...