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how about this: https://create.arduino.cc/projecthub/aleksand1975/intelligen... ?

That looks a lot better, it recognizes the dv/dt condition and has a number of safety features.

Trickle charge (less than 15mA) for NiMH was fine for long-term battery life I thought?

EDIT:

https://data.energizer.com/pdfs/nickelmetalhydride_appman.pd...

>> Finally a maintenance (or trickle) charge rate of less than 0.025 C (C/40) is recommended. The use of very small trickle charges is preferred to reduce the negative effects of overcharging.

C/40 constant trickle-charge is good for NiMH chemistry.

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The simplicity of trickle-charge is one of the defining reasons to use NiMH over other chemistries. 15mA * 40 == 600mA-hrs, which should be safe for even tiny AAA-NiMH cells.

EDIT: Ah, I assumed the article was going for the Trickle Charge technique. Hmmm... it uses a 555-timer for "current limitations" which seems... harder than just using an OpAmp + transistor.

Yeah if you’re going to do this and forget such things you might as well use a diode and resistor and be done with it. It’s no worse than this circuit.