Andy Hall wrote:

NiCd and NiMH batteries can both be charged using the Delta V method
which involves monitoring the battery voltage very accurately to
detect the small voltage drop that occurs just after peak charge and
to stop the charging. With NiMH, the drop is about 2mV per cell, so
implementing chargers is not easy.

It's trivially easy with a ready made chip that does all the hard work
for you. They can be bought 'off the shelf' now. One off price is
only a dollar or two and I'm pretty sure they'd be a lot cheaper in
quantity. The most well known manufacturer is Dallas/Maxim.

--
Chris Green