While I am aware of the risk associated with charging Li-ion below 0 °C and that
the charge acceptance for a NiMH battery decreases with rising temperature,
I am finding conflicting information when it comes to charging NiMH batteries below 0 °C.
It's not a hard line. The battery doesn't just quit at -0.00001 degrees. It depends on the battery internal temperature and the status of the liquid electrolyte inside the battery. If it is frozen, no chemical reaction can take place. As Energized states, it will charge at 14 degrees F, 10 C, but it has half the normal capacity it would.
This is wild speculation, but I suggest that the maximum charge current, and the trickle charge current that the cell can safely handle while fully charged, be reduced at lower temperatures. Possibly proportionally to the low temperature reduced discharge capacity and increased discharge impedance.
There is a catalyst in there to recombine gasses that form during charging, and chemical reactions slow down at low temperatures.