So I started looking more closely at batteries and found out that NiMH cells are negative around their casing while alkaline cells are positive. That's likely what the Rio Volt was counting on above. But since not all brands are made to exact (the same) specs there's likely to be variation causing some incompatibility.

The NiMH cells I have all expose a lot more of the bottom of the battery (negative) than regular cells. However, they all expose exactly the same amount of positive surface (just the center tip) - unlike regular cells which expose nearly the entire top (usually about the same as the bottom).

I'll find out which is the most fool-proof way of checking, but right now I'd lean toward testing the top (positive side).

Bruno