Martin Brown wrote:
Low current devices fail gracefully on rechargables with insufficient
voltage to drive the LCD display to decent contrast.

Another reason for the 'no rechargeables' disclaimer might be the
self-discharge on NiMH. It's no good if an infrequently-used widget is
always flat when you come to use it - and the manufacturer will probably get
the blame for flattening the batteries.

These days we have low self-discharge NiMH so this problem is much less, but
perhaps the manufacturers still put the disclaimer in because people will
use non-LSD NiMH and then complain.

Theo