Are they hybrid NiMH then? They are supposed to hold charge much longer than 'standard' NiMH
(which discharge in a month or two). The problem with NiMH is it covers many slightly different
chemistries, such as the metal M (often but not always lanthanum) can vary, the electrolyte might
be formulated differently, and all these have some effect on the charge/voltage profile. An old
charger might be too fussy for newer versions.