The device uses a NiCd charger that may either undercharge or overcharge NiMh batteries. The former is not a problem but the later will reduce their lifetime.

Even if nowadays NiMh batteries are better, as the OP is not an enthusiast I think he or she will be better served by the closer equivalent that Barkuti posted.

Edit: It would be good to know the model of the device or at least the charging instructions in its manual to figure out what kind of NiCd charger the device uses.