Posted: 22/09/2016 at 22:38
NiMh and NiCd are both OK and good for about 500 charge/discharge cycles (so there's your 18 months!) NiCd are more tolerant of very low temperatures but neither type really like it, so best remove batteries in the coldest part of the year. NiMh should, in theory, last longer (up to 1000 cycles.) They also contain slightly less noxious chemicals (Cadmium is particularly nasty stuff if released into the environment) so always take dead batteries to a proper battery recycling point and never put them in your household waste.