iv always used the mk1 8ft collars or the normal mk1 15ft collars, these micro's take different size batteries iv got loads of the right batteries now, so have sorted it myself, thaks for all of you lot taking you time all jumping in at once to let me know the right size batteries ................... i come home from ferreting and always have to strip off in the back yard i have more mud and god knows what else , it feels like iv brought half the field home somtimes, the misses goes nuts,