the mk 1,,,theres 2 difrent sizes,,,the regular,,,and the micro collar,,,,so make sure you know wich is wich......the mk2 didnt really take off,,there was issues with it,,,,if you mean the mk 3 ,,,like the mk 1 theres two difrent sizes,,,,the mk1 ,,and the mk1m.. ,,confusing i know....
the mk3 is the same size battery as the mk1 micro,,,,,however the mk3m is totaly difrent size to any,,,,i know ,,i have all the difrent collars and recivers