You can leave any decent NiCad batteries in the transmitter. Take them out if they are cheap ones. You can not leave NiCad’s dormant for any length of time because they will lose their capacity and become useless. To keep them healthy they should be cycled at least once a month if you are not using them. If they lose more than 20% of their original capacity or they are approaching three years old it is always a good idea to replace them. As usual, you can do it this way or replace your model whichever comes first. Dan.