You might also want to look for a 'universal USB' charger. These don't have threads but a little rubber grommet that grips the end of the battery to keep it in the charger. Not quite as good as the threaded ones but it'll hold many different models to charge. I don't remember which RN models match which other brand models. I know there is one that's a match for the Joye 510 and another for the DSE901 among others. Obviously if your model matches one of those you'll find lots more options. The only useful thing I can say is that the DSE901 and KR808D-1 have the same threads. So if your rn4081 matches the DSE901 you're golden. If it matches the Joye 510 then you'll need an adapter.