Thanks, but it's the little black round rubber one that seals where the charging cable attaches.  It's attached to the battery via a rubber flap. 

This is the small black rubber plug that you insert into the charging port after you disconnect your charger on a Radrunner Plus (I assume most/all Radpower bikes are the same).  Mine split in two.  Now if my e-bike is ever in the rain, the battery may short. 

I just spent about twenty minutes looking for one of these on line but can't find one anywhere. 

Any ideas?