If you have not drained the battery to a certain level, it will not charge! You can plug it in & no lights will come on. I do not think it would hurt anything if you plug it in & the lights come on & you top it off as these battery's are not supposed to have a memory so partial charges or topping them off shouldn't hurt them. Also you do not want to keep the batteries stored on empty ( flat ) either. You want to charge until the 3rd light starts to flash. I read the best area of charge to leave them for a period of time is around 60%. If you have the new battery firmware, you can fully charge them & if you do not fly for 10 days or so they will start to dissipate. This keeps the batteries from swelling and shortening the life of them. That is just what I read, if not correct, someone will reply.