Storage charge is only required when you're not going to be using them for a long time, a number of weeks for example. At an event this isn't necessary, so just charge them all up the night before, ready for the next day. They need to be in the lipo sack when charging but if leaving them overnight once they're charged, it's fine to store them somewhere such as a box, so long as they're safe. Many choose to leave them in the lipo sacks as a precautionary measure though.