I am new to electric and have built my first electric plane. I am using a 5S battery and have read in a few places that it is not good to use the built in BEC when using more than 3S batteries. Can someone explain the reasoning behind this, is it a valid concern, and what is the risk (i.e. what happens if you do use the BEC with more than 3S)?

I have been researching this and can't seem to find a definitive answer. Being a noob I would like to understand this better.

Thanks for any info or guidance someone could provide.

KB