You wouldn't be writing to a 'binary' file here, since you opened it in text mode.
You seem to be asking, "what sequence of bytes does my system use to end lines in text mode?" Hard to say since we don't know what system you're using. Easy enough for you to figure out for yourself though. All it requires is opening a text file in binary mode created on your system with a linebreak in it and reading in the byte sequence used to indicate the linebreak. That same sequence is what you would write in binary mode to end a line as you would in text mode.