If you are seeing your characters per SMS decreasing when using emoji, it is because SMS does not have a 160 character limit, it has a 140 bytes limit. Usually messages are encoded using GSM 03.38, which uses 7 bits per character so you end up with a 160 character limit.
When you write emoji your SMS app most likely switches to using UTF-16, which uses 2 or 4 bytes per character and which gets you an upper limit of 70 characters. If you only use characters that are encoded as 4 bytes, you can at most write 35 characters. The upper limit can be even lower if you use combining characters (for example diacritic marks).