The main battery life killers seem to be the full-screen always-on display of many smartwatches and GPS. If you can dump those you can do a lot better it seems. Though in my case I have no interest in a fitness tracker without GPS.
I use a Mi Band which I wouldn't really recommend on the whole, but it gets great battery life and one reason might be that AIUI it piggybacks the phone's GPS rather than doing GPS tracking itself.
