Tempo trainer battery
The battery in the tempo trainer is not replaceable because it wouldn't be water proof once you opened it to change the battery. With regular use, they should last about 1 year per Finis. If the one you have "died" prematurely, contact the place you purchased it or Finis directly for an exchange. I've had to do this, because for whatever reason, I do not have a lot of luck with them lasting very long, but that supposedly is not the norm.