FASTSKY is absolutly correct . in a sense ,there is no device that can tell you your batteries are about to fail.
the ESV WILL tell you the state of charge and if there is a dead cell. it will, if used correctly, tell you how well your batteries hold a charge & for how long. case in point ; a TX I had could be fully charged but in use the old weak cells became obvious when placed on th ESV after only two flights. the ESV will also catch a bad pack dierctly off the charger when you think they are ok.you need to charge /discharge packs several times & find out how long that individual pack will last. plus a safety margin under load.