The problem is a litium cell will loose approx 0.5 - 2% of its capacity a year depending on brand. Couple this with the low power draw of wireless devices the curve just drops of completely very quickly.
The difficulty for the manufacturers is at what point do thy report a low battery. Too early and it looks like its running out of batteries, too late and the low battery running time is either very short of non existant.
The low voltage volts will depend on the manufacturer and cannot imo be the same for all manufactures.
I suppose the answer is to have it programmable so the maintainer can choose when it goes into low battery and how long it will function for in tht state before the supervision failure