I must have gone through a dozen of these. Different brands, different models. My conclusion: They're alwarys dead by the time you need them. Apparently we just haven't gotten that far along with battery technology to make a reliable backup unit. The only advancement they do is to make the alarm more annoying and more difficult to deactivate. Either that or the guys at the supply depot recognise my adress and they're like:
 "This guys again! Hey Joe, lets go find him another reject unit."