why is my keypad dead when i cut main supply?

have just fitted texecom veritas alarm, had no problem whatsoever until today, had to cut mains supply and when i did the alarm went off as it should do, but when i went to keypad to stop alarm with my code, the keypad was completely dead. could not put my code in to reset it.went to main panel to check battery, led light was on so battery was working. took front off panel to put tester on battery, battery fine, it should be as its brand new only 2 days old. why have i got no power to keypad when main supply is cut?