Hi Eugene,

Battery backup for an alarm system is always required. It is necessary in case an intruder attempts to disable the alarm by disrupting power to the protected building.

Based on your description, if I were going to try defeating your alarm I would just kill power to the building, set off the alarm, and wait for the buzzer to die. This would be especially effective if the system is not monitored off-site, but if memory serves your other posts suggest that you intend your system to be monitored, so hopefully I would be unsuccessful.

I can tell you from experience that the batteries we used to backup our alarms were 12V 6AH gell cell batteries. The circuit kept the battery charged while the mains power was on, and switched over to battery power when the mains power failed. And, of course, the alarms immediately reported any power failure. They also reported battery condition periodically while on battery power.

It sounds to me as though you are severely underestimating the amount of power a complete alarm system will use. My suggestion is that you stop right now, plan out the entire system, and actually calculate the total power the system is going to require. Then make certain that your battery can keep the whole thing running for no less than twice as long as the longest period you expect the building to be empty, 3 or 4 times as long if the system is NOT monitored off site.