Re: 12v sealed lead acid battery, recharge or repl
Gell cell batteries on float charge do tend to last only about 5 years. Keeping them charged keeps the acid at it's strongest and the usual failure mode is for the plates to dissolve (positive plate, IIRC) in the acid. They don't last much longer (if any) on cyclic charge since they usually are left partly charged (often) and therefore sulfate. The other main failure mode is loss of water from overcharging - that can happen on float charge, also.
Your alarm is expecting you to have a back up battery and in fact has to have one to meet certain fire codes. It's not uncommon to loose your electricity when there is a fire, especially since the electrical system is often the cause. Most of them show some kind of signal when the battery isn't there. Mine for example has an LED that lights up (I don't recall exactly what it does). Anyway, I can tell when the power has been off OR if it is running on the backup battery OR if the battery is dead (run down, not necessarily DEAD). Obviously, your alarm won't work when your power is off (without the battery), including if your burglar disconnects your power (Hey, it happens!).
Home Depot usually has them and any electronic place should, even Radio Shack, as well as electrical distributors since they supply the alarm installers.