I want to build a rechargable power failure alarm system to sound during power failure.I have heard that lithium batteries can be used.Any suggestions?
Lithium batteries could possibly be used but they tend to be expensive and it depends upon your requirements. How much power (voltage and current) do you need to supply during the failure?
Circuits like this are fairly common in alarm and security systems. When mains is available they supply 12 or 24 VDC at some current. They also maintain a 12 volt SLA battery. When mains fails they automatically transfer to the battery. You need to know what the current needs to be?
You have not specified your current requirements? All units like this are built around the voltage and current they need to deliver. This link is a good read on the subject.
Per what KISS posted there are dozens of manufacturers making them, I have a few of the names Kiss mentions lying here in 1, 2, and 3 amp versions. Small boards that use 12 volt SLA batteries in 4 AH and 7 AH versions. Read the link I posted. It should answer all your questions and provides links to schematics that show how these things work.