Make a battery backup for a cordless phone base unit, to allow all handsets to work during a power outage.
I recently started getting telephone service from my cable TV / Internet provider. One of the things to be aware of is the fact that if you lose AC power to your home, you would lose phone service. The cable provider gives some protection for such occurrences by providing about 8 hours of phone service by a backup battery pack inside their phone / internet interface box (known as an EMTA).
I kept that in mind as I was shopping for a new cordless phone system. I wanted a base unit (where the phone line connects), and a number of handsets that only required their AC power dock for keeping them charged. With cordless phones you are always encouraged to keep at least one corded phone around to provide service in case of a power outage.
I thought that it would be useful feature for the cordless phone to have a backup battery pack (like the EMTA), so that all the cordless phones could still make calls in the event of a power outage. There may be some such models out there, but I couldn’t find any. Anyhow, the set I liked had a lot of other attractive features, so I decided to make my own battery backup for the cordless base unit.
The idea is that this battery backup box will connect between your base unit’s AC power adapter and the base unit itself. When you have power to your house, power from the base unit’s AC power adapter is passed through to the base unit. When you lose AC power, a relay is released, and power to the base unit is then provided by batteries.
Although these instructions will tell how I made my unit, you will have to do some technical homework and minor changes for your project.