018. Backup Power Supply

I live in an area that will have power outages for a few hours, two or three times a year. When this happens, I want to still be able to access the internet.

Therefor I want to have a battery backup power supply for my router and cable modem.

When mains supplied power is applied, the relay is energised sending power to the output stage.

Additionally, a separate dc-dc converter is used to charge the backup battery. When mains power fails, the relay will send power from the battery to the output stage.

The output stage is a buck/boost converter, to convert the 19v main supply or the 13ish v battery supply to a stable 12v that can power networking devices.

The cluster of transistors at the top of the diagram is an overvoltage protection circuit in case of failure in the charging buck module. The last thing I need is 19v boiling the electrolyte in a sealed lead acid battery.