I've looked at the other answers here regarding building a low voltage UPS and have come up with the following design.
Requirements: 1) At least a couple of hours of up time; 2) USB power 5V @500mA, DECT phone 9V @300mA, DSL Modem + Router both 12V @250mA; 3) Always connected/auto switch over.
Components BoM: 1) 12V Lead Acid battery + commercial intelligent charger; 2) Use 3 SEPIC converters to supply the 3 different voltages.
Simply connect together and leave permanently plugged in. I need a charger with enough power to keep the battery charged, as well as the devices operational. When the mains power fails, the SEPIC converters go from dropping the 13.8-14.5V charging voltage (typically what I have seen from car chargers) to their set voltage and then maintaining that voltage as the power drains from the battery.
Having tested a decent sized car battery, I know that will power the DSL modem + router for nearly 2 days (just connected straight to the battery, no converter). Maybe overkill but I do have at least one going spare. Those devices didn't like being connected to battery + charger though. Hopefully the SEPIC controllers will provide a steady voltage.
My questions are:
1) What size battery and charger should I go for? Assuming the SEPIC converters are 90% efficient I would need less than 1.5A to power them all full time. One of my car batteries specifies a bench charge rate of 4A, I assume that once fully charged the trickle requirement would be a lot less - it seems to measure less, about 150mA DC.
2) Do I need to add a mains detecting changeover relay to switch from the charger supplying power to the devices to the battery?
Thanks for getting this far.