I think a car battery charger with Wi-Fi is useful, you can remotely check up on it instead of walking back and forth to the garage. Lowest cost would be to mod the 19V laptop adapter to put out less, say 14V or whatever an MCU commands with PWM, but that is beyond a beginner's project. A PWM controller buck-converter or linear regulator might be something basic to add after the 19V PSU.
Purpose of device is a bit different. I am living in country where government supplied electricity lasts max 12-18hours per day, when it go off, in some households private generator supply kicks in (still, it takes 30-60 seconds, enough to reboot router or DSL modem), or if it is less fortunate people - they stay in dark. Sometimes electricity supply is very unstable and might go on/off tens of times per day. Regular offline UPS doesn't last long, too much conversion losses, and need some hacking (hacking away buzzer, hehe).
This device will be connected in the middle, between AC/DC SMPS and wifi router, to provide continuous supply, so at least people can stay online from their mobiles. Most of routers is OK with 14V (step-down buck inside, only problem sometimes caps max voltage on DC IN), but on some old routers i seen LDO, there i i can use cheap LM2596 step-down brick.
ESP8266 will keep an eye on battery voltage, and if it is too low, send data to cloud/owner mobile, cut off router, lower power usage as much as possible (disable RF) until mains appears, and then if still no supply, next level go to deep sleep and wake each 5 minute, to check if power appeared. It will be easy to implement additional functions, battery health monitoring, remaining runtime, programming some router cut-off for night time and etc.
I am considering to make it OSHW, if it will work fine.