Here is an excellent off-the-shelf DC UPS module: »
www.mini-box.com/OpenUPSIt supports both lead-acid and lithium-iron-phosphate batteries for backup. The OpenUPS2 module (»
www.mini-box.com/OpenUPS2) can hold 3 Lithium Iron Phosphate batteries directly on the PCB.
Default is 12V output, so before use, please configure output voltage to 24V using Windows laptop. You can actually make it any custom PoE voltage upto 24V.
You can connect the DC input to AC-DC adapter or solar panel or any other type of DC power source that provides 10-30V range and enough current to power load and float charge the main backup battery.
You can also connect the DC input to a separate huge 12V lead-acid battery, with a float charger connected in parallel to that battery. That gives you backup for your backup battery.
P.S. You can also make DIY versions using DC-DC modules. DROK modules sold at Amazon are really cheap, but works reasonably well.