I've been successfully running OPI ONEs ( smaller sibling of OPI PC ) from power banks with pass-through charging ( charging while delivering power ) as sort of "poor man's UPS". Whether this is viable will depend on capacity of power bank ( draw / charge ) and your actual power needs ( server sounds like attached power-hungry disks ? )
Good luck.
But this solution lacks of information about battery running flat (to make a safe shutdown) and always-charged influence on battery lifespan is unknown (especially for unattended installations).