There is power backup and then there is power backup.

The former is a small power pack (batteries, supercapacitors, whatever)
that will allow the microcell to weather a short blackout or brownout.
We are talking seconds, to bridge switching transits.  To be useful in a
deployment, such a holdover battery needs to be very low maintenance.
(Think about cars that use supercapacitors as a battery replacement --
good for short needs like a single engine start.)

The latter is longer-life power to keep the microcell going for days or
weeks.  It's debatable whether this is mandatory.