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.
