Battery technology was previously discussed in this newsgroup.
The NYT reported that the Phila transit system will use battery packs to pick up power generated by braking subway trains and return it when trains acceleration. (Electric motors can be easily switched to become generators).
Also, some power could be fed back into the general power grid and help control frequency deviations from the desired 60.000 Hz.
Historically, telephone switching offices as well as PBXs had large battery supplies which were kept charged on a 'float current' from power supplied by rectifying commercial power. If commercial power failed, the batteries would be charged by large diesel engines and generators. All of this power plant was a major installation as part of the central office.
It has been suggested that some central offices, remote switching nodes, and cell phone antenna sites, no longer have generators to supply power in a commercial outage after the batteries run down. This is troubling. Plenty of relatively normal nasty snowfalls can bring down commercial lines requiring more an eight hours to restore. A tough weather event would generate greatly increased telephone and datacomm usage, further taxing the power supply.
Hopefully the new battery technology referred to in the article could perhaps allow telephone plant to _economically_ have greater standby capacity, especially when there isn't a diesel engine available.
(Anyone know how many kilowatts of power are consumed by a telephone central office at the busy hour?)