Official EMC position … hope this answers your question !
The Battery Subsystem maintains power for three minutes to the entire subsystem if AC power fails. The system battery backup prevents any loss of data due to a power failure. Symmetrix immediately switches to backup battery power and destages any pending writes in cache to disk. This destage operation takes approximately 20 seconds following which Symmetrix powers down. The entire Symmetrix system is made non-volatile through an onboard battery backup system. The battery backup system provides the means for destaging any fast-write data that might be in cache memory if AC power is lost on both lines to the unit. In addition to providing nonvolatility to the Symmetrix system, the batteries are fully capable of powering not only all electronic components, but also all HDAs during this time. This capability eliminates emergency power off situations, meaning that the disks are always powered down in an orderly manner, extending their useful life considerably. The backup battery subsystem allows the Symmetrix system to remain on line to the host system for 3 minutes in the event of an AC power loss. This 3-minute window allows the Symmetrix system to support multiple power outages, because the battery is able to fully recharge during Initial Microcode Load (IML) time. Symmetrix continually recharges the battery subsystem whenever it is under AC power. The battery backup subsystem also prevents disk device failures due to the sharp power drops that occur during unexpected power interrupts. If power is restored before the battery timer expires, the Symmetrix system becomes available again without an IML.