IBM Storwize V7000 Unified Problem Determination Manual page 259

If power to a node canister fails, the node canister uses battery power to write
cache and state data to its boot drive.
Note: Storwize V7000 Gen2 expansion canisters do not cache volume data or store
state information in volatile memory. Therefore, expansion canisters do not require
battery power. If ac power to both power supplies in an expansion enclosure fails,
the enclosure powers off. When ac power is restored to at least one power supply,
the enclosure restarts without operator intervention.
The battery is maintained in a fully charged state by the battery subsystem. At
maximum power, the battery can save critical data and state information in two
back-to-back power failures. If power to a node canister is lost, saving critical data
starts after a five-second wait. (If the outage is shorter than five seconds, the
battery continues to support the node and critical data is not saved.) The node
canister stops handling I/O requests from host applications. The saving of critical
data runs to completion, even if power is restored during this time. The loss of
power might be because the input power to the enclosure is lost, or because the
node canister is removed from the enclosure.
When power is restored to the node canister, the system restarts without operator
intervention. How quickly it restarts depends on whether there is a history of
previous power failures. The system restarts only when the battery has sufficient
charge for the node canister to save the cache and state data again. A node canister
with multiple power failures might not have sufficient charge to save critical data.
In such a case, the system starts in service state and waits to start I/O operations
until the battery has sufficient charge.
Two light-emitting diode (LED) indicators indicate the state of the battery:
v Status LED - Green
v Fault LED - Amber
See "Procedure: Understanding the system status from the Storwize V7000 Gen2
LEDs" for a complete description of the battery LEDs.
Important: Although Storwize V7000 Gen2 is resilient to power failures and brown
outs, always install Storwize V7000 Gen2 in an environment with reliable,
consistent, and required ac power. Consider uninterruptible power supply units to
avoid extended interruptions to data access.
Design parameters
Consider the following important design parameters:
v The design life of the battery in the Storwize V7000 Gen2 is five years service
after one year on the shelf.
v Each battery is automatically reconditioned every three months to measure the
battery capacity. Batteries in the same enclosure are not reconditioned within
two days of each other. If a battery has a lower capacity than required (below
the planned threshold), it is marked as "End Of Life" and should be replaced.
v Each battery provides power only for the canister in which it is installed. If a
battery fails, the canister goes offline and reports a node error. The single
running canister destages its cache and runs the I/O group in "write-through"
mode until its partner canister is repaired and online.
Chapter 5. Control enclosure
231