IBM Midrange System DS4000 Series Hardware Manual page 96

Backup Battery Unit (BBU)
The Backup Battery Unit provides power to back up the cache memory of each controller onto
flash drives in the event of a power failure. Each battery unit contains a sealed, rechargeable
SMART lithium ion battery. The battery unit contains enough charge to back up the cached
data in each controller to a flash drive in the event of a power failure.
When the unit is powered on the first time or whenever the battery is replaced, the battery
chargers will charge the battery to the programmed level. Then, the controller will start a
battery learning cycle to determine whether the battery current capacity is sufficient.
Note: Data caching starts after the battery is charged to the programmed level.
During the battery learn cycle, the cache will be active if the battery is in good condition. If the
battery fails the learn cycle, it is marked as failed. The battery learning cycle lasts up to three
hours. After the first battery learn cycle, the controller will perform a learn cycle every 8 weeks
to re-calibrate the battery-charging level. The battery unit is hot-swappable. You can remove
the battery unit for servicing and then reinsert it while the DS5020 continues to perform I/O
operations. The battery should been removed using the Storage Manager: Prepare for
Removal procedure (see Chapter 7, "Advanced maintenance, troubleshooting, and
diagnostics" on page 327). However, write I/O caching is disabled when the battery is in a
failed state or removed from the controller chassis. Replace the failed battery as soon as
possible to minimize the time that the write IO caching is disabled. Information about the
condition of the battery unit is conveyed by indicator LEDs on the front of battery unit.
Latch
Figure 3-56 Replaceable Backup Battery Unit (BBU)
To physically remove the BBU, just push the latch, as shown in Figure 3-56, and pull out the
battery unit.
Important: Unlike the batteries for DS4000 storage subsystems, the DS5020 storage
subsystem battery units do not use the expiration dates given in Storage Manager. Do not
replace these batteries after a certain usage period.
78
IBM Midrange System Storage Hardware Guide
Pull handle
Controller A
backup battery unit