IBM Midrange System DS4000 Series Hardware Manual page 62

Smart Battery Backup Units (BBUs)
The Smart Battery Management feature allows the management facilities of the storage
subsystem to take advantage of the capabilities provided by a "smart" battery unit. This
feature provides a mechanism that enables the controller firmware to accurately determine
how much capacity is left in the battery in a battery backup unit (BBU) and to access the
battery's state of health (SOH). This information allows the controller firmware to determine
exactly how long the BBU can hold the cache up and take action as needed, for example,
disable write-back caching if the battery backup facilities are insufficient.
Batteries are used to hold controller power to destage data from cache to flash memory. A
large capacity allows a load of 150W for a minimum of 30 minutes.
The battery can have three states:
Full Charge: The battery is fully charged. The cache is set to Write Caching mode.
Maintenance Charge: The battery is "trickle charging". The cache state is unchanged.
Learn Cycle (for Smart Battery): The battery is testing itself for capacity. The battery will
retain enough charge to flush the cache. The cache state is unchanged.
Learn cycles
To properly calibrate the battery gas gauge, a fully charged smart battery unit must
periodically be taken through a controlled discharge into the discharge load. The battery gas
gauge is properly calibrated when the charge level decreases to a predetermined threshold.
The threshold varies with the specific hardware design of the smart battery unit. Once the
battery is discharged to the predetermined threshold, it is then fully recharged, following any
required rest period. This controlled discharge, followed by a rest period, followed by a
charge, is referred to as a
Learn cycles occur automatically at scheduled intervals. The time between learn cycles is
based on the start time of each learn cycle so that the period remains constant, regardless of
the length of each cycle. The learn cycle interval is scheduled in weeks (the default is
8 weeks), so that the start time for each learn cycle will occur on the same day of the week, at
the same time of day.
Each controller receives backup power from one or more battery components. The controllers
execute learn cycles on their respective set of battery components concurrently. In other
words, controller A discharges and recharges its battery components at the same time that
controller B discharges and recharges its battery components. If a controller receives backup
power from more than one battery component or set of battery cells, the learn cycles for the
battery components associated with that controller are executed sequentially. In other words,
if there are two sets of battery cells that supply backup power to controller A, one set of cells
for that controller is discharged/recharged before the second set of cells is
discharged/recharged for that controller. In a duplex configuration, controller B is performing
sequential discharges and recharges on its battery components at the same time that
controller A is performing sequential discharges and recharges on its battery components.
The batteries are not totally discharged during a learn cycle. There is always power left to
supply backup power for destaging data from cache to flash memory.
A controlled discharge begins once the following conditions are true:
All batteries are fully charged.
No batteries are overheated.
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IBM Midrange System Storage Hardware Guide
learn cycle
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