Rackable Systems OmniStor 4700S Series, User Manual

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Chapter 1 - Getting Started
Battery Backup Unit
13
Battery Backup Unit
The main board of the SCSI-SATA RAID Controller includes battery control circuitry for a
single cell Lithium Ion battery along with a battery pack mating connector. The main
purpose of battery backup is to maintain the cache memory during brief power
interruptions, but is capable of maintaining the memory content for several hours,
depending on the type and size of the memory.
The battery control circuitry has constant current using a constant voltage (CCCV) charger.
The battery charger provides a maximum 250mA charge current. When the charge current
falls below 16mA, the charger determines that the end of charge has been reached, generates
an end of charge indication and shuts itself off. If the battery voltage drops below 3.0V, a
complete battery discharge is indicated.
The battery control circuitry includes a battery safety circuit. The safety circuit protects the
battery by limiting the over-voltage to 4.3V, the maximum discharge current to 3A for
catastrophic events, and the minimum battery voltage to 2.35V. If any of these conditions
exist, the safety circuit disconnects the battery. These conditions will only exist if there is a
hardware fault present, and would never be seen under normal operating conditions. In
addition, the battery pack utilized includes a resettable polyfuse that trips when the current
exceeds 700mA at room temperature. This protects the internal circuitry when, for example,
a partial short exists caused by a component failure.
Lithium Ion batteries have no requirement for conditioning, even after partial discharges. The
current battery pack utilizes a Renata ICP883448A-SC cell, with a nominal capacity of
1150mAh. For a completely discharged battery, the charge time is approximately 5 hours.
Under lab conditions, current draw was measured for different configurations of memory.
The table below shows the results of those tests, and the expected backup time is indicated
for the specified memory configuration. The table shows the absolute maximum backup time
calculated from the current draw measurements. The “Expected Safe Backup Time” is the
absolute maximum de-rated by 50% to account for different operating temperatures and
capacity reduction due to battery charge/discharge cycles. This is the time that should be
used when developing a system level power backup plan.
BBU Battery Hold-Up Times
Configuration Memory Vendor and
Part Number
Measured
Current Draw
Absolute
Maximum Backup
Time
Expected Safe
Backup Time
OmniStor 4711S w/512 MB Kingston
KVR266X72C25/512 27.9mA 41.2 hours 20.6 hours
OmniStor 4712S w/1 GB Kingston
KVR266X72C25/1 48.3mA 23.8 hours 11.9 hours