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The battery control circuitry has constant current, 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, part
number 44-9-95611001, includes a resettable polyfuse that trips when the current
exceeds 700mA at room temperature. This protects the 1 amp rated connector
when for example a partial short exists caused by aa 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
Main board
only w/512 MB
Main board
w/512 MB and
Coprocessor
w/512 MB
Memory Vendor and
Measured
Part Number
Current Draw
Kingston
27.9mA
KVR100X72C2/512
Kingston
48.3mA
KVR100X72C2/512
Chapter 1 - Getting Started
Absolute Maximum
Expected Safe
Backup Time
Backup Time
41.2 hours
20.6 hours
23.8 hours
11.9 hours
Battery Backup Unit
17
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