Battery Electronics Unit (BEU)

NASA Battery Power Workshop 11/27/07 – 11/29/07
Managing the Health and Safety of Li-Ion
Batteries using a Battery Electronics Unit
(BEU) for Space Missions
George Altemose
Aeroflex Plainview, Inc.
www.aeroflex.com/BEU
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Introduction
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Lithium-Ion batteries have become prominent in
space applications, because of their lighter
weight and lower cost than NiH2 batteries
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Lithium-Ion batteries require electronic cell
balancing to reduce the possibility of
overcharging or deep discharging
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Cell balancing is required to achieve the
maximum possible mission life for a LithiumIon battery
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BEU Development History
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The Aeroflex 8627 Battery Electronics Unit (BEU) was
designed, fabricated and tested to meet Boeing specifications
for use with Lithium-Ion batteries on the Boeing 702B and
other satellites
The basic approach for the BEU was developed by Boeing,
and is described in Patent 6,873,134. This patent describes
transformer-coupled DC-DC converters that transfer charge
over a bidirectional Share Bus
Aeroflex has completed the electrical and mechanical design
of the BEU. Engineering Models (EM) and Engineering
Qualification Models (EQM) have been fabricated and tested
In addition to cell balancing, the BEU provides additional
functions:
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Cell voltage monitoring
Battery voltage monitoring
Telemetry (MIL-STD-1553)
Bypass relay driver circuits
Reconditioning load control
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Benefits of Cell Balancing
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Cell balancing equalizes the charge among the cells
in a battery
Unlike other types of cells, Lithium-Ion cells do not
exhibit natural cell-to-cell balancing mechanisms
Lithium-Ion batteries may become unbalanced,
leading to one cell becoming overcharged, causing
cell damage
If cells are balanced, no cell will reach overcharge if
the battery is charged properly
During discharge, balanced cells discharge equally
For satellites with high power or long mission life,
balancing provides:
– Minimum battery size: no additional capacity is required to allow
for imbalance
– Protection from cell damage due to over-charge or over-discharge
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Desirable Features of a Cell Balancing System
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Autonomous operation
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Balancing of all cells to within millivolts of each other
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Balancing current directly proportional to voltage difference
between cells
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Operate continuously during charge, discharge and standby
modes
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Accurate cell voltage monitoring and telemetry
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High reliability
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Fault tolerant, for both cell faults and circuit faults
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Negligible degradation due to temperature, life and radiation
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Protection from Single Event Upset (SEU) faults
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Low power
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Light weight
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Cell Balancing by Share Bus
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Cell Balancing Characteristic Curve
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Functional Description of
Balancing Circuit
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Active DC-DC Converters interconnect all cells,
through a Share Bus
Controlled impedance in each balancing circuit sets
the Transfer Ratio to 1 ohm
Each balancing circuit contains a 1 amp fuse, which
opens if a cell shorts
Balancing is continuous and autonomous
Current flows from the higher voltage cells into the
Share Bus
Current flows into the lower voltage cells from the
Share Bus
Balancing does not utilize voltage measurements or
computations
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Cell Balancing Circuits
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Cell Balancing Circuit Waveforms
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BEU Functional Block Diagram
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BEU Functional Block Diagram
(Primary Side Only)
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Circuit Description – Balancing Circuit
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Forward converter with resonant reset
Resonant frequency is determined by parallel
combination of transformer primary inductances and
circuit capacitances
Transformer secondaries are connected to the Share Bus
by 1 ohm resistors and 1 amp fuses
Balance Clock is generated by Phase Lock Loop (PLL),
and provides zero-loss switching
PLL also generates a Monitor Clock synchronous to the
Balance Clock
1 amp fuses are designed to open in case of a shorted
cell and allows proper balancing among the remaining
cells
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Circuit Description –
Monitoring and High/Low Limits
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Monitor Clock drives Sample-and-Hold (S/H) circuits to derive
monitor voltages equal to the cell voltages
Monitor voltages are connected through Multiplexer (MUX) to
the 12-bit A/D Converter controlled by the ASIC processor.
MUX inputs also include three battery voltage buffers and a
precision 4.000 volt reference
ASIC processor performs auto-calibration (slope and offset
correction) using the precision 4.000 volt reference
ASIC processor checks each cell voltage against preset limits,
which are stored in the PROM. Typical limits are as follows:
– Low Cell: 3.2 volts
– High Cell: 4.2 volts
– Overvoltage Protection: 4.4 volts
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Circuit Description: Custom ASIC
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Fully synchronous design
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PLL and PWM (for power supply clock) circuits
run at 24 MHz
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All other logic runs at 12 MHz
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State machine architecture with no undefined
states
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Guaranteed recovery from single event upsets
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RHA Designator R: 100 Krad (Si)
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Process: Aeroflex UT 0.6 µ CRH Commercial
RadHard Gate Array
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Circuit Description –
SµMMIT 1553 Bus Controller
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Proven technology: has been used
on many previous programs
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Rad hard to 100 Krad (Si)
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Circuit Description – Bypass Relay Drivers
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Provides 2 amp pulses, compatible with NEA
Battery Cell Isolation Switches (referred to as
“relays”)
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Requires three separate and independent 1553
commands to activate a bypass relay.
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Two-fault tolerant with respect to accidental
contact closure
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OR’ed inputs can operate from two redundant
Control Cards
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Circuit Description –
Reconditioning Load Control
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Switches 5 amp battery reconditioning
load (load resistor is located with battery)
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Immune to single-point failures (2 series
FET switches with transformer-coupled
drive circuits)
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Requires two separate and independent
1553 commands to turn on the
reconditioning load
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OR’ed inputs can operate from two
redundant Control Cards
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Circuit Description –
Internal DC-DC Voltage Converter
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Input Voltage Range of 20-36 VDC
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Provides output voltages of +5 VDC (+ 1%)
and + 12 VDC
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Flyback topology with linear post-regulators
on every output
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Immune to spurious SEU outputs to protect
ASIC and SµMMIT from SEU transients
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Fully compliant with MIL-STD-461C EMC
requirements
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Redundancy & Other Reliability
Enhancements
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Redundant balancing circuits (Balancing Card and
Control Card) may be used simultaneously or
individually
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Bypass Relay Driver circuits are two-fault tolerant,
with respect to unwanted turn-on of bypass relay
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Reconditioning Load control circuit is one-fault
tolerant, with respect to unwanted turn-on of the
reconditioning load
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Triple battery voltage monitoring circuits are provided
on each Control Card
– Measured battery voltages may be compared to verify
accuracy of measurement circuits
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Test Results
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Cell balancing tests were performed
using capacitors in place of battery cells
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Monitor voltages were sent by telemetry
over the 1553 bus
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The 1 ampere load was provided on one
cell by placing a 3 ohm resistor across
the cell
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25°C, No Load
Cell Number
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+25°C, 1 Amp Load
Cell Number
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+75°C, No Load
Cell Number
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+75°C, 1 Amp Load
Cell Number
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-35°C, No Load
Cell Number
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-35°C, 1 Amp Load
Cell Number
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Mechanical Design
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Height: 5.25 inches
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Width: 5.25 inches
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Length: 11.5 inches
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Weight: 3.75 Kg
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Analyzed and tested for pyroshock,
vibration and thermal vacuum
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3 machined housings (slices), fastened
with 12 10-32 bolts
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Housings made of nickel-plated aluminum,
painted black for emissivity
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Battery Electronics Unit
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2 Balancing Cards
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2 Control Cards
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1 Bypass Relay Driver Card
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Balancing Card
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24 Cell Balancing Circuits
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Control Card
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ASIC, with PROM and POR
SµMMIT 1553 Interface
A/D, 4.000V Reference, MUX, 3 Battery Voltage
Buffers
Power Supply
On/Off Controls
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By Pass Relay Driver Card
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24 Relay Driver Circuits
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Reconditioning Load Control Circuit
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BEU Performance Characteristics
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Cell Voltage Balancing:
– 10 mv BOL (Beginning of Life)
– 20 mv EOL (End of Life)
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Cell Balancing Current (continuous): 1 ampere
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Cell Voltage Monitoring Accuracy:
– 10 mv BOL
– 20 mv EOL
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Battery Voltage Monitoring Accuracy:
– 0.3% of full scale
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Temperature Range: -34°C to +71°C
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Power (from +30 VDC bus): 9.5 watts
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Currents Drawn by BEU
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Summary
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Cell balancing has been shown to
significantly enhance the mission life of
Lithium-Ion batteries
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The Aeroflex BEU meets the specified
design goals, including autonomous
and continuous cell balancing while
operating in a space environment
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Acknowledgement
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The conceptual design of the BEU was
performed by Boeing Space Systems,
and is described in patent US 6,873,134
B2. The inventors are Stanley Canter,
Winnie Choy and Robert Martinelli
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Aeroflex designed and developed the
original BEU in accordance with Boeing
Space Systems specifications
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Action Space Guy
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