Overcharge Protection and Cell Voltage and Cell Temperature Monitoring for Lithium-Ion Batteries NASA Aerospace Battery Workshop 11/16/10 to 11/18/10 George Altemose Aeroflex Plainview, Inc. www.aeroflex.com/BEU 1 Introduction Lithium-ion batteries are often employed in spacecraft applications, and are commonly charged from solar arrays. It is desirable to attain a high SOC for the lithium-ion battery. However, overcharging even one cell can cause catastrophic failure of the battery. Cell voltage monitoring and Overcharge Protection (OCP) are essential for a reliable lithium-ion battery charging system. 2 BIE Development History The Aeroflex 8675 Battery Interface and Electronics Assembly (BIE) was designed, fabricated and tested to meet specifications provided by Orbital Sciences Corp. for use with lithium-ion batteries in the Commercial Orbital Transportation System (COTS) to deliver cargo to the International Space Station at low earth orbit (LEO). Aeroflex has completed the electrical and mechanical design of the BIE. Engineering Models (EM), Engineering Qualification Models (EQM) and flight units have been fabricated, tested and delivered to Orbital. The BIE provides the following functions: – Analog telemetry for monitoring of cell voltages, battery voltage and cell temperatures – Independent Overcharge Protection (OCP) – Battery on/off control through two series 50 A contactors – Access port for connection to external cell balancing circuit 3 Benefits of Cell Balancing For applications with long mission life, continuous cell balancing maintains equal charge on all cells, allowing high SOC to be achieved, without the possibility of overcharge. For short mission life (typically less than 90 days), on-board cell balancing is typically not required, because the cells typically do not become highly unbalanced. The COTS vehicle does not employ on-board cell balancing, because the mission life is 45 days. The BIE provides access to an external cell balancing circuit. 4 Lithium-Ion Battery Connection Typical Spacecraft Application 5 Key Features of Lithium-Ion Battery Charging Circuit Cells are in series. All cells have identical charge and discharge currents. Battery is charged until the desired total battery voltage is achieved. If all cells have equal cell voltage (cells are in balance), the maximum voltage on any cell is 1/8 of the total battery voltage. If the cells are not in balance, any cell voltage may be greater or less than the average cell voltage. In this case, the maximum total battery charge voltage must be reduced, in order to prevent the highest cell from being overcharged. 6 Lithium-Ion Battery Connection with Battery Interface and Electronics Assembly (BIE) 7 Key Features of the BIE The BIE is part of the Battery Assembly, and provides the electrical interface between the battery and the spacecraft. The BIE contains Analog Conditioning circuits, providing 05V conditioned telemetry outputs for each cell, for the total battery, and for eight thermistor temperature sensors. The OCP circuits monitor each cell voltage. If any cell voltage exceeds 4.500 V, K1 and K2 open, isolating the battery from the charger. This circuit is dual redundant, and no single point failure can cause K1 or K2 to open inadvertently. K1 and K2 may be controlled from external sources, to provide on/off control of the battery. A connector port provides access for an external balancing circuit. 8 BIE Analog Conditioning Circuit BME-2 Card 9 Key Features Analog Conditioning Circuit MUX 1 and MUX 2 provide differential measurement of all eight cell voltages. A software calibration algorithm is provided with the BIE to compensate for resistor tolerances. Using this algorithm, overall accuracy of 0.1% is achieved, including effects of temperature, radiation and aging. MUX 3 provides multiplexed thermistor voltages, in range of 0-5 V. MUX 4 provides multiplexed 1 mA excitation current. Each thermistor receives excitation current only when it is interrogated. Excitation current is identical for all thermistors. A precision 1.000 kohm (+ 0.1%) resistor at an input to MUX 3 allows software calibration of the 1 mA excitation current. 10 Overcharge Protection (OCP) Circuit BME-1 Card 11 Key Features OCP Circuit Two Overvoltage (OV) Sense circuits for each cell, in two groups (Group A and Group B) of eight each. Each OV Sense circuit provides a logic “1” output if its cell voltage exceeds 4.500 V. If any Group A output is high, P-FETs Q2 and Q3 turn on. If any Group B output is high, P-FETs Q4 and Q5 turn on. If Q2 and Q4 turn on, K1 coil is energized. If Q3 and Q5 turn on, K2 coil is energized. If either K1 or K2 is energized, the battery is isolated from the charger, and the cell voltages cannot rise higher. No single component failure can cause either K1 or K2 coils to energize. 12 OCP Circuit Logic Table Any Group A OV Sense On (>4.500 V) Any Group B OV Sense On (>4.500 V) Q2 State Q3 State Q4 State Q5 State K1 State K2 State Battery Connected To Charger No No Off Off Off Off Closed Closed Yes No Yes Off Off On On Closed Closed Yes Yes No On On Off Off Closed Closed Yes Yes Yes On On On On Open Open No 13 Overvoltage (OV) Sense Circuit (1 of 16 on BME-1 Card) 14 Key Features Overvoltage (OV) Sense Circuit Each OV Sense circuit uses an ASIC containing a precision bandgap (BG) voltage reference (trimmed to 2.000 V) and two comparators. The 16 OV Sense circuits are fabricated using the Chip On Board (COB) process. The ASIC and other components are in chip form, and are encapsulated in wells on the BME-1 Card. Each OV Sense circuit trips at 4.500 V, with hysteresis of approximately 200 mV. An input EMI filter prevents nuisance trips due to transients. Test Injection input allows OV Sense circuits to be tested while connected to battery. 15 Immunity to Single Point Failures The BME-1 Card and the BME-2 Card are completely independent. A component failure on one card cannot induce a secondary failure on the other card. The contacts of relays K1 and K2 are in series. A relay failure, e.g. a welded contact or open coil, cannot prevent the BIE from performing its isolation function. The BME-1 Card contains dual redundant (two groups of 8) overvoltage comparator circuits. A single point failure in any comparator circuit cannot cause K1 or K2 to open inadvertently. K1 and K2 are driven by a series/parallel configuration of FET driver transistors. No single FET failure can cause K1 or K2 to open inadvertently. External access to the coils of K1 and K2 are provided by multiple diodeOR paths. No single diode failure can disable the ability to energize the coils of K1 or K2. 16 Front Panel 17 Chip On Board (COB) Module Technology The brown areas contain the active die which are encapsulated after pre-seal inspection. 18 BME-1 Card (Encapsulated) 19 BME-2 Card (Encapsulated) 20 BIE Mechanical Design Height: 11.7 inches Width: 6.95 inches Length: 3.63 inches (excluding connectors) Weight (Complete BIE Unit): 3.0 Kg (6.6 pounds) Weight (BME-1 Card): 0.21 Kg (0.46 pounds) Weight (BME-2 Card): 0.23 Kg (0.51 pounds) Analyzed and tested for pyroshock, vibration and thermal vacuum Fastened to Battery Assembly with eight bolts, 8-32 size Housing made of nickel-plated aluminum, painted black for emissivity 21 Summary Lithium-ion batteries are susceptible to damage from overcharging. A high-reliability charging system for a lithium-ion battery should contain redundant paths to insure that overcharging cannot occur. The Aeroflex BIE contains analog serial telemetry outputs for cell voltage and cell temperature, as part of the battery charge management system. The BIE also contains an independent Overcharge Protection (OCP) circuit, which isolates the battery from the charger if any cell voltage exceeds 4.500 volts. The BIE has a port to allow connection to an external cell balancing circuit. 22 Acknowledgements The BIE project has been a co-development effort between Orbital Sciences Corporation and Aeroflex Plainview. The original technical specifications for the BIE were provided by Orbital Sciences Corporation. These specifications included electrical performance, mechanical characteristics, immunity to single-point failures, interface to the lithium-ion battery, and other characteristics required for the COTS application. Aeroflex wishes to thank Karl Noah and Maggie Figueras of Orbital for their efforts in reviewing this presentation, and for their many helpful suggestions and corrections. 23