Critical Safety Elements in Smart Battery Pack Design Smart Battery Data Expo Portable Design Conference January 15, 2002 Iilonga Thandiwe Staff Engineer Motorola Energy Systems Group
Safety Circuit Design Cell Characterization System Knowledge and Usage Environment Analysis and Test Temperature Shock -40 C/+85 C, 1 hr soak 24 cycles, 48 hrs total 4.325V 4.375V 4.425V 4.475V 4.9V 12v Paxson Pippen Drop w/ Host - 1 Drop w/o Host - 1 POV SOV min Volatility 50mV 425mV min min Smart Battery Temperature Profile Vibration - 1 SMBus Safety Features OVER CURRENT THRESHOLDS AND POWER 110 5.6A 10A=typ 12A 445 fuse will not open at 40A for 8.3ms 50% duty cycle pulse at 100C for over 300,000 100 cycles Over Current Threshold 1605A fuse max rated current capacity =50A, fuse opens in 8ms at 90A [MM1414 Tdelaymax =3ms] 90 short circuit power delivery 15ms max Ta=60C this is the max single pulse power that the FETs can dissipate at 60C for the max OC delay 27W (34A) 102W time of 15ms, if the current is high enough to active the 2nd OC delay 80 (tmax =3ms) then Pdmax=180W 100W this is the maximum power delivered to the FET by Paxson 70 under a short circuit condition all calculations assume 0 trace impedance 93W this is the maximum power delivered to the FET by Pippen (if Z trace = 20mohm then Imax =75A, Pdmax=77W) under a short circuit condition 60 Temp (C) 54W Constant Power Discharge @ 45C ambient 4.0Ah, 4S2P pack Mechanical Actuation - 2 Temperature Cycling Humidity AGREE Drop w/ Profile Host - 2 Drop +70 C, w/o 90%RH Host - 2 88 hrs total 88 hrs total 50 40 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 Time (minutes)
Safety Test Requirements for Li Ion Cells Cell Characterization Cell safety performance data should be thorough Optimum safety circuit design strongly depends on good cell characterization data Testing is crucial Test Failure Description Purpose Referenc Requirements Code Leak Vent Fire Explosion Other Specific safety abuse tests. 1 A Ability to withstand thermal abuse Thermal abuse, oven and test relative cell stability. modified test at 150 C for 60 Minimum survivability is 10 minutes UL 200 C minutes (UL spec). 1a A 2 A 3 A 4 B 5 B 6 B 7 B 8 B 9 B Thermal abuse, oven test at 130 C for 60 minutes Crush, flat plates as per UL 2054 Ability to withstand thermal abuse. Test performed only if cell fails #3 before 60 minutes. Ability to withstand mechanical abuse Impact, round bar and Ability to withstand mechanical weight as per UL abuse from sudden impact 2054 Short circuit, < 0.5 ohm load, 60 C Ability to withstand short circuit Short circuit, < 0.5 ohm load, room temp. Ability to withstand short circuit Overcharge at I 0, V= 5 V Overcharge at 3I 0, Survive minimal level of overcharge Survive faulty charger condition V= 4.5 V Overcharge Determine maximum safe level of mapping: 3 cells at 5 overcharge for safety circuit different test design currents; V= 10 V Forced discharge, C/5 rate to -10 V min for 12.5 hours Ability to withstand deep discharge caused by faulty electronics or cell imbalance in multicell application modified UL/ modified UL/ (UL has different method) 200 C 150 C 150 C Runaway onset voltage > circuit control voltage
Current (amperes) Current (amperes) 2.8 2.4 2.0 1.6 1.2 0.8 0.4 0.0 2.8 2.4 2.0 1.6 1.2 0.8 0.4 0.0 Cells are safe when charged in this region Cells are safe when charged in this region Supplier A Unsafe Region Supplier B Cells are UNSAFE when charged in this region 4.0 4.2 4.4 4.6 4.8 5.0 5.2 5.4 5.6 5.8 6.0 6.2 6.4 6.6 Lithium Ion Cell Failure (overcharged, insulated cells) All cells are not created equally Failure to take into consideration the differences between cells could lead to a safety circuit that is either under-designed or over-designed Safe Flame
Smart Battery Under Voltage Discharge FET Charge FET fuse is thermally coupled to FETs Thermal fuse w/heater B+ Over Current Over Current Sense Regulator Analog Front End, A/D converter Li-ion Protection IC [Over Voltage, Over Current, Under Voltage] Temperature Sensor Microcontroller Core EEPROM Oscillator Fuel Gauge IC Secondary Safety (V, I, Temp) SMBus Interface I/O Over Current Sense 5 FET Over Primary Temp Over Voltage LEDs Fuel Gauge switch Secondary Over Voltage Clock Data B- R sense
Smart charger communicates with battery. Data and status are used by charger to control charging, B+ e.g. battery The T pin serves specifies as a Safety charging Signal voltage preventing and current. chargingwarnings, e.g. of unrecognized and over unsafe temp batteries. alarm, are used to maintain safe operation. Smart System Safety Elements Li-ion Protection IC OC Communication integrity is enhanced through packet error checking. T Reg Temp Core Secondary (V, I, Temp) DATA control Host AFE A/D comm CLOCK OC EEPROM Osc I/O 5 Fuel Gauge/Communications IC Fuel Gauge switch B- Smart Charger Smart Battery Micro core enables additional safety firmware such as use/abuse histograms, failure predictive algorithms, loss of reliability, etc.