Cell Balancing Methods

Transcription

1 Battery Management Deep Dive Nov 7-9, 2011 Dallas, TX Cell Balancing Methods BMS Systems & Applications 1

2 Agenda The Problem-Cell Mismatches Cell Balancing & Implementation Cell Balancing Methods Passive Balancing Cell Balancing: What is Really Needed Active Cell Balancing PowerPump Isolated Bi-Directional DC-DC 2

3 The Problem Cell Mismatches Root Problem Battery cell mismatch Batteries age unevenly Cell Voltage Cell ESR Equivalent Series Resistance Cell Temperature SOC State of Charge Cell History Cell Chemistry Cell Age Accurate SOC calculation and cell balancing is extremely important State-of-Charge imbalances 3

4 What Causes Imbalance? Capacity variation (1~2% cellto-cell for same model) Charging state difference (i.e. SOC difference) Impedance variation (up to 15%) causes voltage difference when charging/discharging Localized heat degrades cells faster than others, particular selfheating at high discharge rate High Cell Count battery systems are more likely to see imbalance due to temperature gradients and cell selfheating at high discharge rates. Cell Balancing Why Balance? Unbalanced cells lead to: Reduced run-time due to... Premature charge termination Early discharge termination Further cell abuse from cycling above or below optimal cell voltage limits Undercharge Capacity Under-used Overcharge Time

5 Cell Balancing Implementation Control Strategy a Layered Approach Balance Cell Terminal Voltage Easiest to understand provides the basis for more complex control Balance Cell OCV estimates Based on Pack current and Cell Impedance measurements Compensates for impedance differences Balance for SOC at 100% Based on how far each cell is from Full Charge Capacity Compensates for capacity divergence Direct Measurements Cell Terminal Voltage, Pack Current (using synchronous measurements) Derived Variables Cell Impedance, OCV, SOC, Qmax Control Action Move energy where and when its needed to minimize global imbalance Passive or Active Balancing 5

6 Battery Cell Balancing Methods Dissipative (Passive) Balance current is dissipated as heat (wasted) Best for very low charge/discharge currents Cannot reclaim capacity lost from mismatch Least Expensive Charge Shuffling (PowerPump ) Balance current is shuffled between adjacent cells Balancing of currents of10ma to 1A and above Balancing during Charge and Discharge Inductive and Capacitive options Isolated DC-DC Balancing Current is moved between individual cells and module Can charge and discharge any cell with high efficiency Fully scalable to very high balance currents 6

7 Passive Balancing 7

8 Passive Balancing Extra charge current is dissipated through a resistor Best for low charge/discharge currents Low Cost Solution Weakest Cell in the Pack Voltage (V) Voltage (V) Cell 1 Cell 2 Cell 3 Cell 4 Cell 5 Cell 6 Cell 7 Cell 8 Cell 9 Cell Cell 1 Cell 2 Cell 3 Cell 4 Cell 5 Cell 6 Cell 7 Cell 8 Cell 9 Cell 10 Voltage Min Max Voltage Min Max 8

9 Cell Balancing: What s Really Needed Approach Higher balance current balance within single cycle e.g. 20% imbalance in 1 cycle Balancing as needed during charge /discharge and /or idle Move energy when and where it is needed rather than bleed it off Benefits Avoid cell abuse which leads to further imbalance Reduce time above 4.20 V Maximum runtime Every cycle Enhanced safety Stay within specified voltage envelope 9

10 Active Cell Balancing 10

11 Active Balancing Extends Pack Life ACB) Nearly 20% more cycles! Notes: 66 Ah 3.6V Cells, 1P96S 95 Good Cells: 3000 cycles to 20% capacity loss 1 Weak Cell: 3000 cycles to 25% capacity loss Charge/Discharge Rate: start of life Linear capacity degradation assumed 11

12 Active Cell Balancing Keeps Cells in Balance For Safety & Performance PowerLAN Multi-Cell Monitoring and Balancing Li-ion cells - 7S1P pack - Discharge/Charge Cycle PowerPump Balancing Enabled after 1st Cycle - Runtime Extended to Maximum Cell Voltage [V] As supplied, cells showed variance in capacity and impedance - resultant multi-cell pack unusable without active balancing Balancing Enabled Cell balance restored within 40 minutes Balancing Enabled All cells achieve same SOC level when charging phase is terminated 3.50 Cell 1 Voltage Cell 2 Voltage Cells now reach undervolt limit together (Balancing terminated by UVL ) Cell 3 Voltage Cell 4 Voltage Cell 5 Voltage Cell 6 Voltage Cell 7 Voltage Overvolt/Undervolt OVL/UVL Status Time [minutes] 12

13 PowerPump 13

14 What is Active Cell Balancing? Within a battery pack, ACB transfers charge from one set of cells to another Allows for true balancing of cells with very little wasted energy TI's patented PowerPump /ACB technology uses inductive strategy for high efficiency and wide balancing current range TI can provide solutions for balancing currents anywhere from 10mA to 1A and above, depending on application Possible applications: Extended life packs Extended runtime packs Dissimilar Cell Capacity Packs 14

15 PowerPump Schematic for 3S pack Factorize the problem V3 V2 V1 Bidirectional PowerPump transfers energy efficiently between adjacent cells Bucket brigade allows redistribution anywhere in pack Move energy where and when its needed to minimize global imbalance PowerPump burst mode is time limited for each control loop iteration Operation of individual PowerPump 15

16 PowerPump Schematic Operation Example : Pumping from Cell 3 Cell 2 P3S frequency is 200 khz, 33% positive Duty Cycle P3S Turns PFET ON DI/DT = V/L : Energy in Inductor builds 16

17 PowerPump Schematic Operation Example : Pumping from Cell 3 Cell 2 P3S Turns FET Off Current continues through NFET (body diode) Energy transfers to Cell 2 Time average Balancing current is 40 to 50 ma 17

18 Supported Balancing Modes Rgs Q1 Rgs Q1 Rgs Q1 Rgs Q1 Q1 L Chf cell3 Q1 L Chf cell3 Q1 L Chf cell3 Q1 L Chf cell3 Rgs Q2 Rgs Q2 Rgs Q2 Rgs Q2 Chf cell2 Chf cell2 Chf cell2 Chf cell2 Rgs Q3 Q1 Rgs Q3 Q1 Rgs Q3 Q1 Rgs Q3 Q1 Q3 L Q3 L Q3 L Q3 L Chf cell1 Chf cell1 Chf cell1 Chf cell1 Rgs Q4 Rgs Q4 Rgs Q4 Rgs Q4 1 & 3 to 2 1 & 2 to 3 2 & 3 to 1 2 to 1 & 3 18

19 Design Considerations for PowerPump External Circuits -Design Size the MOSFET correctly (avoid basing the MOSFET current handling on the average balancing current consider the peak currents) Use very low resistance MOSFET (reduces heat dissipation) Inductor Saturation (Inductor saturation is based on peak currents) Traces width sizing Peak currents cause large voltage drops and spikes that causes damage like high voltage transients Size wires from the cells robustly Lower ESR in the output caps (High voltage ripple can be very spiky) 2-10uf Caps in parallel are better than a single 20uF Cap Avoid long traces between MOSFETs and the Inductor Schottky forward voltage drop should be lower than the MOSFET body diode turn-on Voltage drop on the Schottky determines the max delta between cells 19

20 Design Considerations for PowerPump External Circuits -Layout Peak inductor currents can be as high as 1 to 2 amperes or higher depending on inductor value Use Inductors with ratings 2x of peak to accommodate these high currents Overhead for Temp as tem goes up, saturation currents goes down PowerPump components are located as close as possible to each other and connected with short, wide traces Minimize the impedance of the return path for the high-frequency signals. 20

21 Active Cell Balancing Pros/Cons PROS Reusing available energy Extended runtime (on-the-go use) Extended lifetime (warranty) Significant degree of freedom in system design Solution size/cost can be quite small for the right app CONS Added cost and size of BOM (inductors, schottkys, FETs) Exotic new gauging algorithm development for IT (BattSense) New voltage- or capacity-based balancing algorithms Risks inherent in brand new hardware 21

22 Isolated Bi-Directional DC-DC Active Cell Balancing 22

23 Isolated Bi-directional DC-DC Balancing Current is moved between individual cells and module Can charge and discharge any cell efficiently High performance regardless of chemistry Scalable to very high balance currents Gate Controller Bi-directional DC-DC 23

24 Bi-Directional DC-DC Cell Balance Block Diagram Architecture is based on grouping of up to 7 cells Charge = Cell Module Discharge = Module Cell The chipset combination of EMB1428 and EMB1499 is controlled by a single command via SPI EMB1428 Gate Controller 1 of 7 Switch Matrix EMB1428 Gate Controller 1 of 7 Switch Matrix 4 4 EMB1499 PWM Controller SPI Isolated Bi- Directional DC-DC Charge Flow Module Cell Cell Module EMB1499 PWM Controller SPI Isolated Bi- Directional DC-DC Cell Balancing Engine 24

25 Switch Matrix EMB1428 Gate Controller 12 channel floating NFET driver that is designed specifically for BMS systems Automatically make multiple switch selections based on an input of a simple cell selection command (SPI) From Top Gate Controller (EMB1428) From Top PWM Controller (EMB1499) From Bottom Gate Controller (EMB1428) Vg0' Vg11' Vs0' - Vs11' Cell + Cell - Vg0' Vg11' Vs0' - Vs11' Vg11' Vs11' Vg10' Vs10' Vg9' Vs9' Vg8' Vs8' Vg7' Vs7' Vg6' Vs6' Vg2' Vs2' Vg1' Vs1' Vg0' Vs0' Vg7 Vs7 Battery Connector Goes to sleep mode when STOP command received (<1µA) From Bottom PWM Controller (EMB1499) Cell + Cell - Vg11 Vs11 Vg10 Vs10 Vg9 Vs9 Vg8 Vs8 Vg6 Vs6 Vg2 Vs2 Vg1 Vs1 Vg0 Vs

26 Bi-Directional DC-DC Controller Vsen The EMB1499 is specially designed to control the active clamp forward topology Has the ability to control the charge current in both directions (cell charge or discharge) Current can be adjusted via VSET voltage level +Stack To Module (40 70 V) -Stack Q4 Q3 Q1 Q2 Isen2 Isen1 Driver EMB1412 Gate Decouple Delay GND To Cell via Switch Matrix (2.x 4.x V) 12VF 12V Vsen Isen1 Isen2 EN DIR VSET +Cell -Cell Controller (EMB1499) GNDF Bias Supplies From EMB1428 From microcontroller 26

27 Efficiency More direct cell balancing Much better than other architectures that require multiple transfers DC-DC Typical Efficiency: Charge (Module Cell) transfer efficiency 88% Discharge (Cell Module) transfer efficiency 86% Between any 2 Cells in module, transfer efficiency 76% Example, cell 9 high, cell 1 low Bidirectional DC-DC 2 transfers, eff 76% Switch Matrix Switch Matrix Isolated Bi- Directional DC-DC Isolated Bi- Directional DC-DC 27

28 Control Interface Simple Control Single SPI start or stop command controls the cell selection and DC-DC control EMB1428 has built-in detection of illegal commands EMB1499 has built-in fault detection mechanisms: Over-voltage, under-voltage, over-current and temperature threshold detection Time-out detection Controlled current ramp-down on fault Fault codes reported back to microcontroller Very low microcontroller requirements Gate Controller DC-DC Controller Microcontroller 4 SPI EN DIR DIR_RT DONE FAIL 3 28

29 Pros and Cons Pros Scalable from 6 14 cells Adjustable current level High current for large capacity cells Fast time to balance Near real-time compensation for cell mismatch Higher worst-case efficiency between any cells in module Cons Higher cost per channel High component count Large PCB area 29

30 Cell Balancing Methodology Comparison Bleed Balancing PowerPump Isolated Bidirectional DCDC Cell Count >2 >3 >7 Cost of Implementation Low Mid High Balancing Current Low Scalable Scalable Time to Balance Slow Fast Fast Efficiency Low Mid High 30

31 ACB BMS (Battery Management System) Demo Box BMS Demo Box and GUI for FAE s to demonstrate bi-directional dc-dc active cell balancing 31

32 Thank you! 32