Cell Balancing Methods - Texas...

TI Information – Selective Disclosure

Cell Balancing Methods

1

Battery Management Deep Dive

BMS Systems & Applications

Nov 7-9, 2011 Dallas, TX

TI Information – Selective Disclosure 2011 Dallas BMS Deep Dive

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

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Root Problem

The Problem – Cell Mismatches

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Battery cell mismatch

State-of-Charge imbalances

Batteries age unevenly

Accurate SOC calculation and cell balancing is extremely important

SOC State of Charge

Cell Chemistry

Cell Age

Cell Temperature

Cell Voltage

Cell ESR Equivalent Series

Resistance Cell

History

TI Information – Selective Disclosure 2011 Dallas BMS Deep Dive 4

Cell Balancing What Causes Imbalance? • Capacity variation (1~2% cell-

to-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 self-heating at high discharge rate

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

Time 2 4 6 8 10

4.4

4.2

4.0

3.8

3.6

3.4

3.2

3.0

Overcharge

Capacity Under-used

Undercharge

High Cell Count battery systems are more likely to see imbalance due to temperature gradients and cell self-

heating at high discharge rates.


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

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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

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Passive Balancing

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Passive Balancing

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3.3 3.4 3.5 3.6 3.7 3.8 3.9

4 4.1 4.2 4.3

Cell 1 Cell 2 Cell 3 Cell 4 Cell 5 Cell 6 Cell 7 Cell 8 Cell 9 Cell 10

Volta

ge (V

)

Voltage Min Max

3.3 3.4 3.5 3.6 3.7 3.8 3.9

4 4.1 4.2 4.3

Cell 1 Cell 2 Cell 3 Cell 4 Cell 5 Cell 6 Cell 7 Cell 8 Cell 9 Cell 10

Volta

ge (V

)

Voltage Min Max

• Extra charge current is dissipated through a resistor

• Best for low charge/discharge currents • Low Cost Solution

Weakest Cell in the Pack


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

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Active Cell Balancing

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Active Balancing Extends Pack Life

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: 0.5-0.8C @ start of life Linear capacity degradation assumed

Nearly 20% more cycles!

ACB)

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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

2.75

3.00

3.25

3.50

3.75

4.00

4.25

0 50 100 150 200 250 300

Time [minutes]

Cel

l Vol

tage

[V]

Cell 1 VoltageCell 2 VoltageCell 3 VoltageCell 4 VoltageCell 5 VoltageCell 6 VoltageCell 7 VoltageOVL/UVL StatusOvervolt/Undervolt

Balancing Enabled

Cell balance restored within 40 minutes

Cells now reachundervolt limit together

All cells achieve same SOC levelwhen charging phase is terminated

Balancing EnabledAs supplied, cells showed variance in capacity and impedance - resultant multi-cell pack unusable without active balancing

(Balancing terminated by UVL )

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PowerPump™

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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

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PowerPump™ Schematic for 3S pack

• Factorize the problem…

• 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™…

V3

V2

V1

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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

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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

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Supported Balancing Modes

Rgs

Chf

Rgs

Chf

Chf

cell3

cell2

cell1

Q1

Q2

Q3

Q4

Q1

Q1

Q3

Rgs

Rgs

L

L

Rgs

Chf

Rgs

Chf

Chf

cell3

cell2

cell1

Q1

Q2

Q3

Q4

Q1

Q1

Q3

Rgs

Rgs

L

L

Rgs

Chf

Rgs

Chf

Chf

cell3

cell2

cell1

Q1

Q2

Q3

Q4

Q1

Q1

Q3

Rgs

Rgs

L

L

Rgs

Chf

Rgs

Chf

Chf

cell3

cell2

cell1

Q1

Q2

Q3

Q4

Q1

Q1

Q3

Rgs

Rgs

L

L

1 & 3 to 2 1 & 2 to 3 2 & 3 to 1 2 to 1 & 3

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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

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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.

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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

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Isolated Bi-Directional DC-DC Active Cell Balancing


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-directionalDC-DC


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

-+

-+

-+

-+

-+

-+

-+

Charge Flow

Switch Matrix

Cell Balancing Engine

CellModuleModuleCell

Isolated Bi-Directional

DC-DC

...

EMB1428 Gate Controller

EMB1499 PWM Controller

Switch Matrix


DC-DC

EMB1428 Gate Controller

EMB1499 PWM Controller

1 of 7

1 of 7

SPI4

SPI4


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)

• Goes to sleep mode when STOP command received (


Bi-Directional DC-DC Controller

• 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

To Module(40 – 70 V)

To Cell via Switch Matrix

(2.x – 4.x V)

Controller (EMB1499)

1234

Q2Q1Q3Q4

Vsen

Isen1

Isen1Vsen

EMB1412

Driver

GNDFGND

Delay

Isen2

Isen2

12V12VF

Bias Supplies

ENDIR

VSET

From EMB1428

Gate Decouple

+Cell

-Cell

+Stack

-Stack

From microcontroller


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

-+

-+

-+

-+

-+

-+

-+

Switch Matrix


DC-DC

...

Switch Matrix


DC-DC

– Bidirectional DC-DC • 2 transfers,

∴eff≈ 76%


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

SPI

Gate Controller

ENDIR

DIR_RTDONE

FAIL

DC-DC Controller

3

4Microcontroller


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


Cell Balancing Methodology Comparison

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Bleed Balancing PowerPump™ Isolated Bi-

directional 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


• BMS Demo Box and GUI for FAE’s to demonstrate bi-directional dc-dc active cell balancing

ACB BMS (Battery Management System) Demo Box

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Thank you!

Cell Balancing MethodsAgendaThe Problem – Cell MismatchesCell BalancingCell Balancing ImplementationBattery Cell Balancing MethodsPassive BalancingPassive BalancingCell Balancing: What’s Really NeededActive Cell BalancingActive Balancing Extends Pack Life�Active Cell Balancing Keeps Cells in Balance – �For Safety & Performance�PowerPump™What is Active Cell Balancing?PowerPump™ Schematic for 3S packPowerPump™ Schematic OperationPowerPump™ Schematic OperationSupported Balancing Modes Design Considerations for PowerPump™ External Circuits -DesignDesign Considerations for PowerPump™ External Circuits -LayoutActive Cell Balancing Pros/ConsSlide Number 22Isolated Bi-directional DC-DC BalancingBi-Directional DC-DC Cell Balance �Block DiagramSwitch MatrixBi-Directional DC-DC ControllerEfficiencyControl InterfacePros and ConsCell Balancing Methodology ComparisonACB BMS (Battery Management System) Demo BoxThank you!

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