Download Final Report – Team 1613

Sinusoidal Battery Charger
Team 1613
Faculty Advisor:
Dr. Sung Yeul Park
Team 1613 Members:
Andrew Brooks
Peter Darinzo
Alec Surprenant
Summary
The goal of this project is to design a sinusoidal ripple algorithm and charger to more efficiently charge
rechargeable lithium-ion batteries by monitoring dynamic battery characteristics. By using a sinusoidal
ripple, we can increase charging speed, lower recharging temperature, and extend battery recharge
cycles. This charging framework can be extended into applications including power electronics, electric
vehicles, and battery backup systems.
Background
The most prevalent adaptive charging algorithm currently in use is the constant current, constant
voltage (CCCV) charging algorithm. This charging mechanism holds a constant current with increasing
voltage for the beginning portion of the charging cycle, about 80% charged, and then holds a constant
voltage with decreasing current for the remainder of the charging cycle. An example charging profile is
shown below.
(source: http://www.fveaa.org/forums/index.php?topic=1302.0)
This charging mechanism does not account for all of the electrochemical changes that occur in the
battery as the battery is charged. Some possible negative effects include, cell overheating, charge
capacity loss, overcharging, and lowered recharge cycle life.
Statement of Need
With the growing popularity of rechargeable battery devices and systems is a growing need for more
efficient ways to recharge these batteries. Conventional battery charging algorithms do not consider
the time-varying electrochemical properties of these batteries. The ever-changing electrochemical
properties make for very complex and computationally demanding algorithms. By utilizing an adaptive
algorithm, the potential benefits include increased cycle life, energy savings, faster charging times, and
reduced costs.
Theory
Lithium ion batteries are composed of cells that use intercalation compounds as the electrodes. The
cathode is typically a metal oxide with a layered or tunneled structure on an aluminum current collector,
while the anode is a structure of graphitic carbon on a copper current collector. During charging, the
negative electrode is reduced, and lithium ions are intercalated into the interstitial space. The reverse
occurs during discharge, the negative electrode is oxidized, and the ions are de-intercalated, and
intercalated into the positive electrode.
(Source: http://www.tms.org/pubs/journals/JOM/0809/Fig2.jpg)
Standard charging methods such as CCCV do not take the battery’s electrochemical properties into
account. This has undesirable effects on the battery, such as lithium plating, the formation of solid
electrolyte interphase (SEI), and limited exchange current. Lithium plating occurs when Li+ ions are
deposited onto the battery anode faster than they can be intercalated. The result is a loss of capacity,
and in extreme cases can lead to a short circuit. SEI is a passivation film formed when solvents react with
the electrode surface. SEI spatially separates the electrolyte solvents from the electrode, yet is still
ionically conductive and imparts stability into the system. SEI is critical to battery function, however
continuous growth causes a loss of capacity and increased internal resistance. The combination of these
factors leads to shorter battery life and more losses.
(Source: http://pubs.acs.org/doi/abs/10.1021/jp3118055)
The batteries resistance is the most important characteristic from an electrical engineering perspective.
These phenomena occur during CCCV charging. In addition, a double layer capacitance due to a layer of
discrete charges forming on the electrode surface.
An equivalent circuit of the battery’s internal impedances can be constructed using resistor for ohmic
losses in series with a parallel combination of the charge transfer resistance, and the double layer
capacitance. The double layer capacitance is the result of a discrete layer of charge forming on the
electrode surface, and the charger transfer resistance limits the faradaic currents from electrochemical
reactions between the electrode and the electrolyte.
battery equivalent circuit
Direct current causes the charging of the double layer capacitance. Once fully charged, the entirety of
the applied current flows through the charge transfer resistance. This can be fixed through the
application of a sinusoidal ripple current. The time constant of the circuit can be calculated by the
product of Cdl and Rct. Since it takes several time constants to charge the double layer capacitance, a
ripple current of frequency greater than τ can reduce the effect of double layer charging, and increase
the rate of charge transfer.
Simple EIS curve
(source: http://www.gscsg.com/Electrochemical%20Impedance%20Spectroscopy.html)
The effects of frequency on battery impedance can be measured using a technique called
electrochemical impedance spectroscopy (EIS) by performing a frequency on the battery. At high
frequencies capacitive reactance and charge transfer resistance of the battery’s equivalent circuit are
minimized, leaving only the internal ohmic resistance. Therefore, the specific frequency of the charging
current ripple can be experimentally determined. The minimization of the battery’s internal impedance
results in faster charging, and decreased losses. The application of a sinusoidal charging current can also
mitigate the effects of polarization by reducing lithium plating and SEI. Implementing an algorithm to
detect optimal ripple current frequency can therefore reduce charging time and improving efficiency,
while extending battery life.
Solution
To provide a background of battery charging technology, a CCCV charger will first be developed. This
will serve as the basis for our sinusoidal design. Our charging profile will initially resemble a CCCV
charging profile in that initially the current will be constant at the maximum charging current (C/2).
There will be constant monitoring of the battery to detect when SEI and lithium plating begin. At this
point the optimal frequency is determined and a sinusoidal ripple current is applied by the controller.
This monitoring and ripple adjustment will occur until the battery reaches the constant voltage
conditions.
The basic overall design of the sinusoidal ripple current battery charger is shown in the following block
diagram.
Power Stage
Batteries
Vin
Iin
Vin
VBat
IBat
PWM
Ripple Calculator
Frequency
Calculator
Battery Specifications
For this project we will work with the Valence Energy Storage Solutions U1-12XP lithium iron magnesium
phosphate (LiFeMgPO4) battery. This battery has a nominal voltage of 12.8 V, capacity of 40 Ah,
maximum charging voltage of 14.6 V, and recommended charging current of 20A. A picture and full
specifications are shown below.
Power Stage **ANDREW PLEASE REVIEW THIS SECTION**
The buck converters main advantage over the standard buck converter is efficiency when used to power
a high current load, and moderate duty cycles. The power loss of a diode switch in the converter can be
calculated as the product of its forward voltage, duty cycle, and current. The power dissipated in a nMOSFET of equivalent function is the product of its current squared, its drain-source resistance, and its
duty cycle.
Taking a typical diode bias voltage of 0.7 V, the diode power loss in a standard buck converter under the
parameters of the project would be (0.7 𝑉)(1 − .8)(22 𝐴) = 3.08 𝑊, while the power loss in an
equivalent MOSFET, using 16 mΩ as a saturation resistance, would be (22 𝐴)2 (1 − .8)(.016 𝛺) =
1.55 𝑊, nearly a 50% decrease. Increased efficiency can be achieved using a duty cycle closer to 50%,
and selecting MOSFETs with a lower saturation resistance.
In addition, the synchronous buck can be operated in continuous conduction mode (CCM), while the
standard buck is limited to discontinuous conduction mode (DCM). CCM allows the inductor to become
negative, or reverse polarity. This is unwanted in most applications, however a negative portion of the
ripple current is needed to mitigate the effects of lithium plating and SEI.
buck converter
synchronous buck
(source: http://www.eetimes.com/document.asp?doc_id=1273245)
For our application for project, our only restriction to the design of our synchronous buck converter is
regulating our voltage output according to the four battery in many multiple formations (series, parallel,
or a combination of both). The different combinations will greatly affect the output voltage and the the
current flowing through the batteries. This is where the two MOSFETs come into play. MOSFET 1 will be
ON at time A, MOSFET 2 will be OFF at time A and constantly turning ON/OFF controlled by a square
wave with a certain frequency. Changing this frequency will raise or lower the voltage and the current
across the battery load. In order to have the correct component values, consider the maximum
conditions of the battery load in both series and parallel configurations.
Our battery load constants ask for a maximum voltage of 14.6V with a voltage ripple of about 7% based
on the delta voltage out of 2V. The ideal current through each lithium battery is C/2, which is 20A. This
means the total current of the four batteries in series is 20A. The voltage across these batteries will
13.6V*4 = 54.4V. In a series configuration the calculated values are shown below. Based on the load
requirements, the value of the inductor is 33.75 µH and the capacitor is 4.594 µF.
**I know we need to adjust the numbers in here somewhere**
In a parallel configuration, the voltage across the configuration is just 13.6 V to keep each of the four
batteries at a consistent voltage. The current across the battery configuration should be 20A*4 = 80A.
The calculated values of the parallel configuration are shown below. Based on the load requirements,
the value of the inductor is 0.8 µH and the capacitor is 91 µF.
Taking the maximum value from both cases will allow us to change the configuration of the batteries
and have an effective charger. But then resistance across the load, now becomes a problem. As shown
above, the resistance values are different between configurations. The easiest solution to this is just
sticking with one configuration for our charger. Another, more complex option is to put two resistors in
parallel to reach the lower rated resistance of the load. Adding an ON/OFF switch to the higher resistor
will allow the current to be cut off from that particular resistor and allowing you to do the series
configuration.
The power stage will consist of a synchronous buck converter or a DC/DC step down converter
connected to the battery. The design will utilize MOSFETs instead of diodes due to their high efficiency
with heavy loads and allows for negative currents. The resistance of the load resistor (battery
impedance) will be very critical to our application as we want the charging algorithm to change the
charging characteristics as the load (battery) impedance changes. The synchronous buck converter,
without protection to the battery is shown below. The frequency, V2 is controlled by the PID controller,
which controls the switching of the MOSFETs. As shown in below, these circuit is the series
configuration, parallel configuration, and a combination of both respectively. The PID controller will take
the voltage out and compare the gain with a reduced voltage out and amplify the signal to control the
switching of the MOSFETs.
**WE NEED AN UPDATED PICTURE WITH THE NEW VALUES**
Critical charging parameters, i.e. charging current and battery voltage, will be monitored via closed loop
feedback. These feedback signals will be used by a microcontroller and digital signal processing board to
modify the operation of the synchronous buck converter during charging by changing the switching
frequency to change the current ripple. The final design of the charging stage circuitry will be
implemented on a printed circuit board using the program. The main reason why we choose Altium is
because Professor Park has a license already and it is a simple, easy to use program to get familiar with.
Basic Idea of Altium (Not This Project) **I THINK WE CAN GET RID OF THIS PICTURE SINCE IT IS NOT
RELEVANT TO OUR PROJECT**
Once a sufficient working knowledge of charging technology and electrochemical theory has been
obtained, the project will proceed into the development of a ripple current charging algorithm, and
experimental results can be acquired to validate its performance.
Controller
Here are the transfer functions for the power stage and controller:
Plant open loop transfer function:
0.1125 𝑠 + 6.579 × 106
7.81𝑥10−5 𝑠 2 + 0.4655 𝑠 + 6250
Controller transfer function:
5.549 × 102 𝑠 2 + 4.215 × 1019 𝑠 + 8.00523
1.443 × 109 𝑠 3 + 8.118 × 1014 𝑠 2 + 1.142 × 1020 𝑠
Loop transfer function:
6.242 × 1012 𝑠 3 + 3.655 × 1020 𝑠 2 + 2.774 × 1025 𝑠 + 5.266 × 1029
4.44 × 104 𝑠 5 + 2.855 × 1010 𝑠 4 + 5.69 × 1015 𝑠 3 + 3.78 × 1020 𝑠 2 + 1.353 × 1025 𝑠
Plant Bode Plot
Plant + Controller Bode Plot
The controller will be implemented in dSpace with MATLAB integration. The dSpace controller will
handle all of the analog to digital conversions, PWM signal generation, and monitoring of the system.
The algorithm will be implemented in MATLAB. By using these systems in conjunction, we are able to
quickly adjust values in our algorithm and the time required to implement is reduced. We will continue
to explore using the TI DSP at a later time. The primary focus of this device is for testing and verification
of the theory.
**THE PREVIOUS WAS ADDED. CHANGE AS YOU SEE FIT**
Other Design Components
For this design we also need to incorporate a protection circuit so that we do not create an unsafe
operating environment. This protection circuit will ensure that we operate within the safe ranges for
the batteries we are using. Currently, this area of the design have not been designed, however, safety is
always included in our design.
We also have current and voltage sensors in the lab that need verification with our current design
specifications. These current and voltage sensors were purchased previously and we are not sure if they
are suitable for our design.
Project Plan
Timing
Fall 2016
Spring 2016
-
Develop working knowledge of PCB design software, Altium
Research background theory on electrochemistry, electrochemical impedance, and charging
methods specific to lithium-ion rechargeable batteries
Develop familiarity with microcontroller and its function in digital control systems
o Functions including: GPIO, ADC, PWM, and interrupts
Implement CCCV charger design
Review previous work done by Yong-Duk Lee and others to understand previously
developed algorithms
Use previously developed algorithms for sinusoidal ripple current charging and integrate
additional knowledge from electrochemical research
Timeline
Task
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2015
10
2016
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12
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2
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Project Statement
General Battery Research
Li-Ion Research
SEI Research
Project Proposal
Design Review
Initial Simulations
Parts Ordering
Sensor Verification
Design Prototype
Design Algorthm
CCCV Design
Testing and Verification
Work Organization
The primary responsibilities of the team are as follows:
● Andrew Brooks: PCB design, power stage design
● Peter Darinzo: Battery research, algorithm development, power stage design
● Alec Surprenant: Microprocessor (controller) programming, algorithm development
The team is also supported by two graduate students:
●
●
S M Rakiul Islam
Md Kamal Hassain
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