Battery Charging Redesign for Large Scale Vehicle 2, CUTTHROAT

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Battery Charging Redesign for Large Scale Vehicle 2, CUTTHROAT Powered By Docstoc
					           UNIVERSITY OF IDAHO ELECTRICAL AND COMPUTER ENGINEERING – MOSCOW, ID
                     NAVY ACOUSTIC RESEARCH D ETACHMENT – BAYVIEW, ID




BATTERY CHARGING REDESIGN FOR LARGE SCALE
VEHICLE 2, CUTTHROAT
LSV2 Battery Charging Optimization



                   Christopher Douglas, James Randall, David Hooker
                                 autovolt@uidaho.edu
                                       12/15/2008
Abstract
       Team AutoVolt was formed in order to study alternate charging schemes for recharge of
the all electrical 1/3 scale Virginia class submarine dubbed the Large Scale Vehicle 2 (LSV2),
propulsion batteries in order to increase battery life.
       Increased performances will be achieved by improving capacity retention throughout
the batteries’ life through implementing new charging algorithms. The schemes (discussed in
section 4) are designed to prevent both undercharge and overcharge, thus decreasing
degradation of the batteries. These separate schemes also include other benefits elaborated
upon in their respective sections, which extend the life of the batteries. Battery life will also be
increased through automation of the charge. Automation will also result in better performance
and lower risk of error in the system. The current manual charging done is performed by
technicians, leaving room for human error in adjustments and lack of adequate observation,
both of which can lead to battery degradation. Further disadvantages of the existing system
will be outlined in section 1.
                                                                           3


Useful definitions
VRLA          - Valve Regulated Lead Acid batteries
CI            - Current Interrupt overcharge method
CC/CV         - Constant Current / Constant Voltage recharge method
ZDV           - Zero Delta Voltage overcharge termination method
ORE           - Oxygen Recombination Efficiency
EOL           - End of Life
BOL           - Beginning of Life
VCCS          -Voltage Controlled Current Step Down Charge (also see FC)
FC            - Fast Charge charging method
PSOR          -Partial State of Recharge charging method
OC            - Over Charge
LSV2          - Large Scale Vehicle 2
ARD           - Acoustic Research Detachment
NSWCCD        - Naval Surface Warfare Center Caderock Division
Ah            - Ampere Hours
                                                                                                                                                                  4


                                                                    Table of Contents


Contents
1.        Background ........................................................................................................................................... 5
     1.1.        Motivation for Work ..................................................................................................................... 5
     1.2.        Identification of Need ................................................................................................................... 7
     1.3.        Expected Benefits ......................................................................................................................... 7
2.        Problem Definition ................................................................................................................................ 7
     2.1.        Goals and Deliverables .................................................................................................................. 7
     2.2.        Specifications and Constraints ...................................................................................................... 8
3.        Project Plan ........................................................................................................................................... 9
     4.      Concepts Considered ...................................................................................................................... 10
     4.1.        Zero Delta Voltage (ZDV) ............................................................................................................ 11
     4.2.        Current Interrupt (CI) .................................................................................................................. 11
     4.3.        Partial State of Recharge (PSOR)................................................................................................. 12
     4.4.        Fast Charging ............................................................................................................................... 13
     4.5.        Catalyst Caps ............................................................................................................................... 15
5.        Concept Selection ............................................................................................................................... 17
6.        Future Work ........................................................................................................................................ 18
References .................................................................................................................................................. 20




*due to the fact that the all of the work done to date is research, this section is omitted from
this report.
                                                                                                   5




1. Background
       The Navy Acoustic Research Detachment operates a Large Scale Vehicle (LSV2)
submarine in Lake Pend Oreille to gather acoustic research data. This all electric acoustic
research submarine consists of 1680 VRLA primary batteries used for the propulsion system
batteries. The 2V VRLA batteries are split into four parallel strings of batteries, summing to a
total 840V delivered to the propulsion system.
       In order to charge the batteries, seven chargers are connected to four parallel strings of
60 batteries. The cabling running from the chargers to the submarine are rated for 50A and
main switches are located inside the submarine to reconfigure the batteries for charging or for
an underway. The diagram below displays visually how the chargers are connected.



                       7                                             1-60

                       6                                           61-120

                       5                                           121-180

                       4                                           181-240

                       3                                           241-300

                       2                                           301-360

                       1                                           361-420


                   Figure 1.1


   1.1.Motivation for Work
       The existing charging system for the LSV2 is suboptimal. The charge cycle is run by
technicians who check voltage and current levels every 15 to 20 minutes, which may result in
                                                                                               6


damaging over voltage portions of the charge. The charging routine is explained below and
graphically represented in Figure 1.1.
       1. Commence charging at initial charging rate of 45A per string (180A total).
       2. Continue until battery voltage increases to 2.35V per cell for each string.
       3. Maintain 2.35V per cell by reducing the charging rate as necessary and allow the
            charging current to taper to 6.25A per string (25A total).
       4. Maintain the charging rate for a period of 3 hours and until a minimum of 10%
            overcharge is achieved based on the ampere-hours discharged from the battery. At
            no time during the charge should individual cell voltage be allowed to increase
            above 2.50




            Figure 1.2


As stated in 4, the overcharge portion of this routine is a constant voltage time limited
overcharge, which allows possible overcharge of the batteries, leading to a considerably shorter
lifespan.
                                                                                                    7


   1.2.Identification of Need
       The useful life of the propulsion energy source, made up of the 1680 VRLA batteries, is
replaced every four years. The cost to replace the entire system is roughly $593,000 plus labor
cost and 2 months of downtime where the submarine is out of commission.

   1.3.Expected Benefits
       One expected benefit of this project is the reduction of capacity loss over the batteries’
service life. If the proposed solutions are successful, they will allow the battery to maintain a
high State of Charge (SOC) throughout its cycle life. This allows the LSV2 to maintain longer
data-gathering runs (“underways”) throughout system life. More benefits will be achieved
through automation which must be implemented due to the complexity of the proposed
solutions. Automation will solve the problems linked to human charge addressed in section 1.1
and will allow the technicians to work on other tasks while the charge is in progress. Finally,
the most important benefit will be the extension of the useful life of the batteries, which will
reduce expenses over the long term and reduce submarine downtime, resulting in more testing
time per year. There are several other benefits specific to individual charging algorithms to be
discussed in their respective sections.


2. Problem Definition
       LSV2 is an all electric 1/3 scale Virginia class submarine operated by the Acoustic
Research Detachment (ARD) of the Naval Surface Warfare Center, Caderock Division (NSWCCD)
to study acoustic properties of propulsion systems. The current system’s battery charger
configuration and charging scheme were inherited hardware from an earlier craft, the LSV1.
The ARD would like to optimize and improve battery capacity over the course of the main
batteries’ life cycle, and possibly reduce charge time by improving the method used to charge
the system.

   2.1.Goals and Deliverables
       Maintaining the battery capacity will solve two main problems. First, if successful, we
will be able to offer the ARD longer mission run times as the batteries approach the 4 year
expected end of life. Secondly, if battery life can be extended by what our research states to be
                                                                                                  8


around 50%, the savings to the ARD would be approximately $50,000 per year in battery costs
alone. This cost does not take into account the 2 months of downtime while batteries are being
replaced. This also does not take into account the man hours required to accomplish the
replacement.
       The current algorithm used to charge the LSV2 utilizes a constant current (CC) charge
followed by a constant voltage (CV) charge in order to restore the 1680 Valve Regulated Lead
Acid (VRLA) batteries to a full charge state. This charging scheme has been the standard since
VRLA batteries were introduced in industry. Recent research by several others experienced in
the study of VRLA battery life has been conducted to find better ways of performing a charge
and this is the focus of the project.
       Our research has pointed us to several promising charging algorithms that will be tested
in the course of this project. Among these are Zero Delta Voltage (ZDV) and Current Interrupt
(CI). We will be testing these charging algorithms to determine what benefit would result in
charging the batteries using a new scheme. We are also looking into using a technology called
catalyst caps on new batteries that would potentially extend the battery life regardless of the
algorithm used to charge the batteries.

   2.2.Specifications and Constraints
       The specifications for this project will include documenting the current charging
configuration to show advantages and disadvantages. We will be researching potential changes
to extend the capacity retention. Laboratory tests will be conducted to verify benefits of
proposed charging systems.
       Proposed solutions are subject to several constrains. The first constraint is that any
solution must not reduce the battery capacity, which would lead to shorter mission run time.
Secondly, we are limited by the facility hardware and existing infrastructure. Each charger can
only supply 45 amps per battery string, limiting the charging options that can be used.
                                                                                                   9


3. Project Plan
       Team AutoVolt investigated how the current system was configured to determine what
the strengths and weaknesses were. A visit to the facility enabled us to comprehend the size of
the project and what items would impede our progress towards completion.
       Most of the fall semester was spent researching what methods for charging VRLA
batteries were available and evaluating them in lift of the constraints of the infrastructure to
determine feasibility. Continued research was conducted on potential alternate charging
methods, looking at how they could be tested and modeled.
                                                                                                               10


                     To reduce cost and the amount of physical lab testing, the team utilized part of the fall
             semester to research methods to simulate VRLA battery capacity over the course of their
             intended life. Researching software modeling has proven fruitless, as the simulation of VRLA
             batteries with respect to their capacity retention and state of health over expected cycle life
             has been found to be non-existent. Because of this, the only option available to test actual




             maintenance of performance is to physically test the possible charging schemes against a
             control of the current charging algorithm. In order to test the algorithms, team AutoVolt will be
             designing and constructing a battery cycling testing apparatus to accomplish this task. Figure
             3.1 shows progress of the team through the end of this semester. Once the research portion
             was completed, we moved into conceptualizing the ideas into feasible and non-feasible paths.

             4. Concepts Considered
                    We have researched several new charging schemes for the VRLA battery. These include
             Zero Delta Voltage (ZDV), Current Interrupt (CI), Partial State of Recharge (PSOR) and Fast
             Charging. When analyzing each charge method, the potential gain, cost/benefits, and feasibility
             were analyzed to determine the likelihood of success.




Figure 3.1
                                                                                                    11


    4.1.Zero Delta Voltage (ZDV)
         ZDV [1] charging is simply a method of terminating the system charge at the correct
point during the charging cycle. This will prevent undercharging or overcharging of the battery
cells, which in turn prevents battery performance degradation. Using this charge method
requires that the batteries be charged at maximum current until 70% of the previously
discharged ampere-hours have been returned to the battery string. The batteries are then
charged at 20% of max current until ZDV is reached.
         In order to determine ZDV, voltage measurements are taken on the battery string every
second for 30 seconds. This value is averaged and then subtracted from the previous 30 second
average. This new value is then compared to a limit that will be determined during the testing
phase. If the limit is not exceeded for five consecutive iterations, then the charge is complete.
Figure 4.1.1 shows an example of how ZDV is detected in the charging cycle.




Figure 4.1.1




    4.2.Current Interrupt (CI)
         Current Interrupt (CI) [1] is a charging algorithm that is used during the overcharge
portion of the charging cycle. As stated in [1], research has indicated that CI is typically used
after a fast charge algorithm has restored 100% of the depleted amp-hours. At this point, the
batteries are charged using a pulsed current at approximately one fourth of the batteries rated
                                                                                                  12


current. Pulse durations are set to a 15 seconds on and 20 seconds off cycle as can be seen in
Figure 4.2.1. This charge continues until batteries reach 10% overcharge.




Figure 4.2.1


         Current interrupt overcharging has several advantages. CI can help reduce the gassing
effect. This reduction will prolong the battery life by retaining the chemical composition of the
battery. During the overcharge phase of charging, a certain level of current is required solely to
maintain the oxygen recombination reaction. High pulsed currents will still feed this cycle but
will also provide enough current for continued battery charging. Pulsing the current allows for a
cooling period which will minimize thermal degradation and gives time for chemical reactions
to stabilize resulting in increased charging efficiency.
         As referenced in [1], CI charging was not tested independently of a fast charge
algorithm. Laboratory research will be conducted to determine the stand alone benefits of
using a CI charging algorithm.



    4.3.Partial State of Recharge (PSOR)
PSOR [2] is a charging scheme that limits the amount of overcharge delivered to the batteries.
Using PSOR, batteries are charged to 100% capacity for approximately 9 charging cycles. The
tenth cycle is used as a conditioning charge, charging to 120% of nominal capacity. PSOR has
been shown to extend the operating life of VRLA batteries by a significant amount by limiting
the overcharge the batteries receive.
                                                                                                 13


       Reducing the charge delivered to the batteries will have the negative effect of reducing
operation run time on the LSV2. For this reason, PSOR will not be considered as a viable
candidate for further testing and is not included in section 5.



   4.4.Fast Charging
       Fast charging, also known as voltage controlled current step down charging, provides
large current pulses on the order of 4C where C is the 6 hour Ah rating of the battery. During
these large current impulses the cell voltage is monitored and compared to a voltage limit,
usually about 2.5V per cell. This voltage measurement must also take into consideration the
voltage drop caused by large amounts of current flowing through the batteries’ internal
resistance estimated at the given SOC at which the measurement is taken. This charging
algorithm is further explained and developed in [3] and is summarized below.
       The current impulses follow the maximum charge acceptance curve of the specific
battery. In the case of VRLA batteries, this closely matches an exponentially decaying function.
Looking at figure 4.4.1, it can be seen that the current pulses roughly follow this curve. Not
only does this allow charge in a minimal amount of time, but as can be seen by looking at figure
4.4.2, it extends the life of the battery considerably when compared to CC/CV charging.
                                                                       14


               Fast Charging Algorithm for 12V 70Ah VRLA battery




Figure 4.4.1




               Comparing life of battery using fast charge vs. CC/CV




Figure 4.4.2
                                                                                                 15


       There are several benefits to fast charging, including decreased charge time and
increased capacity retention throughout life, but application to the LSV2 charging system will
be nearly impossible. Given the Ah rating of the propulsion batteries, the initial current values
for this scheme would be upwards of 800A, requiring extremely large wire, and would overload
the power system on base. This is also not a suitable charging method for many batteries in a
confined space because the large amounts of current cause a large amount of heat. Further
comparison of this and other schemes can be found in section 6.

   4.5.Catalyst Caps
       Provided by Philadelphia Scientific as elaborated upon in [4], these devices replace the
native vent caps and introduce a catalyst into the system which aids in the recombination of
oxygen and hydrogen. These devices cost $35.00; since the LSV2 batteries will require 2 per
battery, the total cost will be $70 per battery. Because of the cost, these devices have been
shown to prolong VRLA batteries life by up to 100%. Figure 4.5.1 shows how the Catalyst Cap
decreases the loss of H2 molecules by absorbing them before they are released through the
vent cap.
       The United States Air Force now requires catalyst caps to be installed in all new VRLA
batteries installed in their communication systems. They are also working to replace old
batteries with new ones containing catalysts caps [5]. A more in-depth explanation of catalyst
caps and how the Air Force is using them can be found in [4] and [5].
                                                                        16


               Equation Typical gas cycle of a 100Ah VRLA battery [5]




Figure 4.5.1
5. Concept Selection
       Table 5.1 shows the decision matrix utilized to determine which schemes will be feasible. The solutions shown were down-
selected from the initial research phase. In order to weight each item, they were rated out of a 100% pool from most important
taking up 25%, down to 1% being the least important item to worry about when considering the different algorithms. We
determined that the best course of action for this project was to save money. In order to save money, the system needs to be
improved, meaning longer underways until the batteries are decommissioned, increasing the useful life of the battery system, and
not requiring a major infrastructural upgrade to implement a new method of charging.
       The first option, CC/CV is the current configuration, and the fourth option is the fast charge solution which will not be
considered due to poor scores. Because the other choices, ZDV, CI, and Catalyst Caps are close in scoring, and because with many
unknowns within the matrix, all three have been chosen for lab testing.
 Item                                         Description                                       Method                                          Weigh
                                                                                                                                                t
                                                                                                CC/CV       ZDV    CI       FC       Caps
 Software Complexity                          Less lines of code for greater rating                 0.27     0.2    0.2      0.2       0.3           3%
 Power Requirements                           Lower kW rating for better rating                     0.45     0.5    0.3      0.1       0.6           6%
 Shore Power Considerations                   T = 0/F = 1 - Requires Infrastructure upgrade          1.5     1.5    1.5          0     1.5          15%
 Rewiring of Barge and Vessel                 Less rewiring for better score                         0.2     0.2    0.2          0     0.2           2%
 Difficulty of Implementation                 Replacement with on-hand or new units                 0.45     0.4    0.4      0.2       0.4           5%
 Capacity available for Underway              The more charge available for runs the better          1.2     1.6    1.8          2     1.6          20%
 Expected EOL Capacity                        More capacity at the end of life                       0.3     0.6    0.9          1     0.8          10%
 External Interfacing of Controls             T= 0/F = 1 - Requires external interfacing                ?     ?         ?        ?          ?        8%
 Reduction in Charge time                     Greater reductions for better score                       ?     ?         ?        ?          ?        1%
 Cost of Implementation                       Higher rating for lower costs                             ?     ?         ?        ?          ?        5%
 Long term Costs reduction                    Higher rating for greater reduction possibility           ?     ?         ?        ?          ?       25%
Table 5.1    Higher score is better                                                                4.37       5     5.2     3.4        5.4          100%




 6. Future Work
            Due to the availability of critical components required to test the capacity retention over a battery’s lifetime, team AutoVolt
 has developed a future work plan with two paths. The University of Idaho Battery Lab has given the team access to one channel on
                                                                                                                                                     19


             an Arbin BT2000 battery cycler. It is a 300A testing machine capable of handling any solution proposed. Three choices will be tested
             for 100 cycles as a proof of concept over the months of February to April.
                      The other path involves building a cycler for higher cycle testing. The cycler will include the use of SEADAQ, a data acquisition
             system designed for the SEAJET to collect data during the cycling of the batteries. Because of the complexity of designing a charge
             and discharge testing apparatus, the construction of the system will require a significant amount of the spring semester. Depending
             upon the time constraints, it may be designed to enable all solutions to be tested, or only one system to be tested with the system
             setup.




Figure 6.1
References
  [1] Matthew A. Keyser, et al., “Charging Algorithms for Increasing Lead Acid Battery Cycle
  Life for Electric Vehicles”


  [2] Elizabeth D. Sexton, Robert F. Nelson, and John B. Olson, “Improved Charge Algorithms
  for Valve Regulated Lead Acid Batteries”


  [3] V. Svoboda “The Influence of Fast Charging on the Performance of VRLA Batteries.” June
  2004


  [4] “Catalysts Save US Taxpayers Millions” Batteries + Energy Storage Technology, Winter
  2005 pg 67-70.


  [5] Teresa Hanson “Catalysts on VRLA Batteries Save Air Force Millions of Dollars”
  http://pepei.pennnet.com, Oct 2005

				
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