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1SPW 2012ENNEAD LLC
Complex Modeling ofLi-Ion Cells in Series and
Batteries in Parallel within SatelliteEPS Time Dependent Simulations
Patrick Bailey, ENNEAD, LLC
Aerospace Space Power WorkshopApril 16-19, 2012
Manhattan Beach, CA
2SPW 2012ENNEAD LLC
Dr. Patrick G. BaileyENNEAD, LLCP.O. Box 201
Los Altos, CA 94023-0201
[email protected]/ennead/
14 Years Nuclear Reactor Safety (USAF, LANL, EPRI)25 Years Lockheed (Martin), LMSSC
Retired, Available, EnthusiasticBS, UC Berkeley
PhD, MIT
3SPW 2012ENNEAD LLC
Abstract
A presentation is made of the advanced models and various results that have been obtained tosimulate complex Lithium Ion (LiIon) battery behavior within any satellite Electric Power Systems(EPS). The battery cell behavior is modeled by the publically available Quallion Lithium Ion battery cellmodel, whose voltage behavior is defined to be a complicated function of the cell current, the celltemperature, and the cell state-of-charge. A battery is defined to be composed of a series of cells,whose individual properties in each cell at any given time may be different. The overall EPS battery isthen composed of a number of such batteries connected in parallel. The simulation model allows theproperties of each individual cell in the overall EPS battery to be individually different (for example,each cell at a different state-of-charge and at a different temperature), and each cell can be degradedor dropped out of the battery at any given time. Simulation results are shown for the cases of bothcharging and discharging, to illustrate the effects of varying the temperature between cells, and theeffects of changing the state-of-charge between cells. Studies are also included that show the effectson the overall EPS battery voltage when the individual battery cells are not fully charged (to 100%state-of-charge) during recharging or during rebalancing. In addition, results are shown as cells aredegraded and dropped from operation.
These models and results are very important for complex EPS simulations and predictions. Theinability of cells stacked in series to fully rebalance during recharging can lead to battery voltages lowerthan planned or designed for, and can result in EPS performance that is much less than expected ordesired. Such battery models are planned to be included in the Power Tools Suite (PTS) system ofcodes and tools used at Lockheed Martin Space Systems Company. These models are already beingused in the Satellite EPS Transient Code (Sat-Tran) that has been independently developed byENNEAD LLC for satellite EPS transient simulation, operation, validation, and prediction.
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The Problem
Need for accurate computer simulations of EPS
Time-Dependent Behavior over Mission Life
Many Solar Array Cell Types, Char.s, Models
Many Battery Cell Types, Char.s, Models
Many EPS Designs (architectures, batt. domin., etc.)
Need capabilities for each, including:
• Sizing / Proposals
• Design / with and without Margins
• On Orbit Verification and Planning
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The Solution
Lumped Parameter Models, No Fast C/L/Z Transients
Accurate for 10 second time steps and above
Many Simple Spreadsheets (no generalizations)
A Few General Simulation Packages:
MATLAB & Simulink [Simplistic, Proprietary]
Lockheed Martin Power Tools Suite (PTS) [Proprietary]
ENNEAD Time Dependent Simulations (TDS) [Available]
Models and Results for Verification and Validation
Detailed Documentation
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EPS Simulation - PTS
Time-Dependent Simulation Code for EPS DetailedDynamic Simulations
For Proposals, Sizing, Design
Detailed Battery, Solar Array, Non-Linear Models
Lumped Parameter Models (1 minute time steps)
Excel VB Macros (Same as C++ or FORTRAN)
80,000+ Lines of Code – User Friendly Interfaces
Easily Expandable
Many IECEC Papers and SPW Presentations:
www.padrak.com/pts_pgb/ [Publically released]
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EPS Simulation – PTS (IECEC 2004)
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EPS Simulation – PTS (IECEC 2004)
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EPS Simulation - PTS (IECEC 2004)
-0.5-0.4
-0.3-0.2
-0.10
0.10.2
0.30.6
10
0.2
0.4
0.6
0.8
1
20.000
22.000
24.000
26.000
28.000
30.000
32.000
34.000
36.000
38.000
Ba
tter
yV
olt
ag
e
Discharge Rate (C Fraction)Depth of Discharge
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
1.000 1.100 1.200 1.300 1.400 1.500 1.600 1.700
Battery Model Examples:
Solar Array Model Examples:
* Other EPS components have similar models!
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EPS Simulation - PTS (IECEC 2011)
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EPS Simulation – Using Excel
No s/w application license
Ease of use
Ease of expansion, debugging, verification
Use of VB Function Macros (e.g. Get V from many models)
Stacked functions for various EPS components/types
Function V_Batt_G(soc, curr, temp, age) as Double
… Code …
V_Batt_G = The Result
End Function
12SPW 2012ENNEAD LLC
EPS Simulation - TDS
Time-Dependent Simulation Code for EPS DetailedDynamic Simulations
Developed Independently within ENNEAD LLC
Detailed Battery, Solar Array, Non-Linear Models
Lumped Parameter Models (1 minute time steps)
Excel VB Macros – No Computer App. License Needed
Complex Solar Array and Battery Designs
User Friendly Interfaces
Easily Expandable
Available Now for Various Applications
13SPW 2012ENNEAD LLC
Solar Array Simulation
Simple Wing:
One Cell. N cells in series. M strings in parallel.
Many such wings.
General Wing:
Same cells per string. Many strings in parallel.
Variable number of cells per string, and strings.
Many such wings.
Allows multiple spectral cells for higher SA .
14SPW 2012ENNEAD LLC
Battery Simulation
Simple Battery:
One Cell. N cells in series. M stacks in parallel.
Many such batteries.
General Battery:
Any cells per stack. Many stacks in parallel.
Variable number of cells per stack, and stacks.
Many such batteries.
Allows detailed SOC calculations.
Allows detailed chg. rebalancing simulation
Different: Types, SOCs, Currents, Temperatures, etc.
15SPW 2012ENNEAD LLC
Detailed Battery Simulation
Different Cell Types, Different SOCs, Different Temps
Variable # Cells per Stack, and Variable # Stacks
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Detailed Battery Sim. Looks Easy… Not!
Example:
Given Battery Charging Current
Calculate Current Split into Stacks
Calculate Cell Properties
Calculate Cell New SOCs
Calculate Cell New Impedances
Compare Cell Old to New Impedances
Iterate if Necessary
Converge to Current Splits and Cell New SOCs
17SPW 2012ENNEAD LLC
Sample Detailed Battery Simulations
Sci-Fi LiIon Battery Cell V vs. SOC, 5 Ah Capacity,
2.5 V Full, with V/I and V/T Corrections
(Enlarged to show effects in the results)
30 minute simulation time (1 min. T steps)
Simulation Cases: (Max SOC = 110%)
• 1 – Constant Current Charge
• 2 – Constant Current Discharge
• 3 – Constant Current Cycles
• 4 - Sample Orbit Current Cycles
• Can Cause Cell Dropouts Anytime
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Chosen Battery Cell Model VCell @ I=0
Sci-Fi User Chosen Cell Model (Normalized)
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Chosen Battery Cell Model (V/I, V/T)
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Chosen Battery Model
Same Cell Types, Different SOCs, Same Temps
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Chosen Battery Cell Initial SOCs
User Chosen Stack, Cell, and SOCs
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Given Time Dependent Input Data
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Case 1 – Constant Charge
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Case 1 – Constant Charge
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Case 1 – Constant Charge
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Case 1 – Constant Charge
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Case 2 – Constant Discharge
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Case 2 – Constant Discharge
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Case 2 – Constant Discharge
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Case 2 – Constant Discharge
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Case 3 – Const. Current Cycles
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Case 3 – Const. Current Cycles
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Case 3 – Const. Current Cycles
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Case 3 – Const. Current Cycles
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Case 4 – Sample Orbit Current Cycles
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Case 4 – Sample Orbit Current Cycles
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Case 4 – Sample Orbit Current Cycles
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Case 4 – Sample Orbit Current Cycles
39SPW 2012ENNEAD LLC
Conclusions
Excel allows easy, fast, large, accurate simulations.
EPS Time Dependent Simulation is needed for sizing,
proposals, design mods, and on-orbit validations.
PTS (LM) and TDS are available for general EPS use.
TDS includes detailed (cell) SA and Battery modeling.
Battery cells need to be modeled for individual SOC
and other parameter effects.
Battery cell “not full” recharging and cell drop-outs
are very important in EPS design and use.
Time Dependent Simulations can predict EPS behavior.