using battery modeling as a sizing tool

Using Battery Modeling as a Sizing Tool

IEEE ESSB Summer MeetingChicago – June 12, 2016

2 Using Battery Modeling as a Sizing Tool

IEEE battery sizing

– Sizing standards• IEEE Std 485-2010 for vented and valve-regulated lead-acid• IEEE Std 1115-2014 for nickel-cadmium

– Both use modified Hoxie method• Hoxie, E. A., “Some discharge characteristics of lead-acid

batteries,” AIEE Transactions (Applications and Industry), vol. 73, no. 1, pp. 17–22, Mar. 1954.

• Divide duty cycle into successive periods with constant load• Size by section with progressively more periods

o Section 1 with first period onlyo Section 2 with first two periods o Etc.


IEEE cell sizing worksheet (IEEE 1115)


Limitations for the IEEE method

– Cumbersome for numerous load steps– No consideration for temperature changes during

operation• Ambient changes during prolonged duty cycles• I2R heating effects

– No consideration for ramping loads– No consideration (yet) for new technologies


Battery modeling – an alternative approach

– Various platforms depending on need. Examples:– COMSOL Multiphysics

• ‘Micro’ modeling e.g. electronic & ionic charge transfer• Useful for cell design

– Matlab-Simulink• ‘Macro’ modeling of electrical and thermal characteristics• Scalable to battery level for interactions with dc system• Models for advanced technologies may include battery-

management algorithms


Saft Li-ion Matlab-Simulink models

– Models run same algorithms as battery management systems

– Exactly mimic real battery behavior, including contactor management

– Include aging inputs and outputs


Example 1 – from IEEE 485

– Model requires random load to be in correct location

– Assume ambient temperature of 10 °C


Example 1 – model results

BOL100% SOH

EOL80% SOH


Example 2 – PJM regulation

– PJM Interconnection runs the grid in 13 states• Biggest Independent System Operator in the USA

– PJM power market includes fast regulation service (RegD) typically supplied by battery energy storage systems• Approx. 300 MW in service or being built

– RegD signal is energy neutral and changes every 2 sec.– Sample day:

0 1 2 3 4 5 6 7 8 9

x 104

-1.0

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1.0

Time (s)

P.U

.


Example 2 – model results

0 2 4 6 8 10

x 104

-2000

0

2000

Time (s)

I bat

tery

(A)

0 2 4 6 8 10

x 104

600

700

800

Time (s)

U b

atte

ry (V

)

0 2 4 6 8 10

x 104

25

30

35

40

Time (s)

Bat

Tem

p (°

C)

0 2 4 6 8 10

x 104

-1000

0

1000

Time (s)

Pow

er (k

W)

0 2 4 6 8 10

x 104

0

50

100

Time (s)

SoC

(%)

0 2 4 6 8 10

x 104

97

98

99

100

Time(s)

Ene

rg. e

ff. (%

)


So is modeling the future of battery sizing?

– For Saft, the answer is ‘yes’• But not every battery manufacturer has a model• Most are less sophisticated

– Tabular discharge data does not exist for Saft Li-ion cells• The model is the source of all performance characteristics


Where do we go with sizing standards?

– The IEEE sizing standards have their origins in the time before computer-driven modeling• Needed to avoid use of different sizing methods for the same

project• Continue to be vital for the nuclear industry

– Why do we need sizing standards if models exist?– Is validation an issue?

• How is ‘trust the model’ different from ‘trust the tabular discharge data’?

– Food for thought…

13

Questions? Thoughts?

Using Battery Modeling as a Sizing Tool

[email protected]


‘Bonus’ material – cycle counting

– Question• How many cycles?• …of what depth?

– Answers are important for folks who want to compare cycle count with published cycle-life curves

– The following slides show the Saft method• Others may follow a slightly different approach


Cycle counting technique

– Cycle must be ‘closed’• SOC must return to starting point

– Example with 2 closed cycles• From 80% SOC to 20% SOC and

back again (60% DOD)• From 55% SOC to 50% SOC and

back again (5% DOD)


Modified example

– Still 2 closed cycles• From 65% SOC to 20% SOC and

back again (45% DOD)• From 55% SOC to 50% SOC and

back again (5% DOD)– Cycle starting from 80% still

waiting to be closed• If charging continued (after

pause) up to 80% SOC... • 45% cycle would become 60%

DOD cycle


Another modification

– Now 3 closed cycles• From 65% SOC to 20% SOC and back again (45% DOD)• From 55% SOC to 50% SOC and back again (5% DOD)• From 80% SOC to 30% SOC and back again (50% DOD)

– With Li-ion we count only delta-DOD cycles• Cycle between 20% and 10% SOC counts same as one

between 90% and 80% SOC• Hence no 60% DOD cycle above

using battery modeling as a sizing tool

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