W. Peng, Student Member, IEEE Y. Baghzouz, Senior Member, IEEE

Department of electrical & Computer engineering University of Nevada, Las Vegas (USA)

THE INTERNATIONAL CONFERENCE & UTILITY EXHIBITION 2011 28-30 September 2011 Pattaya City, Thailand

* * Need for battery models * Typical battery discharge curves * Derivation of Steady-State Circuit Model from Manufacturer Data * Steady-State Model verification * Derivation of Dynamic Circuit Model from Laboratory Tests Data and Verification.

* Conclusion

* * Energy storage on the electric power system is becoming an increasingly important tool in * Managing the integration of large-scale, intermittent solar and wind generation. * Shaping the load curve (Peak shaving and valley filling) * Smart Grid designs that call for additional distribution automation and sophistication such as islanding.

* Energy storage in the automotive industry is also becoming important due to the proliferation of Hybrid-Electric and Pure Electric Vehicles.

* There are many types of batteries, each of which has advantages and disadvantages: * the Absorbed-Glass-Mat (AGM) battery - a type of Valve Regulated-Lead-Acid (VRLA) battery that is widely popular in renewable energy storage systems due to its high performance and maintenance-free requirement – is analyzed in this study.

* 89 A 3.7 A 0.75 A

*

I

*

R s

: total resistance (copper and electrolytic) – dependent on rate of discharge.

*

V s

: equivalent voltage source –dependent on rate of discharge and DOD (or SOC).

*

V s

can be replaced by an equivalent capacitance C relation between these two is:

V s



V s

,

o



It

/

C s s

. The

* * Best curve fit:

R s



a

1

I



a

2

* * Best curve fit:

f C s

 (

I

)

a

3 

f a

6 (

I

 ) 

a

7

I a

4

NC f f

(

I

) (

I

)(

DOD

) 

a

5

*

*

*

* 8HR – 9.8 A 4HR – 18.25 A

* * Equivalent resistance split into parts:

R s

from rest):

V drop



R

'

s i

 

R s

' * Total voltage drop due to sudden draw of current

i R t i

 ( 1

R t



e



t

/ 

on

), 

on



R t C t

(starting Sudden voltage drop Exponential Voltage drop

* ►The time constants at turn-on and turn-off are different.

Static component Dynamic component

*

Current Pulse (A)

(1-k)R s

(Ω) 20

0.022

15

0.023

10

0.023

5

0.024

kR s

(Ω)

R s

(Ω)

τ on

(sec)

0.012

0.013

0.014

0.015

0.033

0.036

0.037

0.039

14 15 17 20

τ off

(sec)

96 98 100 101

*

* * A circuit model for an AGM Lead-acid battery was developed for steady-state and transient conditions : * The steady-state model (which consists of two dependent circuit parameters) was derived from the discharge curves provided by the manufacturer. * The dynamic model was obtained by adding a capacitive element across a portion of the series resistance, and the parameter values were obtained from laboratory tests . * The resulting circuit model is found to predict battery performance under both constant as well as variable current discharge with sufficient accuracy.

* The tests in this study were conducted indoors at room temperature. Future work consists of upgrading the circuit model by taking into account battery temperature when operating outdoors.