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 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.

3.7 A 0.75 A 89 A

* 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 s. The relation between these two is: I

* Best curve fit:

8HR – 9.8 A 4HR – A

* Equivalent resistance split into parts: * Total voltage drop due to sudden draw of current i (starting from rest): Sudden voltage drop Exponential Voltage drop

Static component Dynamic component ►The time constants at turn-on and turn-off are different.

Current Pulse (A) (1-k)R s (Ω) kR s (Ω) R s (Ω) τ on (sec) τ off (sec)

* 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.