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Copyright 2003 Operation Technology, Inc.
Battery Sizing &Discharge
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Battery Sizing & Discharge Slide 2
When does a battery kick in?• Load in DC System exceeds the battery
charger capacity
• Output of the battery charger is interrupted
• AC Power is lost [generally the worst casescenario]
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Battery Sizing & Discharge Slide 3
IEEE 485- Definitions• Battery Duty Cycle
– The load that the battery is expected to supply for aspecified period of time (sum of all individual loads)
• Cell Size– Rated capacity of a lead acid battery or number of
positive plates in a cell
• Period– Time interval for which the load is assumed to be
constant
• Vpc– Volts per cell (individual cell voltage)
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Battery Sizing & Discharge Slide 4
Types of Loads• Constant Power
– As battery voltage decreases, their current increases.
• Constant Resistance
– As battery voltage decreases, their current alsodecreases.
• Constant Current
– Load current is invariant to battery voltage changes.
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Battery Sizing & Discharge Slide 5
Duty Cycle Classification• Continuous Load (Non-Random)
– Non-random loads
– Normally carried by the battery charger
– Includes lighting, converters, indicating lights,communications systems, continuously operating motors
• Non-Continuous Load (Random or Non-Random)– Emergency pumps
– MOV operations
– Critical ventilation system motors
– Fire protection systems actuators
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Battery Sizing & Discharge Slide 6
Duty Cycle Classification• Momentary Loads (Random or Non-Random)
– Loads with duration that do not exceed more thanone minute
– MOV with stroke of less than a minute
– Field flashing of generators
– Motor starting currents
– Inrush currents (solenoids, relay coils, etc)
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Battery Sizing & Discharge Slide 7
Individual Load Duty Cycles
• Static Loads
• Motor loads
• DC Lump Loads
• Elementary Elementary Diagram
• DC to AC Inverters
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Battery Sizing & Discharge Slide 8
Guidelines for Battery DutyCycles• All duty cycle periods (constant load) must
be higher or equal to one minute
• First identify the non-random load batteryduty cycle
• Find worst case scenario
• Include the random load duty cycle
• Final battery duty cycle becomes aconservative combination of the two
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Battery Sizing & Discharge Slide 9
Duty Cycle Example A
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Battery Sizing & Discharge Slide 10
Duty Cycle Example B
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Battery Sizing & Discharge Slide 11
Battery Modeling
• Battery voltage based on manufacturerbattery discharge characteristic curves
• Enter battery characteristic curves in batterylibrary
• Interpolation / extrapolation is performed toobtain battery size and discharge voltageprofile
• Different interpolation methods to obtain Vpc
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Battery Sizing & Discharge Slide 12
Characteristic CurvesBattery Characteristic Curves
1.00
10.00
100.00
1000.00
10.00 100.00 1000.00Amps per Positive Plate
Tim
e (m
in.)
1.51.61.671.751.781.811.841.881.921.96
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Battery Sizing & Discharge Slide 13
Interpolation Methods
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Battery Sizing & Discharge Slide 14
Battery Sizing Study Case• Load Summation
– Adds all the load duty cycles
– Generally more conservative for systems with moreconstant resistance loads
• Load Flow Method– Adds all the load duty cycles, but performs a load flow
calculation at every time step to account for systemlosses (more realistic results)
– Generally more conservative for systems with moreconstant power loads
– Load model type based on load type or duty cycle type
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Battery Sizing & Discharge Slide 15
• Load Duty Cycles– 5 different system duty cycles
• Correction factors– Battery minimum temperature– Aging compensation– Initial capacity– Design margin
• Voltage Requirements– Maximum system voltage deviation (charging voltage)– Minimum system voltage deviation (depends on final
Vpc)– Battery minimum discharge voltage (final Vpc)
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Battery Sizing & Discharge Slide 16
Battery Discharge Page
• Vd Calculation Parameters– Time step and maximum voltage limit
• Correction Factors– Same as battery sizing (no design margin)
• Load Flow Parameters– Iterations
– Precision
– Initial conditions and motor load handling
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Battery Sizing & Discharge Slide 17
Duty Cycle Span OptionsOptions
•Two Existing Options Moved from Ini File to Study Case•New Option Added – One-minute Span for Battery only
One-minute Span for Battery only•Calculating Battery Discharging Current – Use One-MinuteSpanned Duty Cycle �To Get Conservative Battery Voltage
•Calculating System Voltage – Use Battery Voltage Calculatedabove to Run Load Flow with User Specified Duty Cycle forIndividual Loads � To Get More Accurate System VoltageProfile
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Battery Sizing & Discharge Slide 18
Displayed Results
• Tabulated results for battery voltage, busvoltage and power flows through the entiresystem
• One-line diagram displayed results (similarto DC load flow)
• Complete set of plots that include batteryvoltage, current, discharged AH and systemflows
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Battery Sizing & Discharge Slide 19
Things to Consider
• Obtain as many battery characteristic curvesas possible
• Obtain specific curves for battery model andtype
• Understand limitations of interpolationmethods
• Use most conservative approach for yoursystem (load flow method or loadsummation)
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