Battery Bank Sizing for Generator Starting Calculator – IEEE, IEC

Accurate battery bank sizing is critical for reliable generator starting and operation. It ensures sufficient power delivery during initial cranking.

This article explores IEEE and IEC standards for battery bank sizing, providing formulas, tables, and practical examples.

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  • Generator power rating: 500 kW, Starting current: 3000 A, Voltage: 480 V, Cranking time: 5 seconds
  • Generator power rating: 1500 kW, Starting current: 9000 A, Voltage: 690 V, Cranking time: 8 seconds
  • Generator power rating: 750 kW, Starting current: 4500 A, Voltage: 600 V, Cranking time: 6 seconds
  • Generator power rating: 2000 kW, Starting current: 12000 A, Voltage: 1000 V, Cranking time: 10 seconds

Common Values for Battery Bank Sizing According to IEEE and IEC Standards

ParameterTypical RangeUnitsNotes
Nominal Battery Voltage (V)12, 24, 48Volts (V)Standard battery module voltages
Battery Capacity (Ah)100 – 2000Ampere-hours (Ah)Depends on battery type and application
Cranking Current (Icrank)1000 – 15000Amperes (A)Peak current required to start generator
Cranking Time (tcrank)3 – 15Seconds (s)Duration of cranking current draw
Battery Discharge Rate (C-rate)0.1C – 10CUnitlessRate of discharge relative to capacity
Battery Internal Resistance (Rint)0.1 – 5Milliohms (mΩ)Affects voltage drop during cranking
Temperature Correction Factor (Ktemp)0.8 – 1.0UnitlessAdjusts capacity for ambient temperature

Key Formulas for Battery Bank Sizing in Generator Starting

Battery bank sizing involves calculating the required capacity and configuration to meet the generator’s starting current and duration demands. The following formulas are essential for accurate sizing, based on IEEE Std 485 and IEC 60896 guidelines.

1. Required Battery Capacity (Ah)

Capacity (Ah) = (Icrank × tcrank) / (V × Ktemp × η)
  • Icrank: Cranking current in Amperes (A)
  • tcrank: Cranking time in seconds (s)
  • V: Nominal battery voltage in Volts (V)
  • Ktemp: Temperature correction factor (unitless, typically 0.8–1.0)
  • η: Battery efficiency (typically 0.85–0.95)

This formula estimates the minimum ampere-hour capacity needed to supply the cranking current for the required duration, adjusted for temperature and efficiency losses.

2. Number of Battery Cells Required

Ncells = Vsystem / Vcell
  • Vsystem: Total system voltage (e.g., 480 V)
  • Vcell: Voltage per battery cell (typically 2 V for lead-acid)

This determines how many cells must be connected in series to achieve the system voltage.

3. Battery Bank Configuration

Nparallel = Required Capacity (Ah) / Capacity per Battery (Ah)
  • Nparallel: Number of parallel strings
  • Capacity per Battery: Ampere-hour rating of a single battery

Parallel strings increase total capacity while maintaining voltage.

4. Voltage Drop During Cranking

Vdrop = Icrank × Rint × Ncells
  • Vdrop: Voltage drop across battery internal resistance
  • Rint: Internal resistance per cell (Ohms)
  • Ncells: Number of cells in series

Ensures voltage remains above minimum required during cranking.

5. Battery Discharge Rate (C-rate)

C = Icrank / Capacity (Ah)
  • C: Discharge rate (unitless)
  • Icrank: Cranking current (A)
  • Capacity: Battery capacity (Ah)

High C-rates reduce battery life; sizing must consider manufacturer limits.

Real-World Application Examples of Battery Bank Sizing

Example 1: Sizing Battery Bank for a 500 kW Generator Starting

A 500 kW generator requires a cranking current of 3000 A at 480 V for 5 seconds. The battery efficiency is 90%, and the temperature correction factor is 0.9. The battery modules are 12 V, 200 Ah each, with an internal resistance of 0.5 mΩ per cell.

  • Step 1: Calculate required battery capacity.
Capacity (Ah) = (Icrank × tcrank) / (V × Ktemp × η)
= (3000 × 5) / (480 × 0.9 × 0.9)
= 15000 / 388.8 ≈ 38.56 Ah

The required capacity is approximately 39 Ah at 480 V.

  • Step 2: Determine number of cells in series.
Ncells = Vsystem / Vcell = 480 / 2 = 240 cells
  • Step 3: Calculate number of parallel strings.
Nparallel = Required Capacity / Capacity per Battery = 39 / 200 = 0.195

Since 0.195 < 1, one string of batteries in series is sufficient.

  • Step 4: Check voltage drop during cranking.
Vdrop = Icrank × Rint × Ncells
= 3000 × 0.0005 × 240 = 360 V

A 360 V drop is excessive; battery internal resistance or configuration must be improved.

Interpretation: The internal resistance is too high for this application. Using batteries with lower internal resistance or paralleling strings to reduce current per string is necessary.

Example 2: Battery Bank Sizing for a 1500 kW Generator Starting

A 1500 kW generator requires a cranking current of 9000 A at 690 V for 8 seconds. Battery efficiency is 85%, temperature correction factor is 0.95. Battery modules are 24 V, 400 Ah, with internal resistance 0.3 mΩ per cell.

  • Step 1: Calculate required battery capacity.
Capacity (Ah) = (9000 × 8) / (690 × 0.95 × 0.85)
= 72000 / 557.175 ≈ 129.2 Ah
  • Step 2: Determine number of cells in series.
Ncells = 690 / 2 = 345 cells
  • Step 3: Calculate number of parallel strings.
Nparallel = 129.2 / 400 = 0.323

One string is sufficient, but check voltage drop.

  • Step 4: Calculate voltage drop.
Vdrop = 9000 × 0.0003 × 345 = 931.5 V

Voltage drop exceeds system voltage, indicating the need for multiple parallel strings to reduce current per string.

  • Step 5: Adjust parallel strings to reduce voltage drop.

Assuming 10 parallel strings:

Current per string = 9000 / 10 = 900 A
Vdrop = 900 × 0.0003 × 345 = 93.15 V

Voltage drop is now acceptable (~13.5% of system voltage). Battery bank configuration: 345 cells in series × 10 parallel strings.

Additional Technical Considerations for Battery Bank Sizing

  • Battery Chemistry: Lead-acid batteries are common, but Ni-Cd and Li-ion offer different performance and sizing parameters.
  • Temperature Effects: Battery capacity decreases at low temperatures; correction factors must be applied per IEEE Std 485.
  • Battery Aging: Internal resistance increases and capacity decreases over time; oversizing compensates for degradation.
  • Safety Margins: IEEE recommends a 20–30% capacity margin to ensure reliable starting under worst-case conditions.
  • Maintenance and Testing: Regular testing per IEC 60896 ensures battery health and performance.

Relevant Standards and Guidelines

These standards provide comprehensive methodologies for battery sizing, testing, and maintenance, ensuring compliance and reliability.

Summary of Best Practices for Battery Bank Sizing

  • Accurately determine cranking current and time from generator specifications.
  • Apply temperature correction and efficiency factors to capacity calculations.
  • Use manufacturer data for battery internal resistance and capacity ratings.
  • Design battery bank with series and parallel configurations to meet voltage and capacity requirements.
  • Include safety margins to account for aging and environmental conditions.
  • Regularly test and maintain battery banks to ensure performance.

Proper battery bank sizing is essential for generator reliability, minimizing downtime and maintenance costs.