NABCEP Energy Storage 3 — Questions and Answers
Question 1: When sizing a battery bank for a 3-day autonomy requirement with a daily load of 6 kWh and a maximum depth of discharge of 50%, what minimum capacity is required?
- 9 kWh
- 18 kWh
- 36 kWh (Correct answer)
- 6 kWh
Correct answer: 36 kWh
Required capacity = (daily load × days) / DoD = (6 kWh × 3) / 0.50 = 36 kWh.
Question 2: What is the term for the gradual reduction in a battery's maximum capacity over repeated charge-discharge cycles?
- Self-discharge
- Calendar aging
- Cycle degradation (Correct answer)
- Stratification
Correct answer: Cycle degradation
Cycle degradation refers to the loss of battery capacity that occurs incrementally with each charge-discharge cycle due to physical and chemical changes in the electrodes.
Question 3: In a DC-coupled battery storage system, the battery charge controller is connected between the PV array and the battery bank. What is a key advantage of this configuration?
- Simpler AC wiring
- Higher overall system efficiency due to a single DC-to-AC conversion (Correct answer)
- Ability to charge from the grid without an additional inverter
- Automatic load transfer during outages
Correct answer: Higher overall system efficiency due to a single DC-to-AC conversion
DC-coupled systems avoid a double conversion (DC→AC→DC) when charging the battery from solar, resulting in higher overall efficiency.
Question 4: Which temperature condition most severely reduces the available capacity of a lead-acid battery?
- High temperatures above 30°C
- Low temperatures below 0°C (Correct answer)
- Constant temperature at 25°C
- Fluctuating temperatures between 20-25°C
Correct answer: Low temperatures below 0°C
Cold temperatures significantly reduce lead-acid battery capacity because the chemical reactions slow down, making less energy available for discharge.
Question 5: A battery-based inverter/charger is classified as a 'multimode inverter.' What capability does this describe?
- Operating at multiple output voltages
- Functioning in both grid-tied and off-grid modes (Correct answer)
- Charging multiple battery types simultaneously
- Supporting both 50 Hz and 60 Hz grids
Correct answer: Functioning in both grid-tied and off-grid modes
A multimode inverter can operate in grid-tied mode, switch to off-grid (island) mode during outages, and transition seamlessly between the two.
Question 6: What safety feature is required by NEC 706.15 at the point where an energy storage system connects to the rest of the electrical system?
- A ground fault circuit interrupter (GFCI)
- A readily accessible disconnecting means (Correct answer)
- An arc fault circuit interrupter (AFCI)
- A surge protective device (SPD)
Correct answer: A readily accessible disconnecting means
NEC 706.15 requires a readily accessible disconnecting means at the point of connection to allow safe isolation of the energy storage system.
Question 7: Which parameter describes how quickly a battery can be charged or discharged relative to its total capacity?
- State of charge (SoC)
- Depth of discharge (DoD)
- C-rate (Correct answer)
- Round-trip efficiency
Correct answer: C-rate
C-rate expresses the charge or discharge current relative to battery capacity; a 1C rate would fully charge or discharge the battery in one hour.
NABCEP Energy Storage 3
When sizing a battery bank for a 3-day autonomy requirement with a daily load of 6 kWh and a maximum depth of discharge of 50%, what minimum capacity is required?