NABCEP Energy Storage 2 — Questions and Answers
Question 1: What is the primary purpose of a battery management system (BMS) in a lithium-ion energy storage system?
- To convert DC to AC power
- To monitor and protect individual cells from overcharge, over-discharge, and thermal events (Correct answer)
- To measure the state of charge using a hydrometer
- To regulate the inverter output frequency
Correct answer: To monitor and protect individual cells from overcharge, over-discharge, and thermal events
A BMS monitors cell voltages, temperatures, and currents to protect lithium-ion cells from conditions that could cause damage or safety hazards.
Question 2: A battery bank has a usable capacity of 20 kWh and a maximum continuous discharge rate of 5 kW. What is the C-rate at the maximum discharge?
- C/2
- C/4 (Correct answer)
- 2C
- 4C
Correct answer: C/4
C-rate = discharge power / capacity = 5 kW / 20 kWh = 0.25C, which equals C/4.
Question 3: Which battery chemistry is most susceptible to thermal runaway and requires the most stringent safety controls?
- Flooded lead-acid
- Lithium nickel manganese cobalt oxide (NMC) (Correct answer)
- Valve-regulated lead-acid (VRLA)
- Nickel-iron (NiFe)
Correct answer: Lithium nickel manganese cobalt oxide (NMC)
NMC lithium-ion batteries have higher energy density but are more prone to thermal runaway if overcharged, over-discharged, or physically damaged.
Question 4: In an AC-coupled energy storage system, where does the battery inverter connect?
- Between the PV modules and the charge controller
- On the AC bus, parallel with the solar inverter and loads (Correct answer)
- Directly to the PV array DC combiners
- Between the utility meter and the main service panel
Correct answer: On the AC bus, parallel with the solar inverter and loads
In AC-coupled systems, the battery inverter connects to the AC bus, allowing it to charge from both the solar inverter output and the grid.
Question 5: What does 'round-trip efficiency' measure in an energy storage system?
- The ratio of energy delivered by the battery to energy required to charge it (Correct answer)
- The number of complete charge-discharge cycles before capacity drops to 80%
- The efficiency of the inverter converting DC to AC
- The self-discharge rate per month
Correct answer: The ratio of energy delivered by the battery to energy required to charge it
Round-trip efficiency is the percentage of energy recovered from storage compared to the energy put in, accounting for losses in both charging and discharging.
Question 6: A lithium iron phosphate (LFP) battery system is rated at 10 kWh with 80% depth of discharge. What is the usable energy capacity?
- 10 kWh
- 8 kWh (Correct answer)
- 2 kWh
- 12.5 kWh
Correct answer: 8 kWh
Usable capacity = total capacity × DoD = 10 kWh × 0.80 = 8 kWh.
Question 7: Which NEC article primarily governs the installation of energy storage systems in the United States?
- Article 690
- Article 705
- Article 706 (Correct answer)
- Article 710
Correct answer: Article 706
NEC Article 706 specifically addresses energy storage systems, covering installation, disconnecting means, overcurrent protection, and wiring methods.
NABCEP Energy Storage 2
What is the primary purpose of a battery management system (BMS) in a lithium-ion energy storage system?