NABCEP Battery Storage Systems & Energy Storage 2 — Questions and Answers
Question 1: In an AC-coupled battery storage system, the battery inverter connects to which bus?
- The DC output terminals of the solar modules
- The DC input of the grid-tied solar inverter
- The AC electrical panel or AC bus (Correct answer)
- The grounding electrode system
Correct answer: The AC electrical panel or AC bus
In an AC-coupled system, the battery inverter connects to the AC bus, meaning solar energy is first inverted to AC before being re-converted to DC for battery charging.
Question 2: Which NFPA standard specifically addresses the installation of stationary lead-acid and lithium-ion battery systems in commercial buildings?
- NFPA 70 (NEC)
- NFPA 855 (Correct answer)
- NFPA 72
- NFPA 101
Correct answer: NFPA 855
NFPA 855, Standard for the Installation of Stationary Energy Storage Systems, provides requirements for battery system placement, separation, fire suppression, and hazard mitigation.
Question 3: What is 'cycle life' as applied to battery storage systems?
- The calendar age of a battery in years
- The number of complete charge/discharge cycles before capacity falls below a specified threshold (Correct answer)
- The maximum discharge current the battery can supply
- The time required to complete one full charge cycle
Correct answer: The number of complete charge/discharge cycles before capacity falls below a specified threshold
Cycle life is the number of full charge/discharge cycles a battery can undergo before its usable capacity degrades to a defined percentage (commonly 80%) of original rated capacity.
Question 4: Which lithium battery chemistry is most susceptible to thermal runaway if overcharged or mechanically damaged?
- Lithium iron phosphate (LFP)
- Lithium nickel manganese cobalt oxide (NMC) (Correct answer)
- Lithium titanate (LTO)
- Lithium polymer with ceramic separator
Correct answer: Lithium nickel manganese cobalt oxide (NMC)
NMC (lithium nickel manganese cobalt oxide) chemistry has higher energy density but is more prone to thermal runaway compared to LFP, which has superior thermal and chemical stability.
Question 5: What is the primary safety concern when installing flooded lead-acid batteries in an enclosed indoor space?
- Electromagnetic interference with PV inverters
- Hydrogen gas off-gassing during charging creating explosion risk (Correct answer)
- Excessive weight loading the floor structure
- DC arc flash from terminal connections
Correct answer: Hydrogen gas off-gassing during charging creating explosion risk
Flooded lead-acid batteries release hydrogen gas during charging, which is explosive at concentrations above 4% in air, requiring adequate ventilation per NEC and manufacturer requirements.
Question 6: For a grid-tied battery storage system to provide backup power during a utility outage, it must include which feature?
- A DC disconnect rated for battery voltage
- An automatic transfer switch or islanding-capable inverter (Correct answer)
- A net metering agreement with the utility
- A separate PV combiner box for the storage array
Correct answer: An automatic transfer switch or islanding-capable inverter
Backup capability requires an automatic transfer switch (ATS) or an inverter with islanding capability to disconnect from the grid and power loads independently during an outage.
Question 7: When sizing a battery system for a residential time-of-use (TOU) rate optimization application, what is the primary design driver?
- Peak solar irradiance at the site
- Load during high-rate utility pricing periods (Correct answer)
- The short-circuit current of the PV array
- The wire gauge of the AC service entrance
Correct answer: Load during high-rate utility pricing periods
TOU optimization aims to store energy during low-rate periods and discharge during high-rate periods, so the battery must be sized to cover the critical loads during expensive on-peak hours.
NABCEP Battery Storage Systems & Energy Storage 2
In an AC-coupled battery storage system, the battery inverter connects to which bus?