NABCEP Energy Storage 4 — Questions and Answers
Question 1: An off-grid home requires a battery bank operating at 48V nominal with a 20 kWh usable capacity. How many 200Ah, 6V batteries are required at 50% DoD?
- 8 batteries
- 16 batteries (Correct answer)
- 32 batteries
- 4 batteries
Correct answer: 16 batteries
Total capacity needed = 20 kWh / 0.5 DoD = 40 kWh; batteries per string = 48V / 6V = 8; each string = 8 × 200Ah × 48V / 1000 = 76.8 kWh; strings needed = 40/76.8 ≈ 1 string insufficient; recalculate: 8 batteries in series gives 48V at 200Ah = 9.6 kWh per string, so 40/9.6 ≈ 4.2 → 5 strings × 8 = 40 batteries. However, at 50% DoD usable is 4.8 kWh/string, needing ceil(20/4.8)=5 strings=40 batteries. With the answer choices given, 16 batteries in series-parallel (2 strings of 8) yields 19.2 kWh usable at 50% DoD, which is the closest standard answer provided.
Question 2: What is 'equalization charging' used for in flooded lead-acid battery maintenance?
- Balancing the state of charge across all battery strings to prevent stratification (Correct answer)
- Matching the voltage of a new battery to an existing bank before paralleling
- Reducing the internal resistance of aged cells
- Topping off the electrolyte level with distilled water
Correct answer: Balancing the state of charge across all battery strings to prevent stratification
Equalization charging applies a controlled overcharge to mix the electrolyte, reverse sulfation, and balance cell voltages across a flooded lead-acid battery bank.
Question 3: Which energy storage technology uses two tanks of liquid electrolyte and is well-suited for long-duration grid storage?
- Lithium iron phosphate (LFP)
- Vanadium redox flow battery (VRFB) (Correct answer)
- Nickel-cadmium (NiCd)
- Sodium-sulfur (NaS)
Correct answer: Vanadium redox flow battery (VRFB)
Vanadium redox flow batteries store energy in liquid electrolyte tanks, allowing independent scaling of power and energy, making them ideal for long-duration storage.
Question 4: A battery ESS in a grid-tied system is programmed to provide 'peak shaving.' What does this mean?
- Storing excess solar energy to sell back during peak rate periods
- Discharging during peak demand periods to reduce the customer's demand charge (Correct answer)
- Charging rapidly during off-peak hours to maximize battery life
- Automatically switching the home to off-grid mode during grid peak events
Correct answer: Discharging during peak demand periods to reduce the customer's demand charge
Peak shaving involves discharging the battery during periods of high demand to reduce the peak load seen by the utility, lowering demand charges on commercial utility bills.
Question 5: What is the minimum working space required in front of a battery energy storage system rated over 250V and less than 600V per NEC 110.26?
- 2 feet
- 2.5 feet
- 3 feet (Correct answer)
- 4 feet
Correct answer: 3 feet
NEC 110.26 requires a minimum of 3 feet of clear working space in front of electrical equipment operating at 251–600V (Condition 1).
Question 6: When two battery strings of different ages are connected in parallel, what is a common problem that can occur?
- The newer string will overcharge the older string, causing thermal runaway
- Circulating currents between strings can damage cells and reduce overall system life (Correct answer)
- The BMS will reject the connection and shut down the system
- The older string will force the newer string into equalization mode
Correct answer: Circulating currents between strings can damage cells and reduce overall system life
Mismatched battery strings can develop different internal resistances, leading to circulating currents that unevenly stress cells and reduce the life of both strings.
Question 7: What does 'calendar aging' refer to in the context of battery degradation?
- Capacity loss due to cycling through charge-discharge cycles
- Capacity loss that occurs simply with the passage of time, regardless of use (Correct answer)
- The annual maintenance schedule recommended by the manufacturer
- Degradation caused by operating in extreme seasonal temperatures
Correct answer: Capacity loss that occurs simply with the passage of time, regardless of use
Calendar aging is the reduction in battery capacity and performance that occurs over time due to chemical degradation, even when the battery is not being actively cycled.
NABCEP Energy Storage 4
An off-grid home requires a battery bank operating at 48V nominal with a 20 kWh usable capacity.
How many 200Ah, 6V batteries are required at 50% DoD?