C. Charging Methods and Estimation of Battery, SOC, and SOH

a) Describe two kinds of Li-ion battery charging methods in reality (5 marks)

Two common charging methods for Li-ion batteries are Constant Current (CC) and Constant Voltage (CV).

·    CC Charging: Supplies a fixed current to the battery until it reaches a certain voltage. It's efficient for fast charging but requires transition to avoid overcharging.

·    CV Charging: Maintains a constant voltage as the current decreases gradually. This method safely completes the charge but is slower in the final stages.

b) Describe the State of Charge (SOC) and State of Health (SOH) of a battery and discuss their importance. (5 marks)

·    SOC: Indicates the current charge level as a percentage of the battery's capacity. It's crucial for managing energy usage and preventing over-discharge.

·    SOH: Reflects the overall condition of the battery, considering factors like capacity and resistance. It's important for predicting battery lifespan and performance.

c) Describe at least two methods for assessing battery SOC and compare their advantages and disadvantages. (5 marks)

·    Coulomb Counting: Measures the charge in and out of the battery. Accurate but can accumulate errors over time.

·    OCV Method: Estimates SOC based on open-circuit voltage. Simple and cost-effective but less accurate due to its dependency on resting time and temperature.

d) Describe at least two methods for assessing battery SOH and compare their advantages and disadvantages. (5 marks)

·    Capacity Fade: Measures the reduction in capacity over time. Direct but requires full charge-discharge cycles.

·    Internal Resistance Measurement: Monitors resistance changes as the battery ages. Can be done without full discharge but needs precise instruments.

D. Battery Control System

a) What are the characteristics that determine a PV-plus-battery system's functionality? (5 marks)

Key characteristics include battery capacity, inverter efficiency, charge controller

effectiveness, and system configuration (AC/DC coupling). These factors determine the system's reliability, efficiency, and flexibility.

b) What is the main difference between AC and DC coupled configurations? (5 marks)

·    AC Coupling: Involves converting DC from PV to AC for use or storage, offering flexibility but with energy losses due to multiple conversions.

·    DC Coupling: Directly stores DC energy from PV in the battery, which is more efficient but less flexible for integrating with existing AC systems.

c) The circuit shown in Fig 1 is a battery equalizer. Assuming the voltage on battery B1 is higher than B2, list at least two steps for operating the switches (Q1, Q2) to equalize the voltage on each battery (10 marks)

1.  Close Q1 to connect B1 to the equalizer, allowing excess charge to be stored.

2.  Close Q2 to transfer the stored charge to B2, equalizing the voltage between the two batteries.