How does the charging mode influence the initial state of charge

The charging mode has a significant influence on the determination of the initial state of charge (SOC) of a battery, as the SOC estimation depends on the battery’s voltage and current characteristics during different charging stages.

How Charging Mode Influences Initial SOC

1. Constant Current (CC) Charging Mode:

• When charging occurs in the constant current mode, the battery is charged with a fixed current. During this stage, the battery voltage gradually increases until it reaches a predefined threshold voltage set by the manufacturer.
• The relationship between SOC and charging voltage during this stage can be used to deduce the initial SOC from the voltage measurements.
• However, with CC charging alone, it is challenging to fully charge the battery without risk of overvoltage or damage, and SOC estimation during this phase is mostly voltage-dependent at increasing voltage levels.

2. Constant Voltage (CV) Charging Mode:

• After the CC stage, the charger switches to a constant voltage mode where the voltage is held steady at a threshold value.
• In CV mode, the charging current decreases over time—rapidly at first and then slowly—until it almost drops to zero when the battery reaches full charge.
• The relationship between SOC and charging current during this CV stage can be used to refine the initial SOC estimation.
• Eventually, the current tapering to near zero indicates full charge, helping to calibrate the SOC to 100%.

3. Combined CC-CV Charging Mode:

• The CC-CV hybrid mode is the most commonly used charging protocol, especially for Li-ion batteries.
• SOC estimation during charging uses the voltage and current profiles obtained in both CC and CV stages.
• The initial SOC can be deduced from the voltage during the CC phase and from the charging current during the CV phase.
• This mode optimizes a balance between charging speed (determined by the CC phase) and capacity utilization and battery health (determined by the CV phase).

4. Open Circuit and Discharge Modes for SOC Reference:

• Outside charging, SOC can also be estimated from open circuit voltage (OCV) if the battery is at rest for a significant duration.
• During discharging, SOC changes can be tracked based on voltage changes at known discharge currents.
• But for initial SOC determination at the start of charging, the charging mode’s voltage and current profiles are key.

Practical Implications

• The charging mode directly affects how the battery voltage and current behave, which are the main observables to infer the initial SOC.
• For example, if a battery is just powered on and the control system assumes 100% SOC without charging, it may miss an actual low SOC state unless the charging mode triggers current flow and voltage changes to recalibrate SOC.
• Different charging modes provide different temporal patterns of voltage and current, which battery management systems use to estimate SOC with varying degrees of precision.

In summary, the charging mode influences the initial SOC determination through the distinct voltage and current signatures it creates during charging. The CC mode affects the voltage rise and provides voltage-based SOC clues, while the CV mode affects current tapering that refines SOC near full charge. The combined CC-CV mode yields the most informative data for accurately deducing the initial SOC from charging data.

References:

• Detailed explanation of SOC estimation during CC and CV charging stages, and their relationships to voltage and current curves.
• Description of CC, CV, and CC-CV charging modes and their impact on battery behavior and SOC estimation.
• General importance of charging current and voltage in SOC estimation accuracy.