Battery Charger IC Fundamentals
Battery Charging Basics Li-ion battery charger ICs are devices that regulate battery charging current and voltage, and are commonly used for portable devices, such as cellphones, laptops, and tablets. Compared to other battery chemistries, Li-ion batteries have one of the highest energy densities, provide a higher voltage per cell, can tolerate higher currents, and do not have to be trickle charged when the battery is fully charged. In addition, Li-ion batteries do not have a memory effect, meaning they do not "remember" a lower charge capacity if they are charged before being fully depleted. However, Li-ion batteries must be charged with a specific constant current and constant voltage (CC-CV) charge profile that is automatically adjusted depending on the battery's temperature and voltage levels. Charging Profile The charging profile is a fundamental aspect of Li-ion batteries, as it describes how a battery's voltage and current vary while the battery is charged. For simplification, charging profiles can be organized as a graph showing time on the X-axis and battery voltage or battery charge on the Y-axes, which offers insight on how to optimally charge a battery while acknowledging safety features. Figure 1 shows the charging profile for the MP2759A, a highly integrated switching battery charger IC designed for applications with 1-cell to 6-cell series Li-ion or Li-polymer batteries.
Li-ion batteries follow a relatively common charging profile, described in greater detail below. Note that if a charger IC provides configuration, the designer may be able to set their own thresholds for these phases. These configurable thresholds are highly beneficial, considering that most battery manufacturers specify certain thresholds for different maximum charge current levels. Configurability can provide an added layer of safety by protecting the battery from over-voltage and over-temperature conditions, as well as overloads, which could all permanently damage the battery or degrade its capacity.
Trickle charge: Generally, the trickle charge phase is only used when the battery voltage is below a very low level (about 2.1V). In this state, the battery pack's internal protection IC may have previously disconnected the battery due to it being deeply discharged, or if an over-current event occurred. The charger IC sources a small current (typically 50mA) to charge the capacitance of the battery pack, which triggers the protection IC to reconnect the battery by closing its FETs. Although trickle charging usually lasts for a matter of seconds, the charger IC should integrate a timer that stops charging if the battery pack is not reconnected within a certain amount of time, as this indicates that the battery has been damaged.
Pre-charge: Once the battery pack has been re-connected or is in a discharged state, pre-charging begins. During pre-charge, the charger starts to safely charge the depleted battery with a low current level that is typically C / 10 (where C is the capacity (in mAh)). As a result of pre-charge, the battery voltage slowly rises. The purpose of pre-charge is to safely charge the battery at a low current. This prevents damage to the cell, until its voltage reaches a higher level.
Constant current (CC) charge: Constant current (CC) charge is also considered fast charging, which is described in greater detail below. CC charging starts after pre-charge, once the battery has reached about 3V per cell. In the CC charge phase, it is safe for the battery to handle higher charge currents between 0.5C and 3C. CC charging continues until the battery voltage has reached the “full” or floating voltage level, at which point, the constant voltage phase begins.
Constant voltage (CV) charge: The constant voltage (CV) threshold for Lithium cells is usually between 4.1V and 4.5V per cell. The charger IC monitors the battery voltage during CC charging. Once the battery reaches the CV threshold, the charger transitions from CC to CV regulation. CV charging is implemented because the external battery pack voltage seen by the charger IC exceeds the actual battery cell voltage in the pack. This is due to the internal cell resistance, PCB resistance, and the equivalent series resistance (ESR) from the protective FET and cell. To guarantee safe operation, the charger IC must not allow the battery voltage to exceed its maximum floating voltage.
Charge termination: The charger IC determines when to terminate the charge cycle based on the current going into the battery dropping below a set threshold (about C / 10) during the CV phase. At this point, the battery is considered fully charged and charging is completed. If charge termination is disabled in the charger IC, the charge current will naturally decay to 0mA, but this is rarely done in practice. This is because the amount of charge going into the battery exponentially decreases during CV charging (the cell voltage is increasing like a large capacitor), and it would take a significantly longer time to recharge the battery with a very little increase in capacity.
Battery Charger IC Fundamentals
Battery Charging Basics Li-ion battery charger ICs are devices that regulate battery charging current and voltage, and are commonly used for portable devices, such as cellphones, laptops, and tablets. Compared to other battery chemistries, Li-ion batteries have one of the highest energy densities, provide a higher voltage per cell, can tolerate higher currents, and do not have to be trickle charged when the battery is fully charged. In addition, Li-ion batteries do not have a memory effect, meaning they do not "remember" a lower charge capacity if they are charged before being fully depleted. However, Li-ion batteries must be charged with a specific constant current and constant voltage (CC-CV) charge profile that is automatically adjusted depending on the battery's temperature and voltage levels. Charging Profile The charging profile is a fundamental aspect of Li-ion batteries, as it describes how a battery's voltage and current vary while the battery is charged. For simplification, charging profiles can be organized as a graph showing time on the X-axis and battery voltage or battery charge on the Y-axes, which offers insight on how to optimally charge a battery while acknowledging safety features. Figure 1 shows the charging profile for the MP2759A, a highly integrated switching battery charger IC designed for applications with 1-cell to 6-cell series Li-ion or Li-polymer batteries.
Li-ion batteries follow a relatively common charging profile, described in greater detail below. Note that if a charger IC provides configuration, the designer may be able to set their own thresholds for these phases. These configurable thresholds are highly beneficial, considering that most battery manufacturers specify certain thresholds for different maximum charge current levels. Configurability can provide an added layer of safety by protecting the battery from over-voltage and over-temperature conditions, as well as overloads, which could all permanently damage the battery or degrade its capacity.
Trickle charge: Generally, the trickle charge phase is only used when the battery voltage is below a very low level (about 2.1V). In this state, the battery pack's internal protection IC may have previously disconnected the battery due to it being deeply discharged, or if an over-current event occurred. The charger IC sources a small current (typically 50mA) to charge the capacitance of the battery pack, which triggers the protection IC to reconnect the battery by closing its FETs. Although trickle charging usually lasts for a matter of seconds, the charger IC should integrate a timer that stops charging if the battery pack is not reconnected within a certain amount of time, as this indicates that the battery has been damaged.
Pre-charge: Once the battery pack has been re-connected or is in a discharged state, pre-charging begins. During pre-charge, the charger starts to safely charge the depleted battery with a low current level that is typically C / 10 (where C is the capacity (in mAh)). As a result of pre-charge, the battery voltage slowly rises. The purpose of pre-charge is to safely charge the battery at a low current. This prevents damage to the cell, until its voltage reaches a higher level.
Constant current (CC) charge: Constant current (CC) charge is also considered fast charging, which is described in greater detail below. CC charging starts after pre-charge, once the battery has reached about 3V per cell. In the CC charge phase, it is safe for the battery to handle higher charge currents between 0.5C and 3C. CC charging continues until the battery voltage has reached the “full” or floating voltage level, at which point, the constant voltage phase begins.
Constant voltage (CV) charge: The constant voltage (CV) threshold for Lithium cells is usually between 4.1V and 4.5V per cell. The charger IC monitors the battery voltage during CC charging. Once the battery reaches the CV threshold, the charger transitions from CC to CV regulation. CV charging is implemented because the external battery pack voltage seen by the charger IC exceeds the actual battery cell voltage in the pack. This is due to the internal cell resistance, PCB resistance, and the equivalent series resistance (ESR) from the protective FET and cell. To guarantee safe operation, the charger IC must not allow the battery voltage to exceed its maximum floating voltage.
Charge termination: The charger IC determines when to terminate the charge cycle based on the current going into the battery dropping below a set threshold (about C / 10) during the CV phase. At this point, the battery is considered fully charged and charging is completed. If charge termination is disabled in the charger IC, the charge current will naturally decay to 0mA, but this is rarely done in practice. This is because the amount of charge going into the battery exponentially decreases during CV charging (the cell voltage is increasing like a large capacitor), and it would take a significantly longer time to recharge the battery with a very little increase in capacity.
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