Charging Modes of a Li-ion Battery

 Lithium is the lightest of all metals available, has the greatest electrochemical potential, and provides the largest specific energy per weight. The inherent instability of lithium metal, especially during charging, transposed research to a non-metallic solution using lithium ions. 

       In 1991, Sony commercialized the first Li-ion, and today this chemistry has become the most promising and flourishing battery on the market. Although lower in specific energy than lithium-metal, Li-ion is safe, provided the voltage and current limits are monitored properly. 

       Li-ion is a low-maintenance battery, having high energy density, an advantage that most other chemistries cannot claim. The battery has no memory and does not need exercising (deliberate full discharge) to keep it in superior condition. The nominal cell voltage of 3.60V can directly power mobile phones, tablets, and digital cameras, offering simplifications and cost reductions over multi-cell designs. 

• Drawback: The need for protection circuits are higher-priced than lead-acid batteries.

Charging Modes:

There are three common procedures for charging a battery.

1. Constant current charging
2. Constant voltage charging
3. Boost or Quick charging

 

• Constant current charging mode:

The current level is approximately 10% of the standard charging rate. Constant current chargers vary the voltage they apply to the battery to maintain a constant current flow, switching off when the voltage reaches a full charge level. The disadvantage of this mode is that the battery may start overheating if overcharged, leading to premature battery degradation and, eventually, replacement.

• Constant voltage charging mode: 

It allows the full current of the charger to flow into the battery until the power supply reaches its preset voltage.  The current will then taper down to a minimum value once that voltage level has been attained.  The battery can be left connected to the charger until ready for use and will remain at that “float voltage,” trickle charging to compensate for normal battery self-discharge. This method of charging consumes more time. 

• Boost or Quick charging:

It is a union of the above two methods.  The charger limits the amount of current to a pre-set level until the battery reaches a preset voltage level. The current then gradually reduces as the battery becomes fully charged. It allows fast charging, isolating the risk of over-charging, and is appropriate for Lithium-ion and other similar battery architectures.

Considering the above plot, we can observe the stages of charging Li-ion batteries (with the traditional cathode materials of cobalt, nickel, manganese, and aluminum). It typically charges 4.20V/cell. High-capacity Li-ion may go to 4.30V/cell and higher. The tolerance is +/–50mV/cell. Increasing the charge current does not hasten the full-charge state by much. Although the battery reaches the voltage peak quicker, the saturation charge will take longer. Stage 1 becomes shorter with a higher current, but the saturation during Stage 2 will take longer. A high current quickly charges the battery to about 70%. On boosting the voltage increases capacity, but going beyond specification stresses the battery and compromises safety. Protection circuits built into the battery pack do not allow exceeding the set voltage. A full charge is reached when the current decreases to 3% - 5% of the Ah rating. Some lower-cost consumer-use chargers may utilize the simplified “charge-and-run” method that charges a lithium-ion battery in 1 hour or less without even attaining the Stage 2 saturation charge. ‘Ready’ appears when the battery reaches the voltage threshold at Stage 1. State-of-charge (SoC) at this point is about 85%.

 

 The electrical behavior of the battery is expressed using three different phases, as illustrated in Figure 4. The constant current mode of charging occurs more rapidly than the constant voltage mode. When “Fast Charging” rates are specified, they usually refer to the constant current mode. The practical charging method uses two types of sources. The constant current charging at the start when the battery is relatively empty. Once the battery reaches a certain voltage near the maximum voltage, constant voltage charging is accomplished. When a lithium battery is nearly empty, we take constant current to charge it. We need to ensure that the charging current is lower than the maximum charging current that the battery can accept. With constant charging, the battery's voltage is slowly increased; when the battery voltage reaches the maximum charging voltage, the charger will ensure the charging voltage is fixed as "constant voltage" and reduce the charging current. This leads to an increase in the lifespan of the battery. This mode's current slowly starts dropping by maintaining a constant voltage. In this state, the charging rate of the battery is extremely meager.

C-Rating:

C-rates govern the charge and discharge rates of a battery. The capacity of a battery is commonly rated at 1C, meaning that a fully charged battery rated at 1Ah should provide 1A for 1 hour. The same battery discharging at 0.5C should provide 0.5A for 2 hours; at 2C, it delivers 2A for 0.5 hours. Losses at fast discharges reduce the discharge time, and these losses also affect charge times. A C-rate of 1C is also known as a one-hour discharge; 0.5C or C/2 is a 2-hour discharge, and 0.2C or C/5 is a 5-hour discharge. Some high-performance batteries can be charged and discharged above 1C with moderate stress. Table.2 below illustrates the typical times at various C-rates.

To summarize, a discharge and charge cycle is commonly understood as the full discharge of a charged battery with subsequent recharge, but this is not always true. Batteries are seldom fully discharged, and manufacturers often use the 80% depth-of-discharge (DoD) formula to rate a battery. This means that only 80% of the available energy is delivered, and 20% remains in reserve. 

Cycling a battery at less than full discharge increases service life, and manufacturers argue that this is closer to a field representation than a full cycle because batteries are commonly recharged with some spare capacity left. Hence, lithium-ion batteries should not be charged fully to increase their lifetime.