Constant current charging

Definition

The capacity of a power battery is measured in ampere-hours (Ah), so the amount of current charged into the power battery within the same period of time determines the charging capacity. Therefore, in a complete charging cycle, a fixed current value is applied to charge the power battery, and this method is called constant current charging.

Overcharged

When the power battery is being charged, the gradual consumption of active substances involved in reactions weakens the driving force of chemical reactions, leading to factors such as a decrease in electrode terminal voltage.

At this time, constant current charging can restore the voltage of the power battery to its nominal value within a short period by using a larger current. However, due to electron movement inside the battery, polarization resistance is formed, resulting in an increase in total internal resistance.

The increase in internal resistance will cause the voltage of the power battery to continue rising. If charging is not stopped at this point, lithium deposition will occur at the negative electrode of the battery, leading to a decrease in capacity. Moreover, under thermal effects (Q=I2Rt), increased internal resistance will generate a large amount of heat and may even lead to explosions when severe; this phenomenon is commonly referred to as overcharging.

Constant voltage charging

Technological incentives

Overcharging of power batteries is mainly caused by excessive charging current, which leads to high battery heat. Therefore, in order to prevent overcharging of the battery, constant voltage charging method is used for power batteries.

Charging initial stage

In the initial stage of constant voltage charging, the battery’s feeding may cause its voltage to drop, which may prevent the voltage from reaching the set constant value. At the same time, the internal resistance of the battery is relatively low during this period, so there is usually a larger available space for receiving charging current in dynamic batteries. Therefore, when using constant voltage charging method, the initial charging current will be relatively high.

Voltage stabilization

When the voltage of the power battery reaches the constant voltage charging stage, as its internal chemical reactions proceed, the capacity will gradually saturate. At this time, the movement speed of lithium ions between electrodes will gradually slow down, and at the same time, the concentration gradient of electrolyte will increase. This may lead to the formation of a solid electrolyte interface (SEI) film on the electrode surface, which will cause an increase in internal resistance of the battery. In addition, as SOC increases, polarization phenomena intensify, especially concentration polarization and ohmic polarization, which also contribute to an upward trend in internal resistance. These factors can have an impact on the lifespan of power batteries.

Fill up immediately

And as the charging continues, when the power battery is close to full, the increase in internal resistance will gradually decrease the charging current. Although low current charging prevents overcharging from occurring, it also means that less electricity is being charged into the battery per unit of time, thus prolonging the overall charging time.

Mixed charging

Integration of the two

As mentioned above, in the constant current charging mode, overcharging mainly occurs during the charging process after the voltage of the power battery reaches the threshold voltage. In contrast, in the constant voltage charging mode, excessive initial current is unfriendly to the power battery. By comparing and integrating both charging modes while eliminating their drawbacks, a charging mode of first constant current and then constant voltage is formed.

Introduction

When using the constant current followed by constant voltage charging method, if the state of charge (SOC) of the power battery is too low, the internal impedance of the battery may increase due to deep discharge. If high current charging is applied at this time, it may cause a rapid rise in internal battery temperature and potentially lead to safety hazards such as thermal runaway.

In this state, the electrochemical reaction activity within the power battery is reduced, and high current charging not only has low efficiency but can also worsen side reactions like lithium deposition and damage to the electrode structure inside the battery. This ultimately leads to accelerated capacity loss and affects its cycle life.

Therefore, in order to protect the power battery and improve its charging efficiency, when the SOC is very low, a “pre-charging” method is adopted. This means that initially, the battery is charged with a small current. Once the battery voltage and temperature reach a certain level, the charging current gradually increases to transition into the normal constant current charging stage. Finally, it enters the constant voltage charging stage. This not only ensures the safety of the battery but also extends its lifespan to some extent.

Three-step charging model

Through further optimization of the constant current and constant voltage charging method, the widely used three-stage process, consisting of a preliminary low-current pre-charge followed by a constant current and constant voltage charging mode when the power battery reaches above the supply voltage, has been formed. The schematic diagram below illustrates this process.

In the application process of the three-stage charging model, when the State of Charge (SOC) of the power battery approaches its maximum value, the internal chemical reactions within the battery become increasingly difficult to proceed. This results in a decrease in the battery’s ability to accept new energy, which is reflected in the charging curve as a gradual reduction in charging current.

When the SOC approaches full value, if constant voltage charging is continued, most of the current will no longer be used for energy storage but will be used to overcome the internal resistance and polarization resistance of the battery, resulting in unnecessary heat generation and power loss, while also accelerating battery aging. Therefore, based on the three-stage charging model, a trickle charge (or float charge) stage is introduced.

At this stage, when the battery is approaching full charge, the charging current is further reduced to a very small proportion of the rated capacity of the battery (such as 0.01C or lower). This weak charging current offsets the loss of electricity caused by self-discharge and polarization, preventing undercharging and irreversible damage to internal active materials in power batteries due to long-term undercharging. It helps extend battery life cycle. Moreover, based on the low current characteristic of trickle charging, it will not cause a significant rise in internal temperature of the battery, avoiding overcharging risks and further improving safety during battery use.

Four-stage charging model

In the four-stage charging model, trickle charging and pre-charging both use small current to charge, but they have essential differences. The pre-charging stage is usually applied in cases where individual batteries (depending on the battery, below 2.5V or 3V) or the total voltage of the battery is too low due to long periods of non-use or deep discharge. The main purpose is to slowly restore the battery voltage to a safe range through small current charging (such as 0.01C~0.1C), activate chemical reactions inside the battery, and prevent damage caused by high current charging.

And trickle charging occurs when the battery is approaching full capacity, typically with a state of charge (SOC) greater than 90% (depending on the strategy), at which point the battery voltage has reached the upper limit set for constant voltage charging. By using a charging current as low as 0.01C or even lower to maintain the battery’s full charge and prevent overcharging, this is considered a supplementary and maintenance charging strategy, with the trickle charging current usually being smaller than that of pre-charging.

Summary

From the previous introduction and process model diagram, we can already understand that in a continuous charging cycle, the maximum charging current that a lithium battery can withstand gradually decreases as the charging process progresses. This is due to phenomena such as polarization and increased internal resistance mentioned above. Additionally, due to the accumulation of charge on the electrode, it causes a voltage shift and makes it higher than the actual normal voltage of the power battery. In normal circumstances, at this time, the battery voltage should be A, but due to voltage offset, it results in this voltage becoming B.

The result of this phenomenon is that the constant voltage charging stage intervenes earlier during the charging process, causing a false full charge situation. The most obvious feeling for users in this situation is that although the meter shows a certain proportion of electricity has been charged, they will find that there is an unusually severe power loss when using the vehicle.

In order to alleviate the occurrence of voltage offset, polarization and other phenomena caused by the accumulation of charges mentioned above, the application of pulse current in the charging model was proposed.

Pulse charging

Pulse charging is a technical method of controlling the charging of power batteries by using a certain intensity of charging current during the charging process, and intermittently forming pulse currents through charge-stop or charge-stop-discharge cycles. This process releases the charges formed by continuous charging in an intermittent manner, thereby alleviating polarization phenomena and ultimately achieving the goal of increasing the charging capacity of power batteries.

Possible stages of application

We know that the accumulation of charge is caused by constant current charging under higher currents, and the early intervention of constant voltage charging is also due to the presence of this charge. The small current charging methods in the first and fourth stages do not cause polarization phenomenon in power batteries, so pulse charging may mainly be applied during the constant current or constant voltage charging stages in conventional charging process models, which are the main stages for charging power batteries.

In this constant current and constant voltage charging stage, the application of pulse current allows for the release of accumulated charge by stopping the charging time (t2) during the charging process. This reduces the charge accumulated at the electrode, resulting in a more accurate detection of battery voltage and enabling more energy to be charged throughout the entire charging cycle. At the same time, releasing charges alleviates polarization phenomena, which benefits the lifespan of power batteries.

By applying multiple dimensions and technologies, the charging process of power batteries can have more advantages in terms of charging efficiency, battery safety, extended lifespan, and energy utilization compared to the original charging model, without changing the performance of the battery itself.