Principle of operation and charging process of lithium ion batteries

Lithium-ion batteries (or Li-ion batteries) are the most widely used batteries in devices such as mobile phones, laptops, and electric vehicles, . Unlike nickel batteries, Li-ion batteries are more expensive. but in return the discharge times are 3 times that of batteries when used for electric vehicles, it has a longer life span, lighter weight, smaller volume and allows for a much faster charge. Due to the special chemical nature, Li-ion batteries have their own charging process, unlike batteries or other nickel batteries.

Overview of Lithium-ion battery charging process

Stable charging current: During the current charging process, the current is kept constant, usually by C / 2-C (where, C is the capacity of [Ah] of the battery). The bigger the charging current, the shorter the steady-state charging process, but the longer the voltage stabilization process will take place; however, the total charging time for both phases is usually no more than 3 hours. At the same time, large current will increase the battery temperature. During charging, it is necessary to monitor the temperature closely as the temperature is too high, which may cause the battery to burn or explode.

Normally, the temperature should not exceed 450C. Some Li-ion batteries use Lithium-Ferro-Phosphate (LiFePO4) technology which can push the temperature when charging up to 600C. If using a fast charger (quick charge) only perform a steady flow pump to the battery (stable charging), therefore, the limit on larger temperatures means higher charging current or fast charging time shorter.

Picture 1 of Principle of operation and charging process of lithium ion batteries
Process of charging li-ion batteries.

Voltage stabilizer: In voltage stabilization mode, charging voltage is usually kept constant by 4.2V / cell. As the capacity of the battery recovers gradually, its electromotive force increases, causing the current to decrease. When the current drops to less than 3% C, the voltage stabilization mode ends. At this time, the battery capacity reached about 99%. During the steady charge process, the voltage on the two battery terminals increased gradually. When the voltage reaches the electromotive force of the battery at full charge, the charger finishes charging properly and switches to the voltage stabilization mode. The entire time of steady current charging usually lasts up to 1 hour (depending on the original remaining capacity of the battery). At the end of the line charging process, the battery capacity has recovered about 70%.

In many cases (quick-charge) people can use it immediately ('charge-and-run' method). This though reduces the charging time and makes the design of the charger much simpler but on the other hand will reduce battery life. In order to ensure the battery life according to the manufacturer's specifications, it is often necessary to carry out the entire voltage stabilization stage - it usually takes a lot more time than the steady-state charging stage.

Unlike nickel or lead-acid batteries, Li-ion batteries do not need and are not allowed to maintain charging pressure after the battery is full (charging current drops less than 3% C) because the Lithium-ion's properties do not allow over -charge; If you try to over-charge, it may heat up the battery and cause an explosion. In addition, according to experts, do not charge Li-ion batteries that exceed 100% capacity as this will reduce the battery life. This issue will be clarified in the next section.

If the battery is fully charged, after stopping charging, the battery's open-circuit voltage will gradually decrease to a stable level of about 3.6 - 3.9V / cell. On the other hand, if charging is only fast (stable charging), after charging stops, the battery voltage will decrease further to about 3.3 - 3.5V.

Because Lithium-ion batteries also have self-discharge properties when not in use (self-discharge), in some cases, to fill the battery, in addition to using stabilization process, voltage stabilization, people often combine Add short pulse charging technique.

For example, when the battery voltage reaches 4.2V / cell, the charging process will stop immediately. At this time, the battery voltage will decrease; When the battery voltage drops to 4.05V / cell the charging system continues to close the 4.2V charging cell / cell to continue the charging process.

Such switching will take place continuously. Thus, the battery voltage will be kept stable in the range of 4.05 - 4.2V / cell, so making the battery charge deeper, avoiding over-charging phenomenon and prolonging battery life.

Over-charging Lithium-ion battery problem

Normally, Li-ion batteries should only be active (charging / discharging) in the designed voltage range (below 4.2V / cell). However, in some cases, when the battery is full and the pump is still injected, the battery voltage will rise above 4.3V. At this time, the battery is called over-charging.

When the battery voltage is outside the safe working area (above 4.2V / cell or less than 2.5V / cell) its operation becomes unstable. Lithium Metallic layers will form on the anode while the cathode will be strongly oxidized to reduce the stability and produce CO2 inside the battery which will increase the pressure on the battery. Normally, for safety, the charger needs to stop charging as soon as the cell pressure reaches 200psi.

If the charger does not have a high pressure monitoring and protection function, because CO2 is constantly being produced, the battery pressure will continue to increase, and the battery temperature will increase rapidly. When the pressure reaches about 500psi, at this time the battery temperature reaches about 130 degrees - 150 degrees, the safety film separates the cells from being punctured and the battery will start to burn even explode.

Therefore, during charging, it is necessary to strictly comply with the temperature and voltage requirements on the cells.

Picture 2 of Principle of operation and charging process of lithium ion batteries
Operation of the battery.

Li-ion batteries in general should not and should not be allowed to over discharge (over-discharge). When the battery voltage drops below 3.0V / cell, it's best to cut the battery off the circuit. If the battery voltage drops below 2.7V / cell the battery protection system of the battery will automatically switch the battery to sleep mode. At this time, the battery cannot be recharged in the usual way but needs to use the 4-stage charging cycle according to the diagram shown in Figure 2.3. Discharge of over-discharged Li-ion batteries

In the 4-stage charging cycle, in addition to the 2 stages of constant current and voltage stabilization, such as the conventional Li-ion battery charging process, 2 stages Pre-charge and Activation are added to restore battery operation.

Picture 3 of Principle of operation and charging process of lithium ion batteries
Lithium ion battery cell Samsung.

First, in the Pre-charge stage, the battery will be pumped into a small current (5-15% C) then the battery voltage is monitored. If after a specified period of time (testing time), the battery voltage does not increase or increase too slowly, the battery is considered unrecoverable. In contrast, if the voltage rises above 2.8V, then the battery is good and can continue to charge. At this time, the charger switches to charging in Activation mode to activate the battery operation again.

In Activation mode, 5-15% C current continues to be maintained until the battery voltage rises above 3V. At this time, the charger switches to operate in normal and stable voltage charging mode.

When manufacturers sell batteries, they usually charge up to 40% of the battery. However, after a while, due to the current self-discharge (self-discharge) battery capacity decreases, meaning that the battery voltage decreases. Therefore, in order to avoid over-discharge, the battery should be maintained periodically by recharging it after not using it for a long time.

Each Li-ion battery cell usually has an open-circuit voltage of about 3.5V. In systems such as electric vehicles, to power the main transmission motor and other electrical equipment in the vehicle, the cells are usually connected in series in parallel until the DC-Bus voltage is about 200VDC or more. 4. Cell balance (cell balancing).

Causes such as the parameters of the cells provided by the manufacturer have certain errors; During operation, the temperature affected on each cell is also uneven or the effect of life makes the properties of the cells uneven. If the cell has a slightly higher voltage, there are cells with a voltage slightly lower than the other cells, or in other words, the cells are not evenly matched.

During charging, a cell with a higher voltage will fill first while some remaining cells are not full. If it continues to charge, the cell will be overcharge so high temperatures and pressures (as discussed above) reduce the life of the entire battery, even destroying the cell. Conversely, during the discharge process, the cell with lower voltage will soon run out. If the discharge continues, the cell will be over discharged, reducing battery life. When a cell is broken, it is common to replace the entire battery system, because, if only the cell is broken (maybe in some cases), the new cell is still different from the other. The remaining cell, meaning the risk of unbalance, can still happen.

The more cells are connected in series, the higher the risk of imbalance and the lower the reliability. Studies have shown that, if the battery system is paired by n cells, the probability of an imbalance increases more than n times compared to only one cell operating independently.

To limit this issue, there are several ways to consider. First, people will try to select cells with relatively equal parameters to pair with each other. The cells will then be paired in series with each other instead of just pairing, so the flow between the cells will help balance the cells together (self-balacing). Then, during use, the temperature must be closely monitored to ensure even distribution on the cells.

However, to completely solve the imbalance of Li-ion battery pressure, in electric cars, battery management system (Battery Management System - BMS), it is necessary to closely monitor each cell's capacity (State of Charge - SOC). If an imbalance is found, the BMS system needs to take certain measures to bring the cells to equilibrium with each other. There are two ways to do this: active balance and passive balance.

Picture 4 of Principle of operation and charging process of lithium ion batteries
36V balance circuit for electric bicycles.

Active balance method will transfer energy from higher capacity cells into lower capacity cells. This method has the advantage of helping the system balance the pressure and there is no loss due to the energy being rotated between the cells. However, designing for each cell an independent charging source is impractical. Pressure balancing is performed sequentially for one or a cell group. Therefore, to fully charge the battery, it takes quite a long time.

The passive equilibrium method is simpler than the active equilibrium method but causes resistance on the resistor. The charger needs to disconnect immediately when a cell is full. After that, the full cell will be discharged through the resistor until it is lower by the cell. After that, the charger continues to power on and the cycle is repeated until all cells are full.

Thus, in the process of charging, in addition to complying with the charging and charging processes, it is necessary to work closely with the BMS system to implement cell balancing techniques to fill the cells, preventing imbalances between them. cell, thereby extending the life of the battery.

The effect of temperature on the Lithium ion battery charging process

As mentioned above, the charging and discharging operation of the battery depends heavily on temperature. In general, all batteries can operate in a fairly wide temperature range. For Li-ion batteries, this temperature range is from 0C - 45C in charging mode and 0C - 60C in discharge mode. Some newer Lithium-based batteries, such as Lithium-Ferro-Phosphate (LiFePO4) or Li-Polimer, allow a slightly wider working temperature range. In this area, battery properties are almost stable, high energy efficiency. But outside of that temperature zone, at very low or very high temperatures, the battery's performance is strongly affected, the chemical reactions within the battery slow down, meaning that the current generated by the battery or absorption will decrease compared to when operating in.

For Li-ion batteries in general, it has been proven that the temperature range of 5C - 45C is the optimal operating temperature range. Under 5C the charging current needs to be reduced and when the temperature drops below 0C (freezing temperature), it is necessary to stop charging.

Conversely, at temperatures higher than 45C battery performance becomes more powerful, meaning that it is possible to discharge or charge more current than the rated current (C). However, both cases (too low temperatures as well as too high temperatures) increase the battery's internal resistance, so if you still try to charge, it will reduce the battery life.

Requirements when using Li-ion batteries

  • Turn off all devices powered by batteries that need to be charged. Meanwhile, the current measurement system, charging voltage will give accurate results, reflecting the charging process parameters.
  • Do not charge when the ambient temperature is too low or too high.
  • Stop charging as soon as the battery temperature rises abnormally
  • Stop charging as soon as the battery capacity reaches about 90 - 99%. So it is better for batteries than charging to 100% or more. Typically, chargers have capacity indicators and self-cut when the capacity reaches 90 - 99%. If not, users need to follow up to cut the charger. This will increase battery life.
  • Before storing unused batteries for a long time, it is recommended to pre-charge it to about 40-50% of the capacity to avoid over-discharge because of a self-dischage battery.
  • Do not try to charge a battery with a voltage below 2.7V / cell (over-discharged) with regular chargers (only stable and voltage stabilized mode) but must use dedicated chargers (support Full 4 modes: Pre-charge, Activation, Constant Current, Constant Voltage.
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