LiFePO4 batteries are the best that the technology has on offer right now. Their long lifespan and highest value for money make users replace alternative batteries with LiFePO4 battery packs.
As it is a newer technology, many owners ask about the LiFePO4 battery balancing. Battery balancing is important for all types of batteries. This article will explore the balancing function of the LiFePO4 battery and what makes it so important.
What is Battery Cell Balancing?
Battery cell balancing means levelling the voltage parameters and State of Charge (SOC) of the different cells within a battery pack. Battery packs generally consist of three, four, or more individual cells. These cells drain and charge at different rates, leading to differing performance characteristics. Battery balancing is helpful to even out this difference between cells.
Why Do You Need to Balance a LiPo Battery?
Balancing is extremely important for prolonging the lifespan of the LiFePO4 battery pack. A battery pack consists of multiple individual cells connected together. The voltage rating of a single battery cell is insufficient for most applications. Multiple cells together provide enough power to run most appliances.
When the manufacturer ships out the batteries, the cells are very balanced. There are only minute differences between the performance voltage parameters of the cell. However, the difference is amplified when the cells go through charge and discharge cycles on the user end. This leads to certain cells having a reduced capacity than the other cells.
During charging, the strongest cell will determine the cut-off voltage of the entire pack. Therefore, the low-capacity cells will be overcharged. Similarly, while discharging the low-capacity cells will be over-discharged. The overcharging and over-discharging of a single cell depreciate the performance of other cells as well.
Therefore, LiFePO4 battery balancing is required to maintain the performance level of all cells and avoid overcharging and over-discharging.
Balancing Extends LiFePO4 Battery Life
LiFePO4 battery balancing does extend its life significantly. In fact, if you follow a proper balancing process regularly, you can extend the lifespan of your battery far beyond what manufacturer-rated lifespan. This is not limited to the Lithium Iron Phosphate battery pack. It also applies to many other types of batteries.
Risks of Unbalanced Cells
Ignoring the importance of cell balancing causes the cells to become unbalanced. In unbalanced cells, some cells have a lower capacity than others. There are many risks to having unbalanced cells in your battery pack. Some of these risks are:
Early Cell Degradation
Cell degradation will definitely occur if your battery pack goes through multiple cycles without proper balancing. This is because of the capacity difference between cells.
The lower-capacity cells tend to get charged the fastest. However, other cells will still pull out the charging current. The voltage of the low-capacity cell will cross the fully charged mark and travel into overcharged battery voltage.
In the overcharging zone, the battery cells begin to lose its health irreversibly. The damage caused by overcharging cannot be fixed later. It deteriorates the capacity of the cell further, pacing the deterioration cycle.
The overcharging caused by unbalanced cells also poses serious safety concerns. The overcharging causes a temperature rise in addition to cell degradation. LiFePO4 batteries have a high energy density. Therefore, the temperature rise brings the possibility of a thermal runaway.
Thermal runaway causes a series of uncontrolled chemical reactions in the battery. These reactions can lead to battery fire or explosions. Therefore, there is a safety risk for the people and property near the battery pack.
Limited Operating Range
LiFePO4 batteries come with a Battery Management System (BMS) to keep the battery in a safe operating zone. This system terminates the charging and discharging whenever the voltage travels in the red zone.
The BMS will disable the charging stage once a single cell reaches the full charge voltage. The low-capacity cell will reach this voltage before the high-capacity cells. Therefore, the high-capacity cells will not reach their full charge when the charging is terminated early.
The same follows for discharging as well. The low-capacity cell will discharge faster than the other cells. The battery management system will terminate battery use when the low-capacity cell is discharged. The other cells will still have some capacity left in them which is wasted.
This results in a very limited operating range of the battery. Only a portion of the battery power is being utilized.
What are the Different Types of Cell Balancing System?
There are two types of LiFePO4 battery balancing system- active and passive. Let us go through the details of each of these systems:
Active Cell Balancing
Active cell balancing transfers energy to cells with a lower charge level from the high-charged cells. Once the charge level of weaker cells becomes equal to the other cells, an equal charging voltage is provided to all cells. This ensures that all the cells are fully charged together.
There are multiple ways to achieve an active balancing function for LiFePO4 batteries. These ways differ in the design of the circuit. The common circuit designs are:
- Single Inductor Circuit: Cheap cell balancing circuit option. It is also one of the smallest sizes.
- Multiple Inductor Circuit: More expensive than single inductor circuit. However, the balancing speed is faster. It also has better efficiency.
- Single Transformer Circuit: Faster than inductor-based circuits. There is a presence of little magnetic losses.
- Multiple Transformer Circuit: Faster cell balancing. However, they are costly, and the circuitry is very complex.
- Single Capacitor Circuit: A single capacitor circuit uses one capacitor for the entire battery pack. Therefore, a lot of switches are required for controlling the charging voltage of each cell. In addition, it needs an intelligent system to monitor the switching function.
- Multiple Capacitor Circuit: There are a number of capacitors for every battery cell. The charging voltage is maintained by choosing the appropriate capacitor for unequal voltage distribution. There is no need of complex sensors and controls.
Power Electronics Circuit
- Flyback Converter: A flyback converter stores the energy of the high-capacity cell in the transformer. The transformer has an air gap in the transformer for this purpose.
- Forward Converter: The forward convertor stores the energy of the high-capacity cell in a storage choke after the transformer.
- Full Bridge Converter: Full-bridge convertors transfer the energy from the high-capacity cell to the low-capacity cell in real time. They do not require storing the charge. This makes them a very fast active cell balancing option.
Benefits of Active Cell Balancing
- Active balancing has very good efficiency. All the energy of the high-capacity cells is transferred to the low-capacity cell with minimal losses.
- This cell balancing works well even when the energy difference between the storage cells is vast.
- The cell balancing process with the active method is very fast.
- Lifespan of the battery pack increases significantly.
Limitations of Active Cell Balancing
- The balancing circuit is complex.
- It can only transfer charge from the high-charged cell to the low-charged cell.
Passive Cell Balancing
Passive cell balancing is entirely different from its active counterpart. In passive balancing circuit, there is no transfer of charge from the high-cell to the low-cell. Instead, all the extra energy of the high cell is dissipated in the form of heat. This is done by adding resistors in front of the cell. The extra energy is burned till the State of Charge of the high-cell matches SOC of the low-cell.
There are two types of passive cell balancing circuits:
In fixed-shunting balancing, the resistors limit the maximum voltage the cells can reach. This is done to prevent any cell from overcharging. Whenever any cell reaches the maximum voltage, the extra energy supplied is given to the shunting resistor. It dissipates this energy in the form of heat.
Switch Shunting Balancing
Switch shunting balancing works by using several switches and a voltage sensor. The voltage sensor is installed in each cell to monitor the charge levels. There are two modes- continuous and voltage sensing. The continuous mode lets the cell operate in normal working conditions. The sensing mode turns on the voltage sensors and balances any inequality with the resistors. Switch shunting balancing is a good option for battery cells with low charging and discharging currents.
Advantages of Passive Cell Balancing
- Passive cell balancing only comes into effect when cells have unequal SOC. If the cells are working perfectly, passive cell balancing is disabled.
- The extra energy is burned off only after the cell is fully charged. There is no balancing operation before the charging is complete.
- It is a cheap cell balancing option.
- Passive balancing can fix any differences in self-discharge currents.
Disadvantages of Passive Cell Balancing
- The extra energy is burned off as heat. Therefore, passive balancing has a very low efficiency.
- It increases the temperature of the battery pack and battery management system.
- There can be cases of thermal runaway if the temperature becomes too high.
- High energy losses mean you will incur extra electricity costs for the charging process.
How to Do Cell Balancing?
There are two different approaches of cell balancing: top balancing and bottom balancing. Each of these approaches has its working steps and requirements. In addition, you cannot use both of these approaches together. They will negate the effect of each other if used together.
Let us go through them one by one:
Top balancing emphasises the charging characteristics of the cell. The main factor at play here is the cell voltages. The main goal here is that at the fully charged voltage, the cell capacity of all units in the battery pack is the same. Top balancing aims to equal the capacity of the cells when they are at the fully charged voltage.
Steps to Top BAlance the Battery
- Disconnect the individual cells from the battery pack.
- Set up the cells in a parallel connection. Battery balancing should never be done in a series connection.
- Preset the charging voltage to 3.5 V in a DC supply. Double-check to see the polarity of the cells is correct.
- Monitor the supply voltage and the cell voltages from time to time.
- Set the DC supply voltage to 3.65 V as the cell voltages near 3.5 V. Monitor it constantly at this point.
- When the current falls down close to 0 A, the cells are charged. Turn off the DC power supply.
- Wait for some time for the cell voltage to become stable and redistributed among each other.
- The cells are now top-balanced. Check the voltage of each cell with a voltmeter or a multimeter. You should find the same voltage of each cell to be 3.65 V. Any small variations of some millivolts are considered negligible and nothing to worry about.
- Connect the cells back into your battery pack.
Bottom balancing emphasises the discharging characteristics of the LiFePO4 battery pack. This is done by balancing the battery capacity. A bottom balance is required when there is a considerable difference in the capacity of all the battery cells. You cannot use the battery management system (BMS) after the bottom balance is done.
Steps to Bottom Balance the Battery
- Discharge all the cells of the LiFePO4 battery to zero capacity. The zero capacity comes at 2.5V. The current output at this voltage level is zero.
- Disconnect the Battery Management System from the battery pack.
- Connect all the cells in series. Double-check to see that you have the polarity of the cells right.
- Start the battery charging with your li-ion battery charger.
- Monitor the cell voltage of each cell. Whenever any one of the cell reaches 3.65V, disconnect the battery charger.
- Note down the voltage reading of the other cells at this point. This is the maximum voltage For charging your battery.
- Connect the BMS to the battery. Turn off the BMS balancing feature or it will nil the effect of the bottom balance.
What are the Causes of Cell Imbalance?
Many factors contribute towards cell imbalance. Some of these factors are inherent to every battery and unavoidable. Others can be avoided by following healthy habits for using the battery. Here are the causes of cell imbalance:
No two products are identical, regardless of how precise the manufacturing processes are. This is because the intricacies on a microscopic level are outside human control. This is just as true for LiFePO4 batteries. There are minute inconsistencies in the battery cells even when they are new. These consistencies are amplified when the battery goes through its cycle life.
Manufacturing errors cannot be completely avoided. However, it is possible to ensure that there is a very low possibility that these errors will affect you. This can be done by buying the LiFePO4 battery from a reputed brand. Eco Tree Lithium is one of the most reliable manufacturers in this regard.
Overcharging a battery pack is harmful to any battery. Overcharging occurs when the battery is left in the charging stage even after it is fully charged. Overcharge causes unwanted chemical reactions in the cell. These reactions are uncontrolled and cause varied deterioration in the cells. This causes cells to become unbalanced. The damage due to overcharging is irreversible. However, the cells can be balanced to avoid future overcharging and optimize leftover capacity.
Over-discharging similarly affects the cells as overcharging. Overcharging occurs when the cells are drained beyond their rated capacity. Over-discharging causes irreversible damage to the cells. These damages are unequal and result in cell imbalance.
It is common for cell imbalances to occur as the battery goes through its lifespan. Usage wear is more visible near the end of the battery life. Usage wear affects each cell differently. This causes cell imbalance. Balancing the battery can prolong its life and limit usage wear.
Equipment for LiFePO4 Battery Balancing
Many devices can be used for balancing a LiFePO4 battery. These devices are:
- Battery Management System (BMS): BMS is created to ensure the safety of the battery. One of the safety measures is the cell balancing function. A BMS ensures balanced cells during charging and discharging. Not every battery comes with a BMS with a balancing function. Fortunately, Eco Tree Lithium batteries come with it preinstalled.
- Battery Charger with Balancing Feature: Advanced Li-ion battery chargers are available with the balancing feature inbuilt. These chargers can be useful for ensuring that the battery cells are balanced whenever you charge them. However, they do not ensure battery balancing during discharge.
- Protective Circuit Module (PCM): PCM can help balance the battery cells. However, it is more of a protective measure than a corrective one. It works by disconnecting the voltage supply whenever it reaches unsafe levels.
- Balancing Circuit: A balancing circuit is a simple tool for getting balanced cells. However, it has no protective measures for overcharge or discharge.
BMS is unarguably the best way to achieve battery balancing along with many protective measures. It also displays all the important data about the battery parameters.
Balancing With a Battery Management System
A BMS is the best way to ensure the balancing of a LiFePO4 battery. The other balancing equipment can balance the battery. However, the BMS does the job of battery balancing and protecting the battery from the elements that cause imbalance.
For instance, Eco Tree Lithium batteries come with an integrated BMS. This BMS has multiple protection levels along with cell balancing feature. Once the cell is balanced, the overcharge and over-discharge protection eliminate additional imbalance in the cell. There are also thermal sensors to eliminate the possibility of a thermal runaway.
This system also collects and reports all the battery data. This i useful for owners who want to micromanage the battery functioning or optimize the battery usage characteristics.
How Long Does It Take to Balance a LiFePO4 Battery?
It can take up to a week or more to balance a LiFePO4 battery. The balancing time depends on the balancing method, the capacity of cells, the number of cells, and the balancing current. For instance, when top balancing 4 cells of 150 Ah each with a 10 A supply, the time will be:
Time = Number of Cells x Ampere Hour Rating ÷ Balancing Current
= 4 x 150 ÷ 10
= 60 hours
Even if you spend 15 hours each day balancing, it means a time period of 4 days. This time is significantly increased as the cell Ah rating is increased.
This is a long time for most users. Therefore, BMS is the preferred battery balancing method. It balances the battery whenever the battery is being used.
LiFePO4 Battery Balancing- Things to Remember
Here are some of the things you should keep in mind when balancing a LiFePO4 battery:
- Make sure the connections are tight. Loose connections can cause sparking and fire.
- Check the polarity of the cells, whether you are connecting them in series or parallel.
- Balancing is a long time so it is okay to leave the battery unattended for intervals. However, you should monitor the battery continuously when the cell voltage is near 3.5V.
- The balancing system should be turned off at night since you cannot regularly check on it.
- Turn off the balancing system if there are unexpected occurrences during the balancing process.
How to Maintain a LiFePO4 Battery and Keep It Balanced?
Keeping a LiFePO4 battery balanced is easy if you follow a proper maintenance protocol. Maintaining a LiFePO4 battery is a very easy task. Following the maintenance steps for LiFePO4 battery will prolong the life of your battery and keep the cell balance intact in the process.
LiFePO4 battery balancing can seem like a daunting task. However, with the right battery and BMS, it is quite easy. Eco Tree Lithium batteries have management systems that can handle your cell balancing requirements. Therefore, you won’t have to spend days working on keeping your battery the right way.