Cell Balancing and Passive Cell Balancing
What is cell balancing
In a multi-cell lithium-ion battery pack, cells are connected in series to achieve a higher voltage. Some configurations may also have cells connected both in series and in parallel. While production techniques have extremely tight tolerances there are still slight variations in manufacturing and assembly. This is because during construction, we don’t really work on the materials at an atomic level as they impose unreasonable costs.
Therefore, during usage cells can develop differences in their state of charge (SOC) and capacity. This is reflected in their terminal voltage. Some secondary effects of this are differences in discharge capacity, impedance and in internal temperature. This imbalance can lead to reduced overall battery performance, safety risks, and shortened lifespan.
To mitigate this, packs with cells in series or series-parallel configurations use a technique called cell balancing. This helps equalize the voltage between the different constituent cells in the pack.
Figure 1 shows an illustration of how cell balancing results in equalized potential across multiple cells in the same battery stack.
Advantages of cell balancing
Cell balancing helps in the following ways
Improve battery performance
A balanced battery pack can deliver consistent power output and longer runtime. This is because if the cell voltages across the stack are very unequal, then the utilization of the pack by the load is limited by the performance of the weakest cell. The weakest cell reaches its minimum voltage the quickest by depleting its capacity sooner than the remaining cells. Therefore, the usable capacity will be limited by that cell. Cell balancing mitigates this by keeping the cells within a tight range.
Enhance battery life
By preventing overcharging or over-discharging of individual cells, cell balancing extends the overall lifespan of the battery pack. If the cell voltages are not equal both charging and discharging present challenges
Charging: During charging voltages deltas cause weak cells to reach the max charging voltage faster than the rest of the pack. Due to this, it is possible to overcharge such cells. Overcharging can cause chemical and physical changes in the cells that increase the cell resistance and reduce the battery lifespan.
Discharging: During discharge, the weak cells in a stack reach the minimum manufacturer recommended voltage the quickest. Once it drops below the manufacturer's recommended minimum voltage chemical processes will accelerate the degradation of the cell again affecting lifespan.
Reduce safety risks
Some secondary effects of unbalanced cells are due to overcharging and overdischarging.
Overcharging increases can lead to thermal runaway, a dangerous condition where excessive heat can cause a battery to catch fire or explode. This is because overcharging causes Lithium ion deposition on the anode where they form needle like structures called dendrites. As they grow, if they pierce the separator, they can cause internal shorts. These internal shorts result in uncontrollable internal cell currents that generate substantial heat. This becomes an exothermic self-sustaining reaction that can cause increased internal pressures, fire and in some cases an explosion.
Overdischarging damages the SEI layer, which is a thin interface that develops on the anode of the Li-ion cell. The reason that Li-ion batteries are rechargeable is because the movement of Li-ions is reversible with high coulombic efficiency. SEI damage causes increased resistance affecting the coulombic efficiency. Overdischarging can decompose the electrolyte within a cell, this directly causes a loss of charge carriers and reduces the capacity of the cell.
For these reasons it is imperative that overcharging and overdischarging are avoided.
Fortunately, there are protective circuits that can prevent both these conditions. So, we can now turn our attention to cell balancing.
Methods used for cell balancing
There are 2 methods that are used for cell balancing. They are passive and active cell balancing.
Active cell balancing
In active cell balancing, energy from cells that are at a higher potential is moved to cells at a lower potential. This is done using switching circuits that use FETs along with inductors or capacitors to move the energy between the cells. Some of the architectures are bidirectional buck-boost, direct transformer based and switched-matrix transformer based. These circuits are highly efficient and help to better use the battery energy as they reduce wastage by directly dissipating the energy to ground as is done in passive balancing. The decision of which active balancing architecture is used depends on reliability, efficiency and cost factors. I will discuss this in a follow-up article.
Passive cell balancing
Passive cell balancing does not switch or move energy between cells at different potentials. It equalizes the cells by burning off the excess energy from cells at potentials higher than the cell with the lowest energy in the stack. This is done by using a balancing or a bleed-off resistor that ties the positive terminal of the cell to be balanced to a FET or switch that bleeds the current to the ground.
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Figure 2. illustrates a simple passive balancing circuit. In this R1, R2, and R3 form a network of passive balancing resistors that are used to limit the balancing current across cell1, cell2 and cell3. The switches in the circuit S1 to S3 are usually present in the controller or the BMS that is connected to the cells. They are typically MOSFETs that can be switched on or off depending on the logic used by the controller.
Figure 3 shows how this cell balancing circuit works. When the BMS controller determines that cell 1 has exceeded a certain threshold in comparison to the reference level, then it turns on an internal MOSFET that is shown by S1 in the circuit. This reference can either be the voltage delta or the state of charge that is determined by the capacity measurement internally computed by the BMS.
Now the cell current is determined by the voltage across cell 1 and the value of the resistor R1. If the switch is an internal MOSFET that has an RDSON then that value is also added to R1 to determine the series resistance seen in the circuit. If the BMS uses voltage to determine the balancing threshold, then the cell voltage is monitored continuously until the delta between Cell 1 and the reference voltage falls below a minimum threshold. If charge is used, then the amount of charge that is to be balanced is then calculated using the duty cycle of the switch. Cell balancing times are typically indicated in s/mAh or seconds per milliamp hour.
Limitations
Efficiency
Passive balancing bleeds off energy from cells at a higher potential. This means that the energy in the circuit is dissipated as heat instead of being transferred to other cells at lower potential. Therefore, it reduces the efficiency of the battery pack.
Current Limits
Passive balancing uses both resistors and FETs to bleed current. The balancing current is limited by the resistor value and its power dissipation capability. In most passive balancers the switch that controls the balancing is typically a FET that’s internal to the BMS controller. This is because passive balancing makes sense when the energy levels are low. Since the cell taps on the controller are basically sense inputs rather than the power path for load currents, the FETs are sized for switching efficiency and low power. This means that they cannot be safely operated at high balancing currents without increasing the package heat or without the possibility of an excursion from their Safe Operating Region (SOA). So, passive balancing currents are usually limited to low values.
Speed, Heat and Accuracy
Since balancing current is limited to low values, cell balancing speed is also reduced.
In addition, cell balancing resistors also have I2R losses that increase the heat in the battery pack that can have negative effects on battery performance especially in high temperature environments.
Finally, in dynamic loads with intermittent charging or with low power chargers, cells may not be in OCV conditions. This means that cell voltages might not be indicative of the actual state of charge or depth of discharge of the cells. Using voltage thresholds for balancing in such cases can cause errors in estimation by over-discharging or underestimating the level of discharge needed.
Other considerations – when to balance cells
Now that we have seen how cell balancing can be advantageous to the performance of a multi-cell system, designers and users may want to think about when to balance cells. BMS systems can typically be in charging, rest or relaxation or in discharging states.
Normally, it is beneficial to use passive cell balancing during charging. This is because when the pack is connected to a charger and actively receiving energy, the energy loss due to balancing is less small when compared to the absolute energy received by the pack. Secondly, in charging both the ?BMS on the pack and the charger exercise control over the charging current. It is safer in comparison to balancing during discharge.
Passive balancing can also be done during periods of rest or when the battery has relaxed into a steady state OCV. In applications that have long rest periods, when a battery pack reaches its OCV state, the state of charge of each individual cell in the pack can be determined more accurately. Therefore, the threshold of charge to be dissipated for balancing is also more accurately found. So, when state of charge or depth of discharge is used as a reference, it may present a more efficient method to balance the pack.
Passive balancing is usually not recommended during discharge. This is because many BMS systems do not control discharge unless an overcurrent or short circuit is detected in discharge. Also, the demands of a load might be unpredictable, therefore loss of capacity during discharge is not preferred.
In the next article, I will discuss active cell balancing and how that is implemented in different high power BMS applications.
For any questions, comments, or opportunities, please contact me on LinkedIn.
My previous articles can be found here
Part 1: https://lnkd.in/g7b2zM9d
Part 2: https://lnkd.in/guFvSnYQ
Part 3: https://lnkd.in/gJihbyW4
Other articles on BMS authored by me on EEPower: https://lnkd.in/g4u_HzTt
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