Week 8 Multi cell Battery Pack

Week 8 Challenge - Multi Cell Battery Pack

Q) How weakest cell limits the usable capacity of the battery pack? What is the solution?

ANS:

Theory

The individual cells in a battery pack may have different capacities and hence different relative states of charge (SOCs) and this can be due to a variety of reasons e.g. manufacturing variances, assembly variances and usage history. The weakest cell of a battery pack is the one that 'rules' the battery pack. This is because the weakest cell imposes a limit on the overall operation of the battery pack and so it can be considered as the weakest link. This battery can be said as being 'as strong as the weakest link'. When charging a battery pack, theoretically all the cells should get charged with the same current and voltage and should get charged at the same rate thereby all getting fully charged at the same time. The same is assumed during the discharge mode in that all the cells get discharged with the same current and voltage at the same rate and therefore will all get fully discharged at the same time. In addition to this, we also assume that the temperatures of all the cells in the battery pack are the same respective of the charging or discharging mode. However in reality, this doesn't have to be the case. There are instances when an individual cell or cells gets charged or discharged at a different rate from the other cells and this can cause them to exceed the safe operating limits. For example, a cell may get charged quicker than the other cells in the pack and may reach the over voltage limit and get over charged which is not advisable. The reason for this is twofold. The first is that there is a safety hazard that comes along with a lithium ion battery operating outside of its safety limits. If the voltage of a lithium ion battery exceeds the full charge voltage by even a few millivolts, the battery can explode or catch fire due to the thermal runaway effect. The second reason is that putting the cell outside its safe operating limits causes the degradation of the its usable capacity over time and accelerates its ageing affect. When this occurs, the shortcomings of the individual cell(s) causes the battery pack to be imbalanced i.e. unsynchronized.

The solution for cell imbalance is cell balancing. A balanced battery is one in which all the cells have the same state of charge (SOC) at some point in their charge / discharge cycle. Cell balancing is defined as the process of equalizing the voltage and state of charge of all the cells in a battery when they are fully charged. Practically, no two cells have the same voltage or SOC when charging or discharging and this occurs with cells that are of the same electrochemistry or same manufacturer. These small discrepancies can cause a divergence in the voltage over time. Battery chargers nowadays detect whether a battery is fully charged from checking if the voltage of the cells in the pack have all reached the voltage regulation point. The allowable limit of the variation in the cell voltage is up to the point that the voltage do not exceed their safety limits. However, the weaker cells i.e. the cells with lower capacities, tend to have higher voltages across them than compared to the other cells connected in series when they are fully charged. The higher voltages across the weaker cells exceed the over voltage limit and this causes further degradation of their capacity and performance. If the maximum allowable voltage of a cell is exceeded by as small as 10%, the rate of degradation of that cell will be increased by approximately 30%. During discharge, weaker cells have lower voltages across them than compared to the other cells connected in series and this can be due to a higher internal resistance or faster rate of discharge (due to reduced capacity). So, if any of the weak cells dips below the under voltage limit while the battery pack voltage is still sufficient to run a system, the full capacity of the battery pack will not be utilised as it will stop the discharge of the whole pack.

Cell balancing of the cells in a battery pack is performed by battery management systems (BMS) implementing a cell balancing network. A BMS will use its features of monitoring, computing, protecting, communicating and optimizing to perform cell balancing to maximize the life of the cells in the battery pack. The cell balancing network will use the necessary electrical and electronic components to control the flow of current in the circuits depending on the situation and cell requirements. Cells in a battery pack are considered to be balanced when two conditions are met. The first condition is that if cells have the same capacity, then they are balanced when they have the same relative SOC. The second condition is that if the cells have different capacities, they are still considered balanced when they have the same relative SOC. In order to maintain cells wih the different capacities at the same SOC, cell balancing must provide differential amounts of current to the series connected cells in charging / discharging.

Components

In order to perform cell balancing, the battery pack needs certain components. The battery pack needs to be able to perform the following tasks:

• Accurately monitor the voltage of each cell - this will require differential amplifiers that can operate on a supply of atleast 20V and have an input range between 2V and 4.5V and an accuracy of atleast 10mV.
• Determine the cell with the highest voltage and compute the voltage differential between all the cells - this will require hardware comparators and an algorithm which uses an analog to digital converter.
• Make decisions from the individual cell voltage and voltage differential data and be able to initiate the cell balancing process - this will require a microcontroller.
• Control the cell balancing FETs.

Methods

There are two main categories of cell balancing that are most commonly used. They are active cell balancing and passive cell balancing.

Passive Cell Balancing

This is the type of cell balancing involves discharging the over charged cells via dissipation through a dissipative bypass route. This bypass route may be integrated with the IC or may be externally connected to it. This method is simpler and is preferred in low cost applications. However, the disadvantage of this method is that the all of the excess energy stored in the weak cells is dissipated as heat and so is wasted without being used. As a result, the efficiency of the cell balancing process wil be lower. Also, this method is less preferred during discharge mode because the extra energy is not used and so the running time of the battery pack is affected. This type of cell balancing will require components such as zener diodes, FETs and resistors. Passive cell balancing is only implemented when a cell is charging.

There are a few different schemes for passive balancing.

One method is charge shunting. In this method, the voltage of each of the cells connected in the series arrangement is levelled up to the rated voltage of a good cell. Once a cell has reached the rated voltage, the full current will bypass the fully charged cells and this continues until the weak cells reach the rated voltage. This method is quick and allows for maximum energy storage, but requires high current switches and high power dissipating resistors.

Another method is charge limiting. In this method, the charger is switched off when the first cell in the series arrangement reaches the fully charged voltage during charging. Conversely, the battery should be disconnected when the first cell's voltage goes below the cutoff voltage during discharge. The disadvantage of this method is that it stops charging of the batteries before they have been fully charged which can reduce the capacity of the battery and its life.

Active Cell Balancing

This type of cell balancing involves transporting or 'shunting' the extra energy in an over charged cell to a cell with a lower charge level. The obvious advantage of this method is that none of the extra energy is dissipated as heat energy, and is instead reused by the other cells connected in series in the battery pack. This will increase the overall efficiency of the cell balancing process and also the battery pack. However, the tradeoff is between efficiency and cost as this method is more costly and more complex than the passive method. This type of cell balancing will use components such as capacitors, inductors, switches and maybe even a DC - DC converter.

There are 2 schemes that use active balancing.

The first is charged shuttle charge distribution. In this, a capacitor is switched sequentially across each cell in the series arrangement. The capacitor will average the charge level in each cell by taking the charge from the cells with a higher than average voltage and supplying it to the cells with lower than average voltage. This can be sped up by programming the capacitor to repeatedly transfer charge from the highest voltage cell to the lowest voltage cell.

The second is inductive shuttle charge distribution. In this, a transformer is used and its primary winding is connected to the battery whereas the secondary winding is switched between the different cells in the series arrangement. The transformer will take pulses of energy from the full battery instead of from the cells with higher than average voltage to supply the remaining cells. Like the charged shuttle charge distribution scheme, the charges in each cell will be averaged but the inductive shuttle method avoids small voltage differences between the cells and is faster as well.

Cell Balancing Circuit

The circuit technique used to perform cell balancing is shown with the following circuit diagrams below:

Figure 1

In this figure, assume that the cells 'Cell1' to 'Cell3' are cells connected in series in a battery pack. Each cell has its own resistor and transistor (FET), which are all connected to a battery monitor device. The resistors (R1, R2, R3) and transistors (T1, T2, T3) are assumed to be externally connected to the battery monitor device as opposed to being integrated, even though this is possible. Assume that 'Cell1' needs to be balanced. To perform cell balancing when the battery pack is in the discharge mode (with the charger off), transistors T2 and T3 are switched off whereas T1 is switched on.

Figure 2

With the transistor T1 only on, the circuit in Figure 1 will look like the circuit in Figure 2. The discharge route of Cell 1 through T1 will not take away any current from the other two cells. So transistor T1 will only discharge Cell 1 and similarly, each transistor will only discharge its respective cell. Although, the discharge route is dependent on the load resistance of Cell 1. If the resistance of the load is greater than the combined resistances of the resistor R1 and the transistor T1, the discharge current will flow through transistor T1. However, if the load resistance is lower than the combined resistances of the resistor R1 and transistor T1, the discharge current will flow through the load instead and this will make the cell balancing process inefficient.

Figure 3

From Figure 3, we can see that the equivalent discharge resistance of the 1st circuit (with T1 switched on) is equal to the resistance in the other 2 circuits. The equivalent resistance can be expressed mathematically as:

`R_d = [(R_1 + R_(T1))* R_l]/(R_1 + R_(T1) + R_l)`

where,

Discharge resistance - `R_d`

Resistance of Resistor 1 - `R_1`

Resistance of Transistor 1 - `R_(T1)`

Load Resistance - `R_l`

To decrease the discharge time, a resistor and transistor of low resistances are selected since we cannot control the load resistance. As mentioned before, it is preferred that the load resistance be higher than the sum of the resistances of the resistor and the transistor. This is so that the discharge current will flow through the resistor and transistor instead of through the load.

Figure 4

The above figure shows the flow of current that occurs in cell balancing while charging. The charger will supply a current that is given by the formula below:

`I_(char) = I'_(char) + I_l`

where,

Current delivered by charger - `I_(char)`

Current that charges the Cell - `I'_(char)`

Load current - `I_l`

The current that charges the cell is at its maximum value when the load resistance is disconnected. If the load resistance is connected during charging, then some of the charging current will flow through the load. While balancing Cell 1, the current that charges the cell can be expressed as:

`I'_(char) = I_1 + I_2`

Issues

There are several issues with the cell balancing technique that occur whether it is used in the charging or discharging mode.

The discharging mode issues are:

• Balancing done on a cell when discharging will dissipate power as heat which does not get used
• The time taken to balance a cell while discharging is very long
• Balancing during discharge within a reasonable time period requires an external transistor with a very low resistance, which is possible with FETs, MOSFETs etc.
• Fast balancing requires a low resistance resistor connected in series with the transistor which requires a large wattage device which increases cost

The charging mode issues are:

• Measurement of cell voltage while charging is not that accurate and this can cause premature cell balancing - so the charging will have to be stopped every once in a while to take the voltage reading
• Balancing done while charging also requires an external transistor with low resistance which again requires a large wattage device which increases cost.
• Since the unbalanced cell has low resistance, all the charge current can be shunted to it, causing some current to flow through the other balanced cells as well. Cell balancing should begin at a lower voltage level to give the cells enough time to balance.

Conclusion

Cell balancing is a very important procedure that should be implemented by a cell balancing network that is controlled by a BMS in the battery packs of EVs, laptops etc in order to ensure that the cells are always operating in their safety limits (as mentioned in their specifications). By doing so, the performance and the working life of the cells and hence the battery pack will be maximized. In addition, the battery will be safe to use, without having to worry about the hazards associated with lithium ion batteries. There are benefits and drawbacks that come with the use of the cell balancing method, however their usage is vital to get the maximum usage and hence the maximum efficiency of the batteries and so are necessary.