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Large Li-ion battery packs: Active balancing improves many parameters - Part 1
Author: Jack Marcinkowski
Source From: EETimes
Posted Date: 2010-11-30
The electrification of automobiles has finally reached the tipping point as indicated by the rapid growth of the number of hybrid-electric (HEV) and electric vehicles (EV) entering or planned to be introduced to the market.
The major hurdle to the success of the EVs is the battery. Recent progress in Li-ion battery technology increased the power and energy density of battery cells and reduced their cost. In order to mitigate the “range anxiety” plaguing early EVs and being the main concern of potential customers, further improvements are needed.
The Battery pack is one of the most expensive and potentially least reliable components of the EV. high performance Battery Management System (BMS) is the answer to the weaknesses of HEV and EV battery packs.
As stated by the design team of the Chevy Volt - “Through the course of development, the team has learned that the battery management system is the key to extracting the best life and performance out of the battery.” (http://green.autoblog.com/2009/11/17/gm-provides-update-on-volt-vehicle-and-battery-development/)
Challenges and the passive balancing solution
The first challenge the designers of battery pack management systems are facing is charging of large strings of cells and avoiding overcharging of individual cells. The Li-Ion cells are sensitive to overvoltage condition which causes degradation of the cell performance leading to its destruction. The cells are inherently different in terms of their parameters. In addition, they may contain different residual charge before recharging. In result, some cells reach their maximum voltage sooner than others. This leads to some cells reaching over-voltage condition and getting destroyed.
In order to allow charging of all cells to their full state of charge (SOC) a method of diverting the current through a bypass resistor has been introduced. This method known as passive balancing prevents overcharging of the cells by dissipating the excessive energy in resistors. Power dissipation limits the amount of current that can be diverted from the cell.
The discharge of the pack must stop when the “weakest” cell is depleted. This results in significant waste of energy which cannot be used but is still accumulated in the “stronger” cells of the battery. This is the main reason for the uncertainty of the driving range and the “range anxiety” - one of the main consumer concerns affecting the market adoption of EVs.
Passive balancing cannot help during discharge. A different solution is needed.
Even the perfectly balanced cells contain different amounts of charge. This phenomenon is known as capacity mismatch. Even the cells with equal initial capacity may have different effective capacity as a result of larger internal losses. The cells have inherently different parameters already while coming off the manufacturing line. Cell and battery pack manufacturers often screen and select the cells for best match before installing them in a battery pack. This results in additional cost due to the testing time and cell rejects. As the cells age they lose capacity and their parameters diverge further. Uneven ageing of cells is typically caused by temperature gradients within the battery pack. Thermal management is very critical and expensive.
The “weak” cells with lower effective capacity are being exercised the hardest. They are always discharged deeper, to a lower SOC. In result, they age faster and loose more capacity with time. Their life-time is shorter and so is the life-time of the entire battery pack.
The active balancing solution
Active balancing is the answer to the challenges posed by the Li-Ion battery packs. Instead of bypassing the cells and dissipating power, the active balancing system transfers charge between cells by the means of DCDC converters. The charge can be transferred during charging, discharging or idle state. The cells can always be kept balanced. Unlike in the case of passive balancing, high balancing currents are possible because the transfer of charge is very efficient. The cells are being equalized faster and higher charging current rates are possible.
During the idle state, even the perfectly balanced cells lose charge at a different rate due to different internal leakage resulting from temperature gradients. The self-discharge rate doubles with every 10oC rise in cell temperature. The active balancing allows rebalancing of the cells during idle state. Keeping the cells in balance is critical to being able to utilize all the energy stored in the battery pack.
Figure 1. Comparison of active and passive balancing methods. For full resolution, click here.
Figure 1 illustrates the advantages of active cell balancing by comparing balancing of cells with different capacities. With passive balancing, the total amount of energy equal to the difference between the maximum cell capacity and the lower cells capacity is dissipated during charging.
The cells can only be discharged to the level determined by the cell with the lowest capacity resulting in unusable energy left over in the remaining cells. The effective charge capacity (SOC) of the battery pack is reduced.
Active balancing method allows cells with different capacity to be charged to their full SOC with minimum power loss thanks to high efficiency of the power converters. The charge that would be dissipated during passive balancing is being transferred to the cells with higher capacity. Similarly, during discharge, all cells can be fully discharged with no residual charge left in the battery pack because the energy from cells with larger capacity can be redistributed to the cells with lower capacity. The effective SOC of actively balanced battery pack is larger than that of the passively balanced pack.
The performance of active balancing system depends on the amount of balancing current relative to the charge and discharge rate of the battery pack. The higher the cell imbalance and the higher the charge or discharge rate, the more balancing current is required. Figure 2 shows the amount of cell capacity mismatch that could be compensated during charging or discharging with an active BMS assuming constant balancing current.
Figure 2. Capacity mismatch compensation with active balancing
Charging of battery packs consisting of tens and in some cases thousands of cells w/o overcharging a single cell while balancing all cells during charging, idle and discharging states and mitigating the effects of cell charge and capacity mismatch requires state-of-the-art active BMS.
Active BMS lowers the overall cost of battery pack by eliminating the need to oversize the pack to account for the effects of cell-to-cell parameters variation and uneven ageing. It plays a crucial role when the old cells are replaced with new ones and the pack becomes mismatched. It allows using cells with larger spread in the parameters what results in increased manufacturing yield. Cost of warranty and maintenance is also reduced. The active BMS enhances performance, reliability and safety of the battery pack while helping to reduce the cost.
Active BMS is a key enabler of the HEV and EV revolution.
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