Cell Balancing
In multicell batteries, because of the larger number of cells used, we can expect that they will be subject to a higher failure rate than single cell batteries. The more cells used, the greater the opportunities to fail and the worse the reliability.
Batteries such as those used for EV and HEV applications are made up from long strings of cells in series in order to achieve higher operating voltages of 200 to 300 Volts or more are particularly vulnerable. The problems can be compounded if parallel packs of cells are required to achieve the desired capacity or power levels. With a battery made up from n cells, the failure rate for the battery will be n times the failure rate of the individual cells.
All cells are not created equal
The potential failure rate is even worse than this however due to the possibility of interactions between the cells. Because of production tolerances, uneven temperature distribution and differences in the ageing characteristics of particular cells it is possible that individual cells in a series chain could become overstressed leading to premature failure of the cell. During the charging cycle, if there is a degraded cell in the chain with a diminished capacity, there is a danger that once it has reached its full charge it will be subject to overcharging until the rest of the cells in the chain reach their full charge. The result is temperature and pressure build up and possible damage to the cell. During discharging, it is even possible for the voltage on the weaker cells to be reversed as they become fully discharged before the rest of the cells resulting in failure of the cell.
Balancing is less of a problem with parallel chains which tend to be self balancing.
See Interactions Between Cells for more details.
The problems caused by these cell to cell differences are exaggerated when the cells are subject to the rapid charge and discharge cycles (microcycles) found in HEV applications.
While Lithium batteries are more tolerant of micro cycles they are less tolerant of the problems caused by cell to cell differences.
Because Lead acid and NiMH cells can withstand a level of over-voltage without sustaining permanent damage, a degree of cell balancing or charge equalisation can occur naturally with these technologies simply by prolonging the charging time since the fully charged cells will release energy by gassing until the weaker cells reach their full charge. This is not possible with Lithium cells which can not tolerate over-voltages. Although the problem is reduced with Lead acid NiMH batteries and some other cell chemistries, it is not completely eliminated and solutions must be found for most multicell applications.
Once a cell has failed, the entire battery must be replaced and the consequences are extremely costly. Replacing individual failed cells does not solve the problem since the characteristics of a fresh cell would be quite different from the aged cells in the chain and failure would soon occur once more. Some degree of refurbishment is possible by cannibalising batteries of similar age and usage but it can never achieve the level of cell matching and reliability possible with new cells.
Cell selection
The first approach to solving this problem should be to avoid it if possible through cell selection. Batteries should be constructed from matched cells, preferably from the same manufacturing batch. Testing can be employed to classify and select cells into groups with tighter tolerance spreads to minimise variability within groups.
Pack construction
Another important avoidance action is to ensure at all times an even temperature distribution across all cells in the battery. Note that in an EV or HEV passenger car application, the ambient temperature in the engine compartment, the passenger compartment and the boot or trunk can be significantly different and dispersing the cells throughout the vehicle to spread the mechanical load can give rise to unbalanced thermal operating conditions. On the other hand, if the cells are concentrated in one large block, the outer cells in contact with ambient air may run cooler than the inner cells which are surrounded by warmer cells unless steps are taken to provide an air (or other coolant) flow to remove heat from the hotter cells. See also Thermal Management
Cell equalisation
To provide a dynamic solution to this problem which takes into account the ageing and operating conditions of the cells, the BMS may incorporate a Cell Balancing scheme to prevent individual cells from becoming overstressed. These systems monitor the State of Charge (SOC) of each cell, or for less critical, low cost applications, simply the voltage across, each cell in the chain. Switching circuits then control the charge applied to each individual cell in the chain during the charging process to equalise the charge on all the cells in the pack. In automotive applications the system must be designed to cope with the repetitive high energy charging pulses such as those from regenerative braking as well as the normal trickle charging process.
Several Cell Balancing schemes have been proposed and there are trade-offs between the charging times, efficiency losses and the cost of components.
Active balancing
Active cell balancing methods remove charge from one or more high cells and deliver the charge to one or more low cells. Since it is impractical to provide independent charging for all the individual cells simultaneously, the balancing charge must be applied sequentially. Taking into account the charging times for each cell, the equalisation process is also very time consuming with charging times measured in hours. Some active cell balancing schemes are designed to halt the charging of the fully charged cells and continue charging the weaker cells till they reach full charge thus maximising the battery's charge capacity.
Passive balancing
Dissipative techniques find the
high cells in the pack, and remove excess energy through
a resistive element until their charges match the low cells. Some passive balancing schemes stop charging altogether when the first cell is fully charged, then discharge the fully charged cells into a load until they reach the same charge level as the weaker cells. Other schemes are designed continue charging till all the cells are fully charged but to limit the voltage which can be applied to individual cells and to bypass the cells when this voltage has been reached.
Charge limiting
A crude way of protecting the battery from the effects of cell imbalances is to simply switch off the charger when the first cell reaches the voltage which represents its fully charged state (4.2 Volts for most Lithium cells) and to disconnect the battery when the lowest cell voltage reaches its cut off point of 2 Volts during discharging. This will unfortunately terminate the charging before all of the cells have reached their full charge or cut off the power prematurely during discharge leaving unused capacity in the good cells. It thus reduces the effective capacity of the battery. Without the benefits of cell balancing, cycle life could also be reduced, however for well matched cells operating in an even temperature environment, the effect of these compromises could be acceptable.
All of these balancing techniques depend on being able to determine the state of charge of the individual cells in the chain. Several methods for determining the state of charge are described on the SOC page.
The simplest of these methods uses the cell voltage as an indication of the state of charge. The main advantage of this method is that it prevents overcharging of individual cells, however it can be prone to error. A cell may reach its cut off voltage before the others in the chain, not because it is fully charged but because its internal impedance is higher than the other cells. In this case the cell will actually have a lower charge than the other cells. It will thus be subject to greater stress during discharge and repeated cycling will eventually provoke failure of the cell.
More precise methods use Coulomb counting and take account of the temperature and age of the cell as well as the cell voltage.
For batteries with less than 10 cells, where low initial cost is the main objective, or where the cost of replacing a failed battery is not considered prohibitive, cell balancing is sometimes dispensed with altogether and long cycle life is achieved by restricting the permitted DOD. This avoids the cost and complexity of the cell balancing electronics but the trade off is inefficient use of cell capacity.
Whether or not the battery employs cell balancing, it should always incorporate fail safe cell protection circuits.
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