Week 8 Multi cell Battery Pack
A)1.In multi-cell 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.
2.The more cells used, the greater the opportunities to fail and the worse the reliability.
3.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.
4.The problems can be compounded if parallel packs of cells are required to achieve the desired capacity or power levels.
5.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.
6.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.
7.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.
8.The result is temperature and pressure build up and possible damage to the cell.
9.During discharging, the weakest cell will have the greatest depth of discharge and will tend to fail before the others.
10.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 also resulting in early failure of the cell.
11.With every charge ,discharge cycle the weaker cells will get weaker until the battery fails.
12.Various methods of cell balancing have been developed to address this problem by equalising the stress on the cells.
solution: Cell balancing
Self Balancing method:
1.Unbalanced ageing is less of a problem with parallel chains which tend to be self balancing since the parallel connection holds all the cells at the same voltage and at the same time allows charge to move between cells whether or not an external voltage is applied.
2There can however be problems with this cell configuration if a short circuit occurs in one of the cells since the rest of the parallel cells will discharge through the failed cell.
3.The problems caused by these cell to cell differences are exaggerated when the cells are subject to the rapid charge and discharge cycles.
4.Once a cell has failed, the entire battery must be replaced and the consequences are extremely costly.
5.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.
6.Equalisation is intended to prevent large long term unbalance rather than small short term deviations.
Cell selection
1.The first approach to solving this problem should be to avoid it if possible through cell selection.
2.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.
Large versus small cells
1.The high energy storage capacities needed for traction and other high power battery applications can be provided by using large high capacity cells or with large numbers of small cells connected in parallel to give the same capacity as the larger cells.
2.In both cases the large cells, or the parallel blocks of small cells, must be connected in series to provide the required high battery voltage.
3.Using large cells keeps the interconnections between cells to a minimum allowing simpler monitoring and control electronics and lower assembly costs.
4.When the cells are used in a serial chain, cell balancing is essential to equalise the stress on the cells, caused by these manufacturing variances, to avoid premature cell failures.
5.Using small cells connected in parallel to provide the same voltage and capacity as the larger cells results in many more interconnections, greater assembly costs and possibly more complex control electronics
6.When large numbers of cells are connected in a parallel block, the performance of the block will tend towards the process average of the component cells and the self cell balancing efect will tend to keep it there.
7.The parallel blocks will still need to be connected in series to provide the higher battery voltage but the tolerance spread of the blocks in the series chain will be less than the tolerance spread of the alternative large capacity cells, leaving the cell balancing function with less work to do.
Pack construction
1.Another important avoidance action is to ensure at all times an even temperature distribution across all cells in the battery.
2. 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.
3.The temperature across the battery pack should be the first considered in order to minimise the need for cell balancing.
Cell equalisation
1.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.
2.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.
3. 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.
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
1.Active cell balancing methods remove charge from one or more high cells and deliver the charge to one or more low cells.
2.Since it is impractical to provide independent charging for all the individual cells simultaneously, the balancing charge must be applied sequentially.
3.Taking into account the charging times for each cell, the equalisation process is also very time consuming with charging times measured in hours.
4.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.
a)Charge Shuttle (Flying Capacitor) Charge Distribution
With this method a capacitor is switched sequentially across each cell in the series chain. The capacitor averages the charge level on the cells by picking up charge from the cells with higher than average voltage and dumping the charge into cells with lower than average voltage. Alternatively the process can be speeded up by programming the capacitor to repeatedly transfer charge from the highest voltage cell to the lowest voltage cell. Efficiency is reduced as the cell voltage differences are reduced. The method is fairly complex with expensive electronics.
b)Inductive Shuttle Charge Distribution
This method uses a transformer with its primary winding connected across the battery and a secondary winding which can be switched across individual cells. It is used to take pulses of energy as required from the full battery, rather than small charge differences from a single cell, to top up the remaining cells. It averages the charge level as with the Flying Capacitor but avoids the problem of small voltage differences in cell voltage and is consequently much faster. This system obviously needs well balanced secondary transformer windings otherwise it will contribute to the problem.
Passive balancing
1.Dissipative techniques find the cells with the highest charge in the pack, indicated by the higher cell voltage, and remove excess energy through a bypass resistor until the voltage or charge matches the voltage on the weaker cells.
2.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.
3.Others 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.
4.This method levels downwards and because it uses low bypass currents, equalisation times are very long. Pack performance determined by the weakest cell and is lossy due to wasted energy in the bypass resistors which could drain the battery if operated continuously. It is however the lowest cost option.
Charge Shunting
The voltage on all cells levelled upwards to the rated voltage of a good cell. Once the rated voltage on a cell has been reached, the full current bypasses fully charged cells until the weaker cells reach full voltage. This is fast and allows maximum energy storage however it needs expensive high current switches and high power dissipating resistors.
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.
Lossless balancing
1.Recent developments have produced a superior way of cell balancing by means of software control which is both simpler and lossless and avoids the different problems
2.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
3.The simplest of these methods uses the cell voltage as an indication of the state of charge.
4.The main advantage of this method is that it prevents overcharging of individual cells, however it can be prone to error.
5.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.
6.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.
Redox Shuttle (Chemical Cell Balancing)
1.In Lead acid batteries, overcharging causes gassing which coincidentally balances the cells.
2.The Redox Shuttle is an attempt to provide chemical overcharge protection in Lithium cells using an equivalent method thus avoiding the need for electronic cell balancing.
3.A chemical additive which undergoes reversible chemical action absorbing excess charge above a preset voltage is added to the electrolyte .
4.The chemical reaction is reversed as voltage falls below the preset level.