By Spyros Loukakis
Α Li-on cell works at about 4, 2V, but in the applications where are used these cells such as EV, portable electronics, laptops, Power Banks etc. a much higher voltage than the nominal voltage is required.
That’s why the Battery Pack designers combine more than one cell in a series to form a higher voltage value battery. When the batteries are connected in series, the voltage increases. For example, when 4 cells Li-on 4.2 V are connected in series, the output voltage of the resulting battery will be 16.8 V.
Connecting multiple cells in a row is like putting a lot of horses on a chariot. Only if all the horses run at the same speed the chariot will be driven with maximum efficiency. Of the four horses if a horse runs slowly, then the other three must also reduce their speed, thus reducing efficiency and if a horse runs faster, it will eventually damage itself by pulling the load of the other three horses. Likewise, when four cells are connected in series, the voltage values of all elements should be equal for the operation of the battery pack at peak performance. The method of maintaining equality of trends of all elements is called Cell Balancing.
“A cell balancing would be mandatory to utilize the battery pack to its maximum efficiency”
Why we need Cell Balancing
The Cell Balancing is a technique in which the voltage levels of each individual cell connected in series to form a battery pack are retained to be equal in order to achieve maximum performance of the pack. When different cells are combined together to form a battery pack, we must be sure that they have the same voltage value. But when the batter pack is charging and discharging, the voltage values of the individual cells tend to vary due to various reasons.
This variation in voltage levels causes the cell unbalancing to result in one of the following problems:
The worst thing that can happen is the thermal runaway. The Li-on components are very sensitive to overcharging and over discharging. In a pack of 4 cells if one has a voltage of 3, 6V while the other are 3, 1V the charger will charge all the cells together since they are in series and will be charged this with the voltage of 3.6 V more than the recommended voltage since the other components are still in a state that Charge.
When the Li-on cell overcharges, even slightly above the recommended value, its performance and lifecycle decreases. For example, a slight increase in charging voltage from 4.2 V to 4.25 V will degrade the cell faster by 30%. So if Cell Balancing is not accurate, even too little overload will reduce battery life time.
Incomplete charging of battery pack
As the battery pack get older, some cells become weaker. These cells will be a huge problem, as they will charge and discharge faster than a healthy cell. When charging a battery pack, the charging process should stop when even one cell reaches the maximum voltage. In this way, if, for example, two cells of a battery pack become weak, they will be charged faster and the remaining cells will not be charged to the maximum.
Incomplete utilization of Pack energy
Similarly, when the battery pack is being discharged, the weaker cells will be discharged faster than the healthy cells and will reach the minimum voltage faster than the other cells. The battery pack will be disconnected from charging, even if an cell reaches the minimum voltage. This leads to the inadequate use of the energy of the package.
Accounting all the above possible disadvantages in consideration, we can conclude that a cell balancing would be mandatory to utilize the battery pack to its maximum efficiency.
A battery pack that has been used, its cells tend to lose their initial equilibrium state for the following reasons:
SOC imbalance (State Of Charge)
Measuring the SOC of a cell is complicated. As a result it is very complicated to measure the SOC of individual cells in a battery pack. An ideal Cell Balancing technique should match cells of the same SOC instead of cells with the same voltage levels (OCV). And because it is practically impossible for the data to be of the same SOC, the imbalance of SOC leads over time to a change in OCV.
Internal resistance variation
is very difficult to find cells with the same internal resistance (Internal Resistance). IR determines the current flowing through a cell. Since the IR is varied, and the voltage getting the cell also varies.
The charging and discharging capacity of the cells depends on the ambient temperature. In a large battery pack, the cells that form it do not have the same ambient temperature, resulting in a cell being charged or discharged faster than the rest causing an imbalance.
From the above it is clear that we cannot prevent the cells of a battery pack from being unbalanced if we do not interfere with an external system that causes the cells to be Balanced.
There are many different techniques (Hardware and Software) used for Cell Balancing which are classified in the following 4 categories:
- Passive Cell Balancing
- Active Cell Balancing
- Lossless Cell Balancing
- Redox Shuttle
In a following article we will explain in detail the above ways Cell Balancing.