The Purpose of Cell Balancing for Battery Packs
By Anton Beck, Battery Product Manager
Epec Engineered Technologies
Battery packs can have anywhere from 1 to hundreds or more cells to power equipment or applications. Energy is pulled from one cell to the next in the series. When a battery reaches its lowest allowable discharge, recharging begins.
However, each cell in the battery pack will have a different state of charge, as well as temperature ranges, self-discharge rates, impedance, and capacities. A battery that has neared its lowest state of charge will trigger the recharge phase of the pack, even when the other battery packs are still at full capacity. This serious issue can cause damage to the cells and possible thermal runaway.
Cell balancing prevents these issues by redistributing charging functions. Higher capacity cells will first go through charging and discharging cycles before lower capacity cells. This method prevents damage to the battery pack and ensures that larger battery packs can be designed with higher capacities.
What is Cell Balancing?
Cell balancing prevents weaker batteries from being discharged past their lowest limit and prevents higher-capacity cells from being overcharged during the charging and discharging phases. Before cell balancing occurs, the state of charge of each cell in a battery pack can vary significantly, leading to the potential overcharging of higher-capacity cells and over-discharging of weaker cells, which cell balancing aims to prevent. (Figure 1).
Figure 1: Example of battery cells before balancing.
Due to the cells not being identical in their capacities, temperature ranges, impedances, self-discharge rates, and other aspects, cell balancing ensures that there is enough power in the individual cells to perform the functions. It does this through two different methods: active balancing and passive balancing.
Active Balancing
Active balancing (Figure 2) is where the charge is moved to different batteries based on the amount of capacity/state of charge within each individual cell during charging and discharging cycles. If there are 5 batteries and one battery has a lower capacity than the rest during discharging, then integrated circuits, capacitors, transformers, or inductors are used to move the charge from higher capacity cells (the ones with a higher state of charge) to the lower capacity cell. This method ensures that the cells are not draining at uneven rates throughout the series.
Figure 2: Example of battery cells with active balancing.
For charging the battery, the charge from the cells with the highest capacity is moved to the cells with the lower capacity. This shuttling of the charge helps to ensure there is more power balance between the batteries without one specific battery dictating when charging and discharging cycles occur.
When designing a battery pack using capacitors, inductors, transformers, or power electronic interfaces, the number of parts used can vary. Single capacitors, inductors, and transformers can be used on multiple cells within the pack. However, more electronics will need to be attached -- such as switches and controllers for capacitors. Using multiple capacitors, inductors, and transformers can offer fast balancing and efficiency yet may become complex based on the number of cells used in the pack.
Passive Balancing
Passive balancing (Figure 3) involves burning off the excess energy in the higher-capacity cell until all cell charges match. This procedure is done using a resistor. For example, you may have a 4-cell battery pack where 3 cells have a 3.6v capacity and one cell has a 4.1v capacity. In a passive system, the excess energy in the 4.1v cell is burned off as heat until the capacity reaches 3.6v.
Figure 3: Example of battery cells with passive balancing.
While you have cells that are capable of higher capacity in the pack, this type of cell balancing keeps the state of charge the same between them all. There are two different resistors that may be used with this method. switch shunting resistors and fixed shunting resistors. With switch shunting resistors, they operate on two different modes of continuous and sensing. The continuous mode will turn all switches on/off while the sensing mode relies on a real-time voltage sensor.
For fixed shunting resistors, they are attached to the cell's fixed shunting to prevent an overcharge. This method helps control the limit level for the cells.
Advantages & Disadvantages to Passive and Active Cell Balancing
There are many advantages to using passive and active cell balancing designs. For both methods, they may provide cost-effective ways to prolong the batteries to increase energy efficiency and tackle the issue of getting cells that have varying states of charges in the pack. With passive balancing methods, the cells only balance when they reach full energy, and this offers a low-cost way to achieve this advantage. For active cell balancing, this method improves energy efficiency, and capacity usage, and extends the life expectancy of the battery pack.
There are a few disadvantages to both methods. Active cell balancing may cause roughly 10% to 15% energy losses. An active method also can be costlier and have more complex designs when all the cells are connected to multiple power electronic interfaces.
In the case of passive cell balancing, burning off the excess energy causes poor thermal management with the cells. There may also be a higher energy loss amount due to switching losses and the electrical energy transformed into heat.
Why is Cell Balancing Important?
Cell balancing allows for the entire battery pack to be used efficiently. It is often used for lithium-based chemistries as these types are impacted by cell imbalances more than the other chemistries. Cell balancing can also prevent unsafe environments within the battery pack, such as thermal runaway, cell degradation, and incomplete charging cycles. Since batteries from other manufacturers may be present in a single pack, the state of charge could be drastically different.
Summary
Preventing overheating and overcharging while having the battery pack last for long lengths of time is highly desired for every customer's application. Cell balancing can achieve this while achieving the right battery pack performance.
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