2nd September 2019
Cell Balancing for Maximum Battery Pack Performance
Electric vehicles (EV) are widely viewed as an essential technology for energy-saving and environmentally sustainable transportation. As the new traction requirements, critical energy sources of EV, lithium-ion (Li-ion) battery pack is drawing a vast amount of attention for its advantages, such as compact volume, large capacity, higher energy density, and better safety.
Single battery cells are serially and parallelly connected to make a battery stack to achieve higher voltage and capacity. However, the charging and discharging process need to stop as soon as any cell reaches its maximum limit or working threshold (below absolute threshold). Due to this, the capacity of the battery pack is limited by the imbalance in the cells of the pack. This reduces the energy usage efficiencies and shortens the lifetime of the battery pack.
Therefore, battery cell balancing is a basic, but essential function of a Battery Management System (BMS) and is necessary for battery packs. The following image provides a basic idea about cell imbalance for a series-connected 10S pack.
Figure 1: Cell Imbalance & Balance
There are two popular ways of balancing, namely passive balancing and active balancing. The conventional passive balancing method prevents the bleeding of excess energy from cells into heat, while in active balancing there is a transfer of the excess energy into energy-depleted cells. Both typologies have their respective pros and cons. This blog will cover the important aspects of complete cell balancing.
Before diving into balancing, it’s important to understand the basic concepts of battery packs. The state of charge (SoC) is an important parameter for cell balancing. The SoC is nothing but the level of charge of an electric battery relative to its capacity. The balancing decisions are taken based on SoC. The aim is to have the same SoC for each and every battery at any given time. There are multiple algorithms used in BMS software to calculate accurate SoC to make decisions about balancing parameters.
Passive Cell Balancing
Figure 2: Typical Shunting Resistor Passive Cell Balancing
The above figure shows the basic diagram of a typical shunting resistor passive cell balancing. This topology supports the simultaneous balancing of multiple cells and achieves the balancing by dissipating energy from energy-filled cells in the respective resistors.
The important factors for consideration of this architecture are, the Resistance sizing with balancing current and balancing time. Also, the thermals for heat dissipation need to evaluate to eliminate the degrading of the system. In general, the on-board passive balancing current ranges from a few hundreds of milliamperes to around 1 Ampere, and hence the effective balancing time calculations heavily depends on balancing current. Also, the balancing current is directly proportional to cell voltage and hence its not constant throughout charge cycle.
There are possible tweaks in the basic architecture to enable it for larger balancing current but eventually its a trade-off for balancing current and required balancing time with the cost. The overall architecture and implementation of passive balancing is very economical with reduced complexity.
Active Cell Balancing
Active cell balancing topology manages to transfer the charge from energy exceeded cell to energy-depleted cell via bi-directional DC-DC power converter circuits. There are several types of active balancing methods based on the type of energy transfer.
The energy transfer can be from one cell to the whole battery, from the whole battery to one cell, or from cell to cell. The following figure is typical circuitry for the cell to pack energy transfer section. The overall complexity of active cell balancing dependents on two factors – the control logic behind the energy transferring scheme and the complexity of energy transferring bi-directional DC-DC converter section.
The balancing current and balancing time is completely dependent on the control logic and efficiency of DC-DC converters. In general, there is no theoretical limit for balancing current but the balancing current per cell should be below the typical discharge current given in the datasheet (most of the Li-Ion cells recommends 1C as ideal discharge current). The overall active balancing is relatively more expensive than passive and needs a lot more complex hardware and firmware.
The Real Need of Balancing
It is a well known and accepted fact that cells will always have parameter dispersion, and that will lead to an imbalance in the pack with battery aging. The overall imbalance with aging affects battery efficiency and performance. Hence, the battery management system should provide a balancing feature.
The real concern is what topology to select and whether it will able to handle the overall imbalance or not. With ION Energy we worked extensively on this problem. This experience leads us to understand that the newly built batteries typically have less than 0.2% parameter variations and with new researches in the battery domain and improved battery manufacturing qualities, these variations are going down.
With proper battery pack integration and improved thermal management, the battery pack experiences lesser imbalance in a lifetime and the overall requirement of balancing is not very extensive but a must-have feature. The passive balancing with the proper board-level thermal management can handle all balancing requirements with cost-effective solutions.
ION Energy has FS-CT and FS-LT platforms with on-board passive balancing capabilities, the platforms together cover a wide range of pack configurations from 3Cells to 25Cells in series.
ION Energy also has a distributed master-slave based platform called FS-XT where the slave has passive balancing capabilities till 420mA balancing current.
Active cell balancing is an amazing way to handle the cell balancing problem, but, in terms of system integration and optimization, this approach takes a hit on many fronts like cost and complexity. With improved battery cells and new cell chemistry, the battery variation is stabilizing. Hence, passive balancing is a widely accepted approach for most of the applications, while having some exceptions.