Battery management deep dive on-demand technical training

Hello and welcome to this Texas Instruments course covering the introduction to battery management. This is the first in a five-part series that covers some of the key aspects of battery technology, and each of our modules is going to take about 15 to 20 minutes.

And my name is Maria Cortez. I'm the Marketing Manager for Battery Management Solutions at TI. And this training course is based on the technical presentation material by Upal Sengupta, who is the Applications Manager for Battery Management Solutions.

Here are the five parts of this course series. In the first part, we'll start by looking at the common types of rechargeable batteries that are used in various types of applications. Second, we will look at how battery monitors, also known as fuel gauges, operate. And then we'll look at the ways that you can implement lithium ion battery charge circuits.

We'll also discuss some of the special issues for large battery pack designs for applications such as power tools or other high-power systems. And finally, we'll take a brief look at the new emerging technologies for wireless power transfer.

So let's get started with our overview of battery technology. The applications for rechargeable batteries have grown dramatically in the last several years. Just until the 1990s, there were only two common types of rechargeable systems, either lead acid or nickel cadmium. But since then, advances in battery technologies have enabled many types of applications that we take for granted today.

Nickel-metal hydride was first developed as a higher-performance alternative to NiCd in the late 1980s, but since the advent of lithium ion batteries in the early 1990s, we've seen the growth of mobile computing, communications, and entertainment devices that would have been impossible without a small, lightweight, high-performance battery technology.

And just more recently, lithium ion batteries have been optimized for other applications such as power tools, UPS power backup, and even electric vehicles. While lithium ion batteries have many performance benefits, they do require electrical monitoring and control systems, so we'll go further into the details of how battery management electronics operate, and that'll be in the later sections of this tutorial.

This chart shows the relative energy density by weight and volume. Any battery can be measured by the small amount of energy it delivers. Amp hours and watt hours are typical terms used to describe the capacity of a given battery. An ideal battery would have a very high or infinite energy density.

So for real batteries, the more energy that can be packed into a small volume, the better-- the better lightweight it is. However, there are many other aspects of battery performance that need to be considered, such as the types of materials that's used, the cost, the high-current capability, self-discharge, and even safety.

You can see here that while lead acid technology has very low energy density compared to lithium ion, it is still used in many applications because it's low cost and durable. It's also well-suited for many fixed applications like UPS or energy storage, and also for vehicle starter batteries.

Lithium ion, on the other hand, is extremely lightweight, and it has high energy density. So it's ideal for portable computers, mobile phones, and other handheld devices such as tablets, also.

Lithium ion has become very popular because of its many performance benefits. One of the key concerns with NiCD, nickel-metal hydride, is their self-discharge characteristic. A full battery when left in storage may not retain its charge for very long, even if it's not used. So this self-discharge behavior is aggravated at high-temperature storage conditions.

Lithium ion provides higher energy in a smaller package and much better capacity retention in storage than the nickel-based systems. But the main drawback of lithium ion batteries is that they require precision electronics for monitoring and control, and this is needed to ensure safe and reliable operation. But despite this drawback, lithium ion has become the most popular choice for a huge variety of portable electronic devices.

So why is lithium ion so popular? You only have to look at where lithium sits on the periodic table compared to lead or nickel to guess that lithium ion would be a much lighter battery. But in brief, a lithium ion battery for its given size will provide more energy than a corresponding nickel- or lead-based battery, but it's also going to be lighter. Lithium polymer systems use the same chemical materials for anodes and cathodes, such as conventional lithium ion batteries, but they can also be packaged in a very flat, thin form factors, which makes them ideal for handheld devices such as mobile phones, tablets, or wearable medical devices.

So since the introduction of the lithium ion systems in the early 1990s, there's been steady progress in the energy density of these products. The single-most common cell type, even today, is the cylindrical 18650 design, and this number comes from the dimensions of the cell. It's 18 milimeter in diameter and 65 millimeter in length.

The typical construction used for most rechargeable cells is shown here. The two electrodes, the anoid and the cathode, have a nonconductive separator layer in between them. And these three sheets are wrapped in what is referred to as a spiral-wound type of device. The materials are put into the cylindrical housing, and there is a conductive fluid, or electrolyte, and that's poured in between to fill the space.

The physical design of the case includes a pressure relief valve, and also, a positive temperature coefficient, or PTC, element, which will disconnect the cell in case of extreme high temperatures. When the cells are charged and discharged, the internal chemical reaction may result in gas pressure generation inside the housing. And in addition, there may be some temperature increase due to ohmic losses or exothermic reactions. The pressure relief valve and the PTC prevent continued current flow to the external circuit in case the cell temperature and the pressure get too high to ensure safe operation.

So in many cases, though, when cell failures have been analyzed, the root cause is often due to small particulate contamination or medical particles that may have punctured the separator, causing an internal short. And when there is an internal short, disconnecting the cell from the external circuit cannot remove the fault. So high quality in the manufacturing process is essential to prevent contamination during cell construction. Reliable cell manufacturers will have a strict quality control procedure to ensure that their cells provide consistent, reliable performance and minimal chance of failure when they leave the factory.

So as we discussed before, battery systems-- and in particular, lithium ion systems-- require some form of electronic battery management to ensure safe, reliable, and efficient operation. The basic functions needed are the precise control of charge current and voltage, the prevention of abuse conditions-- over-charge or excessive discharge, overload current or short circuits or temperature extremes-- and the monitoring-- you have to monitor the capacity level of the battery. And additionally, smart battery systems will communicate important information about the battery capacity, the health, and status to the host system for display to the user.

So while there are many variations of pack configurations and capacities for different applications, the basic functions of battery management circuitry are roughly the same. However, smaller packs generally have simpler circuitry, while larger applications like electric vehicles can have very complex designs due to the larger packs and many cells and higher load current levels.

This illustration shows a typical arrangement of the different components for a battery pack in a portable system. For a multi-cell lithium ion pack, it's important to monitor each individual cell within the pack. And as we'll see in later sections, keeping the cell in a multi-cell pack in a matched or balanced condition is essential.

The charging IC is typically external to the pack and regulates the current and voltage from an external DC source to the precise levels required for a lithium battery. External power conversion circuits translate the variable voltage from the battery into the specific regulated levels required by different subcircuits in the electronic device.

For medium- and larger-size battery packs, it may also be recommended to implement a second or redundant safety circuit as a backup in case the main safety circuit malfunctions. In case of a fault condition, the primary safety circuit can disconnect the battery from the external circuit and reset the protection when the fault condition is removed. The secondary protection circuit typically blows a fuse when a fault condition is seen because by that point, the primary circuit has already failed, and the pack should be removed from service.

This chart shows the discharge voltage curve for a typical lithium ion cell at different constant current load levels. The x-axis shows the effective capacity in milliamp hours that's delivered from the pack when it is discharged from full, which 4.2 volts, to empty at 3 volts. At lighter loads, the pack can deliver more energy before reaching the end-point voltage because the internal losses due to internal resistance of the cells are reduced.

Another interesting phenomenon is seen in the high-current discharge curve at the bottom. When the high-current discharge is started, the battery terminal voltage drops quickly, as one might expect. But after a few minutes, the heat generated due to current flow actually causes a reduction in the internal resistance of the cell, and so the voltage actually rises for a short period of time, even though the battery continues to lose capacity as it's being drained. Eventually, of course, the continued discharge of the cell will reduce the voltage down to the end point of 3.0 volts.

So while the resistance of the battery is a relatively complex parameter, and it varies with age, temperature, and state of charge for a given battery, it can be approximated to a first order as an ohmic resistance. The displacement between the curves at different current levels can be used to estimate the internal resistance of the battery, as shown in the formula delta v equals delta i times resistance of the battery.

This chart shows the discharge of the same type of cell in the previous chart at a fixed current, but for different temperatures. Because the internal resistance is lowered at high temperatures, the high-temperature curves have higher output voltage than the low-temperature curves. The same self-heating can be seen in the cold temperature examples near the beginning of this discharge cycle.

So we know from the previous charts that the impedance effect is most noticeable at high currents and low operating temperatures. Here, we show the effect of aging or cycling a battery many times. As a cell is charged and discharged repeatedly, it can begin to wear out as the internal impedance gradually increases each time the cell is cycled. At 100 full cycles, we see that an older cell will not deliver the same energy as it did when it was brand new.

This illustration shows the importance of precision in the output regulation point for a lithium ion battery. Let's say we start out with 950 milliamp hour nominal battery and charge and discharge it hundreds of times. Each time it's recharged, the terminal voltage is regulated to precisely 4.20 volts.

Under these conditions, after 500 cycles, the cell can still deliver about 850 milliamp hours at the same load current as it did when it was new. This is a relatively mild degradation and shows that a battery like this could provide good service for many months or even years, depending on how often it's used.

But if your charge circuit regulates just 100 millivolts too high on each recharge, as in the 4.3 volt curve, after about 200 to 250 cycles, the degradation curve becomes very steep, and the cell quickly becomes unusable. So a precision regulator circuit is required for the charge in order to maintain battery performance and extend the battery's service life.

Another critical degradation mechanism for lithium ion cells is high temperature storage. In particular, if a cell is stored at a fully-charged condition in a high-temperature environment, the effective output capacity will be decreased. So for best long-term stability, lithium ion cells are best stored in cool, room-temperature environment at around a 50% state of charge.

And finally, we can also see that charging a cell too fast can degrade its performance over time. For a 900 milliamp hour rated battery, the C rate is 900 milliamps. C stands for capacity, so the C rate is the rate of current that the battery could supply that would fully drain it in one hour.

In particular, for most lithium ion batteries, it's recommended to recharge slightly lower than the 1C rate so that the overall charge time may be in 90 minutes to two hour, in that range. However, some new cells are optimized for high-power applications, and they allow a high-rate charging. So this data may not apply to all battery types.

Especially in the last few years, the variety of lithium ion battery types have increased significantly. The original lithium ion batteries used a graphite, or soft carbon, anode structure, and a lithium cobalt oxide cathode. This combination provided a good energy density and a high capacity but is relatively expensive, and it uses cobalt, which is a relatively rare and hard to obtain material.

But as lithium ion batteries became more popular, there was a need to find lower cost materials, as well as optimize their performance for new types of applications. Some of the other materials which are now used include lithium manganese oxide, lithium iron phosphate, lithium nickel metal cobalt blends, or lithium nickel cobalt aluminum blends.

All of these variations will have somewhat different voltage discharge curves, with some of the typical voltages shown here. Some of the combinations, such as lithium iron phosphate, have lower voltage and corresponding lower energy density. However, these types of cells are optimized for very high peak power output and also can recharge cycle hundreds of times or even thousands of times. So they're well-suited for high-power applications, where cycle life is important.

So to review, there are now a wide variety of cell types, even within the family of lithium ion, that are designed to optimize the type of performance parameters that are needed for very specific applications. Choosing between high peak power and high long-term energy density at relatively lower currents is a typical design trade-off for battery cells.

So high-energy, low-current cells are used for lower-power hand-held products like phones and ultrabook PCs, while higher power cells are used in power tool applications and eBikes. There are now even mid-rate type of cells designed for applications where the system needs a balancing between long-term energy delivery and instantaneous surge current capability.

Here are some examples of the types of battery performance parameters that are important for different types of applications. When starting a car, the battery is rarely drained of its full capacity, so the amp hour rating is less important. But the ability to deliver large surge currents for shorter durations, especially across a wide temperature range, is extremely important. And given that the battery is not being carried by hand but mounted in a vehicle, the size and weight are relatively less critical, and this is why lead acid batteries, even after hundreds of years, remain dominant for this application, even though other battery types might have lighter weight and higher capacity.

Cordless power tools have greatly increased in popularity in the past several years. And in the 1990s, and up until about 2005, they were almost all powered by NiCd or nickel-metal hydride batteries, and this is because high-current capability was required, but lead acid would be too heavy for a hand-held tool.

In recent years, however, the new types of lithium ion power cells that can provide high-output current have started to take over this application. Since the new lithium ion cells are so lightweight yet high voltage, power tools can now be made more powerful without becoming too heavy.

And finally, for small handheld devices like portable computers, tablets, mobile phones, digital cameras, et cetera, the high-capacity, small form factor cells that are available using lithium ion technology are critical to allow the small, thin products that are now the norm.

Here's an example comparing the discharge curve of a lithium phosphate battery in red, with the traditional lithium cobalt battery in blue. The phosphate cell operates at a lower voltage and has a much flatter discharge curve, so the task of estimating battery capacity for this type of cell may be more challenging.

And also, the safety thresholds that define overcharge and excessive discharge levels have to be tuned differently for each type of battery. We'll talk more about how to estimate capacity of a battery and the battery management electronics in general in the next sections.

So in summary, the basic functions of all lithium ion pack-monitoring circuits are the same, regardless of the application. We want to maintain safe operation of the battery under all conditions, extend and maintain the service life of the battery as much as possible, ensure complete charging and utilization of the pack without violating safety thresholds. The level of sophistication and the features implemented will vary depending on the application.