This article will provide an overview of how lithium-ion batteries are designed. It will examine the two main components of a battery: the battery itself and the electronic device, comparing lithium-ion battery chemistry with other types of chemistry on the market, such as sealed lead-acid (SLA), nickel metal hydride (NiMH), and nickel-cadmium (NiCd), and how this affects design. We will delve into the safety aspects of lithium-ion batteries and how the battery management system (BMS) ensures that the battery is used in a safe manner. Future articles will explore each of these aspects in greater depth.
Battery Overview
In today's world of portable electronics, you can't live without lithium-ion rechargeable batteries. These batteries power your phones, smartwatches, tablets, laptops, cars, bicycles, homes, and airplanes. Unfortunately, recent news about electric vehicle fires and battery issues has highlighted the dangers of lithium-ion batteries. Market pressure to push for as much energy as possible in the smallest possible volume exacerbates safety concerns. Lithium-ion chemistry is inherently unsafe, so battery selection, manufacturing processes, and the electrical and mechanical design of the battery are crucial to ensuring its safety.
A battery cell is the power source of a battery. Batteries come in many different sizes, shapes, and chemical compositions. A primary goal of electronic devices is to ensure that the battery operates under its safe conditions. The casing provides a enclosure for the battery and circuit board to achieve mechanical integrity.
Battery Chemistry
Regardless of their chemical properties, batteries share a very similar basic structure. They consist of four main parts: the anode, the cathode, the separator, and the electrolyte. The goal of a battery is to generate a flow of electrons, thus producing an electric current, which can be used to power LEDs, light bulbs, electronic devices, motors, and more.
The electrodes, consisting of a cathode and an anode, are made of two different materials. One is more willing to give up electrons, while the other is more willing to accept them. A separator ensures that the anode and cathode do not come into contact (short circuit) and allows lithium ions to flow through. The electrolyte is used as the medium to generate the oxidation/reduction process and the flow of ions between the anode and cathode.
When the circuit is complete, it produces a chemical reaction that discharges the battery. The anode material releases its electrons at the negative terminal and flows around the external circuitry towards the cathode. Lithium ions simultaneously flow from the anode to the cathode through the electrolyte material. The reverse occurs during charging.
The differences in the cathode, anode, and electrolyte constitute different types of batteries. Each element has its unique properties, which affect the battery's current carrying capacity, voltage, cycle life, storage life, safety, and operating temperature.
Lithium-ion batteries
Due to its very high energy density, low maintenance requirements, and excellent cycle life performance, lithium-ion chemistry is winning the competition to become the preferred battery chemistry for the future of the electronics industry. Lithium-ion batteries come in three main shapes: cylindrical, prismatic, and polymer.
Cylindrical batteries come in two common sizes: 18650 and 26650. These batteries are made of steel or aluminum. The first two digits of the size indicate the diameter of the can in millimeters, and the next three digits indicate the length in millimeters. Therefore, an 18650 battery has a diameter of 18 mm and a length of 65.0 mm. These batteries are commonly used in laptop batteries and are widely used in power tool batteries. They are also found in many electric vehicles. Several battery manufacturers are exploring a new cylindrical size, the 21700, to meet the higher capacity (over 4 Ahr) required by electric vehicles.
Prismatic batteries are rectangular batteries that were very popular in older mobile phones (flip phones). Many applications that used prismatic batteries are being replaced by polymer batteries.
Polymer batteries are used in many mobile phones, tablets, laptops, and wearable devices. They come in a variety of shapes and sizes. The lack of industry-standard polymer sizes leads to usability issues, as these batteries quickly become obsolete without significant demand. Most polymer battery sizes are custom-made for specific applications. Polymer batteries can have very high energy densities and can also provide very high current rates. Careful mechanical design considerations are required when designing polymer batteries. Mechanical protection is needed around polymer batteries to ensure they are not punctured or crushed. Furthermore, the casing needs to account for expansion over time.
Lithium-ion batteries are composed of a variety of different anode and cathode materials. Each type offers specific advantages over others. Careful selection of the battery is necessary to ensure that the optimal lithium-ion chemistry is used for the application and use case.
Electronic products
While lithium-ion chemistry offers many advantages over other battery chemistry methods, its biggest drawback is its safety. Lithium-ion batteries can experience thermal events if used improperly. Robust electronics and fuses need to be incorporated into the battery design to ensure safe operation.
If the battery encounters overcurrent, overvoltage, undervoltage, overheating, and/or overheating conditions, the primary protection will shut down charging and/or discharging. The secondary protection circuit is a redundant protection function that can blow a fuse and permanently disable the battery in the event of overvoltage. The overvoltage setting for the secondary protection is higher than that for the primary protection. The secondary protection circuit will only be triggered if the primary protection fails due to a design/manufacturing defect. Batteries requiring UL2054 testing typically require secondary protection.
In addition, some batteries may have a fuel gauge to measure their remaining capacity. Several techniques exist for doing this. The most basic and simplest method is to correlate voltage with state of charge. This has several drawbacks because it doesn't account for temperature, discharge current, and battery life, all of which affect battery capacity. In practice, very few batteries use this method.
Another approach is to use coulomb counting, which tracks the current flowing into and out of the battery. An initial capacity is programmed into the battery, and as it discharges, it subtracts the remaining capacity by measuring the current and time. If the battery is charging, it adds it back. This is a more accurate method of capacity measurement than relying solely on voltage measurements. The drawback of coulomb counting is that it requires periodic full discharges so that it "knows" the battery needs to recalibrate its capacity.
The most common measurement method is to use a fuel gauge based on impedance tracking. These ICs actually measure the impedance on the battery and track it over time, temperature, and usage patterns. These ICs tend to be more accurate in reporting capacity than their counterparts based on voltage or coulomb counts. They do require prior battery characterization and loading the data into the fuel gauge.
Battery balancing is another function employed in batteries to ensure that they remain balanced throughout their lifespan. Battery imbalance can result from uneven current consumption, uneven thermal load, and/or poor battery matching at the factory. Battery imbalance can shorten battery life and, in some cases, pose safety concerns. There are two common battery balancing methods: passive and active. Passive battery balancing circuits discharge the battery with the highest charge, while active balancing circuits transfer charge from one battery to another. Extensive validation is required to ensure proper battery balancing. If incorrectly configured, it can actually cause battery imbalance over time and prematurely degrade battery performance.
In summary, lithium-ion batteries are volatile chemicals with numerous safety concerns compared to other battery chemicals, but their high energy density, long cycle life, and maintenance-free operation make them very attractive for powering electronic devices. However, safety concerns can be mitigated by selecting the right lithium-ion battery for the application and by managing the battery through appropriate electrical and mechanical design.
