It is common to explore different power supply options when designing your applications. However, one that often gets neglected is the differences between the types of battery cells in your portable applications. There are a lot of similarities between battery cells, but also very many differences that make certain cells more efficient than others when it comes to application.
Let us explore the characteristics that define the cells:
- Chemical Capacity
- Power Capability
- Self-Discharge Rate
- Durability, Cycle Life, and Shelf Life
- Monitoring and Safety Measures
- Battery Weight
- Cost
Chemical Capacity
Chemical capacity refers to how long is the charge available before the cell cannot power the application anymore. As simple as it gets, this relates to the capacity in milliamp-hours. Power capability refers to how much power can be pulled at once at a given state of charge, which often relates to discharge current x battery internal impedance. The self-discharge rate is just as it sounds – at what rate does a cell discharge when it is not in use.
Various battery cells with battery management systems.
Durability, cycle life, and shelf life relate to the rate at which cells age with use, or with lack of use. Monitoring and safety measures relate to the additional circuitry necessary to ensure that the pack functions as expected, and that neither the consumer nor the pack is at risk of physical damage or permanent failure. Battery weight and cost should not necessarily need explaining, but they do also play a critical role when considering how much the pack will weigh or whether it will fit into the project’s budget. Let us compare our four main types of battery cells regarding the above characteristics.
Lead Acid Cell
The first type of cell we can examine is lead acid. The construction of a lead acid cell is one electrode is composed of lead and the other is lead dioxide. The electrolyte is sulfuric acid, which is converted to water during discharge, which may evaporate in high-temperature settings. The chemical composition of this battery is susceptible to degradation when discharged, and susceptible to corrosion if constantly charged. The cycle life if maintained properly is around 350 cycles.
Let us first examine voltage, lead acid can store a nominal charge of 2.1V, so it would require 6 cells in series to output 12V. Lead acid can store charge typically for 2 years with low self-discharge. Typically, a cell can have roughly 3Ah capacity. Lead acid is capable of high discharge rates (1-10C) however when the cells discharge, lead acid becomes unstable as previously mentioned, allowing for plate corrosion or sulfation which is not easily repairable. Extending the capacity by connecting cells in parallel is possible, however, it is also required to tie in as many cells needed in parallel in series to achieve the desired power. If 12V was required (6 cells in series) and triple the capacity was required (3 cells in parallel), 18 total cells would be required to meet the criteria. Weight is a big concern to this battery chemistry as the energy density is one of the lowest of all the options at 90 Wh/L. However, lead acid is very cheap and typically does not require a battery management system (BMS) to monitor charge and discharge current unless the battery requires methods for fast charging techniques.
The most common applications for lead acid are automotive batteries, where the battery is recharged via alternator after its initial jolt of high current to turn on the automobile. Other applications include backup systems for computer servers or telecommunications. The applications where lead acid is really utilized are those in which the weight of the battery is a non-factor.
Nickel Cadmium Cell
The next cell to compare is nickel cadmium (NiCad). The construction of a nickel cadmium cell is two electrodes, the positive electrode being nickel oxyhydroxide and the negative electrode being cadmium and during discharge, the positive electrode becomes nickel hydroxide, and the negative electrode becomes cadmium hydroxide. Potassium hydroxide is the alkaline electrolyte. Nickel cadmium has a very long cycle life of 2,000 cycles and can be rapidly charged and discharged (10-20C) without damaging the cell nor requiring a battery management system. However, like lead acid, nickel cadmium cells are susceptible to plate corrosion. This is generally caused by high temps and is not repairable.
Another degradation method is being fully charged, which allows the cathode material to crystallize so the method of preventing this is to avoid storage at full charge. The cells have a high self-discharge rate, so while it is recommended not to store fully charged, it is also recommended not to fully discharge either to avoid deeply discharging.
The voltage of the NiCad cells is much lower than lead acid at 1.2V nominal charge. Capacity can vary based on the size of the cell, ranging from as little as 300 mAh to 7000 mAh. NiCad cells are typically cylindrical-shaped ranging from AAA-F sizes. Due to the differing shape and capacities of the sizes, the Nickel Cadmium cells have an energy density in the range of 150 Wh/L. If this is used for a 18V power drill, and we want at least 3000 mAh capacity, this could require up to 150 (15 series x 10 parallel) cells. Generally, these cells are light (depending on size) and extremely cheap for their very long cycle life. The most common application for Nickel Cadmium cells is for power tools and consumer electronics.
Nickel Metal-Hydride Cell
Nickel metal-hydride (NiMH) cells have the same alkaline electrolyte and positive electrode as nickel cadmium cells. The only difference is the negative electrode, as it is converted from a metal hydride to a combination of metal and water. NiMH and NiCad are nearly identical except that NiMH has nearly double the capacity and discharges at a much slower rate than NiCad (1-2C). These cells are most useful for their high capacity, long usage, and low cost so they are prevalent in low-power consumer electronics where high capacity is most important.
Lithium-Ion Cell
Lithium-ion cells are composed of lithium as the anode of the cell, or even a lithium compound as the cathode. Compound meaning a more stable combination of lithium and other elements such as oxygen or manganese, etc. There are many variations of the compounds that make up the cells. These variations create changes in conductivity and stability so no two cells are the same, however the pros and cons of each cell type will not differ much within the lithium-ion family. The electrolyte must be organic, and the separator is constructed of a microporous material which allows for very low self-degradation. lithium-ion cells are most recognized as the cylindrical 18650 cells with a liquid electrolyte, but they also come as pouches, which use a polymer rather than the liquid electrolyte. 18650 refers to the size as 18-mm diameter and 650-mm height. The polymer batteries use a gel or solid electrolyte which allows for a more compact design and can be attributed to the batteries in our mobile devices.
Lithium cells typically have a nominal voltage of 3.7V and capacity can vary from 2.2Ah to 3Ah. Some cells are made for high discharge rates or high capacity, so it varies (2-10C). Discharging a cell past its low voltage threshold may reduce the cycle life from the typical 500 cycles to 50 cycles. In the same sense, properly charging and discharging the cells within the given ranges can allow the cell to survive past 500 cycles.
Unlike the other cells, lithium-ion cells degrade much faster in both fully charged and discharged states. When fully charged, the electrodes are susceptible to corrosion and when deeply discharged, degradation is drastically accelerated due to expansion and contraction in the Solid Electrolyte Interphase (SEI) layers. For these reasons, it is recommended to store lithium-ion cells at 50% charge.
Another factor apart from charge to consider is temperature. Temperature (both high and low) also accelerates degradation which may be the most significant factor when considering battery lifespan. For this reason, a BMS is required to keep the cells operating within specified ranges. Monitoring cell voltage and temperature are also critical to ensure that there is no chance of overheating or overcharging which could both cause combustion or worse, an explosion. Safety and monitoring circuits are required as these cells do possess a lot of power. The energy density is in the range of 350 Wh/L which greatly outpaces the other cell chemistries. For this reason, it must be budgeted to spend money on the development of cell monitoring and safety circuitry, as well as testing and validating.
The most common applications for lithium-ion cells are hand-held electronics, tablets, electric vehicles, power tools, video cameras, and other consumer electronics which require high power or high capacity.
Summary
Each battery cell type has its own advantages given the specific application, however, lithium-ion is the main trending cell chemistry. It is costly to develop lithium-ion, but it is well worth it as the advantages greatly outweigh the other cell chemistries. Lead acid is ruled out of most applications that are not stationary applications. The other cells can be compared by the combination of weight, voltage, capacity, and durability to determine which is the best for the application. If you are unsure which cell chemistry is best for your application, contact Epec to get some advice. We would be happy to assist through any step of your battery pack development.
