You have probably been hearing a little about Lithium Ion battery fires as of late. Boeing has gotten a lot of press for it lately. In my blog I highlighted some of the various fires that have occurred in the last few years. Only some of these fires can be attributed to the batteries themselves, and the majority of them to various issues regarding the packaging and handling of the batteries. What many people, including design engineers, don’t know is that there are many different Lithium battery chemistries available, all with differing precautions for handling and use. So what are the major types?
One of the most prevalent and also earliest Lithium battery introduced is the non-rechargeable (disposable) type that are used in watches and small electronics. These use lithium or lithium compounds as an anode. These vary in voltage from 1.5V to almost 4.0V, and can be found in higher voltages (such as a replacement for the common 9V battery) via the stacking of lower voltage cells internally. There is a dizzying array of exact chemistries, though the majority of these cells are Li-MnO2 (Lithium Manganese Dioxide). These are some of the hardiest of lithium batteries, though typically are found in lower-cost and low-drain applications. The environment and handling is not much of a concern, though they do self-discharge in high heat, and can overheat or explode if short-circuited or too rapidly discharged. Overall, they are fairly robust.
The rechargeable types of lithium batteries come in a similarly wide range of chemistries. Most handheld electronics use LiCoO2 (Lithium Cobalt Oxide). These are relatively inexpensive and offer excellent capacity as a function of cost. These are particularly sensitive, though, to rapid discharge and faults (short circuits, physical damage) and can create tremendous heat and burst under these conditions. Lithium Cobalt Oxide also generally has a much shorter cycle life (number of charge/discharge cycles). This can vary but will generally be in the hundreds of cycles (<500).
Growing in use are Lithium Iron Phosphate (LiFePO4), Lithium Manganese Oxide (LiMn2O4), and Lithium Nickel Manganese Cobalt Oxide (LiNiMnCoO2). These have found their way into a wide variety of products, but are becoming the darling of the transportation industry. These chemistries trade a little capacity for a dramatic increase in life (>2000 cycles). Additionally, these are some of the most stable chemistries available, being less reactive in nature, being less reactive with water (most Lithium batteries are very reactive), and “failing” in a much less pyrotechnic manner if shorted or breached (a “nail” test where a nail or spike is driven through the cell is common).
How do rechargeable lithiums compare to “traditional” rechargeable batteries? There are a few key differences, some known to anyone who has worked with other rechargeable chemistries, and some that are new:
Cell Life: The lifespan of a rechargeable lithium cell is generally expressed by the manufacturer as the number of charge-and-discharge cycles before degradation of the cell life. This typically is expressed as a discharge from 100% of cell capacity to 20% (described as an 80% “Depth of Discharge” or DOD), then a recharge back up to 100%. Degradation is typically defined as 80% of cell capacity. So, for example, a 1000 mAh cell with a cycle life of 500 can be discharged to 20% and recharged to 100% 500 times and at the 500th charge the cell will still deliver 800 mAh. Do you understand now why your cell phone battery seems to give up the ghost after 1-2 years?
Charging: Lithiums, like other rechargeable chemistries, require some intelligence while charging. Unlike other chemistries, they are much less forgiving to improper charging (risk of internal damage or leakage). Due to charge rate limitations (see below) the cells must first be charged in a constant current mode. As the cell voltage increases to near the fully charged state, the charger must switch to a constant voltage mode. When multiple cells are used or charged in series, cells must be balanced individually (see more below).
Charge and Discharge Rates: Typically rated in “C” (the Ah capacity of the battery), this is a measure of what current level the cell should be charged or discharged at and is specified by the manufacturer. For example, a 10Ah Lithium Iron Phosphate cell may allow for charging at 1C (10Ah), continuous discharging at 2C (20Ah), discharging for 10 seconds at 5C, and a pulse (less than 1 sec) of 10C.
Operating Voltage: As mentioned, a supplier will provide a recommended operating range for cell voltage. This is critical to cell life and safety. As such, active systems need to be employed to monitor the cells and manage charging. For small individual cells, there are many charging ICs available for engineers to select from that incorporate these functions on a single component with a few support components. For applications with many cells (such as EVs) a Battery Management System is required (more below).
Determining % Charge: For lead-acid and other chemistries, cell voltage is a good indicator of cell % charge. The cell voltage is fairly linear with this % value and is typically just translated by a simple voltage measurement. Not so with lithiums. Lithium chemistries can be somewhat linear in the middle of their voltage range, but with very little margin. For instance, a lithium cell might be considered fully charged at 3.6V and fully discharged at 3.2V. At 0.4V difference between the two, that’s less than a 10% swing in voltage that has to translate to a 0-100% scale. As a result, most charging or battery management ICs or systems usually resort to “coulomb counting” to accurately track battery %. A cell (or cells) will be fully charged based on max voltage, then an IC in the device or management system will track and report coulombs (discharging or then charging) to indicate % charge.
Temperature: Like most components, lithium cells have recommended storage and operating temperatures. It is important to note that this can be a huge impairment in operation. Most lithium chemistries lose a lot of capacity near or below freezing (as much as 20%). Interestingly, this is a function of the chemistry and a fully charged cell brought down in temp and then back up will still retain a full charge. A heating system may be required to realize full capacity at low ambient temperatures. Heat is also generated at high discharge rates (typically only an issue in automotive traction batteries or other high-draw applications) and will need to be dealt with to keep cells within their recommended temperature range.
Battery Management System (BMS): As mentioned, there are many factors to consider when using lithium cells. As a result, charge controller ICs (for one or a few cells) or BMSs (for larger arrays) have been employed to manage charging, voltage and temperature monitoring, and charge status. In some cases these incorporate shunts to allow an array of cells in series to shunt charge current around them once they are full (to better balance the pack). As you may know, in a series array of cells your total charge capacity (in Ah) is only as great as the weakest cell. If you continue to draw current or run a series array past what the lowest common denominator can deliver, you may damage that cell.
In all, lithium chemistries offer some great advantages over others, but at a cost of greater care and management. Even lead acids need the above precautions to maximize their life, but with their cost so low it is far less a concern in most consumer applications if there is a failure.