Battery Power - March/April 2013 - (Page 12)

Feature Managing Lithium-Chemistry Batteries: It’s Mostly About Their Temperature Robin Tichy, Senior Marketing Manager Electrochem Solutions, Inc. Batteries based on lithium chemistry have certainly been getting a lot of attention lately. There’s no doubt about it. While we don’t know the specifics of the cause(s) of the problems with the battery subsystem on the Boeing Dreamliner 787, we do know that this class of batteries has had issues in the recent past. Several years ago, there were well-documented cases of self-initiated fires in some notebook PCs that were not on nor connected to their chargers. These were eventually traced to internal short circuits in the cells that allowed very large currents to flow, and I2R heating took over from there. Despite these potential risks, what is it about lithium batteries that we like and is driving their increased use in everything from smartphones, to laptop PCs, hybrid/all-electric vehicles (HEV/EV) and even to very large aircraft? The answer is simple: it’s our never-ending quest for increased energy-storage density of rechargeable batteries (and non-rechargeable ones, as well, of course), measured both by weight and volume. Though exact numbers vary depending on the specific lithium chemistry used, some representative values for energy density by weight and by volume are shown in Table 1 for Li-Ion, lead acid, nickel cadmium (NiCd) and nickel metal hydride (NiMH) batteries. Compared to any of these, Li-Ion cells are superior by factors of approximately two-times to five-times. Keep in mind that in a design world where 5 percent and 10 percent improvement is considered noteworthy, this shows that lithium’s numbers are truly impressive. Battery Chemistry Energy Density by Weight Energy Density by Volume Whr/kg (kJ/kg) Whr/L (MJ/L) Li-Ion Lead Acid NiCd NiMH 110 to 160 (1056 to 1536) 30 to 50 (288 to 480) 45 to 80 (432 to 768) 60 to 120 (576 to 1152) 400 to 450 (3.84 to 4.32) 90 (0.864) 120 (1.15) 240 (2.30) Table 1. However, lithium cells require very carefully managed charge and discharge cycles, along with temperature management. Excessive temperature rise is a critical issue for this battery chemistry. In general, Li-Ion batteries must not be charged above 45°C (113°F) nor discharged above 60°C (140°F). It’s not hard to reach those temperature in many real-world installations (it is usually possible to exceed these limits slightly, but cycle life will be reduced). If cells exceed these temperature limits, they may start to vent, leading to battery failure at best and a cell fire at worst. The problem is aggravated by the trend to using larger battery packs for more power, which produce much more heat than smaller packs, but in a more concentrated physical package with 12 Battery Power • March/April 2013 less opportunity and surface area for cooling and dissipation. To properly monitor and manage the temperature, designers must understand the various sources of heat and what control, if any, they have over them. These sources include: 1) The external ambient temperature. 2) The inefficiency and resultant heat generation of the charging circuit, which is usually located close to the battery packs. 3) Ohmic losses and heat due to high currents, from both internal cell impedance as well as connector, conductor and protective-component resistances. 4) Finally, the chemical nature of the charge/discharge cycle: batteries are electrochemical devices on-going chemical reactions; charging is endothermic and absorbs heat, while discharging is exothermic and generates heat. Let’s look at these sources in more detail. 1) The ambient temperature is generally outside of your direct control, unless you are in a well-controlled environment. Even when this is the case, you have to assume that the ambient may rise above the target maximum for any one of many reasons, ranging from short-term fluctuations to longer-term failures. 2) Charging circuits typically have inefficiencies is the 10 percent to 30 percent range. Given the amount of power they are providing to the battery, this produces a significant amount of heat to be dissipated. While the solution may seem straightforward, just locate the charging circuit away from the batteries, this is sometimes impractical and more often undesirable due I2R loss from source #3. 3) Basic losses, and thus heat, from inherent internal cell resistance, which is on the order of 100 mΩ are the first problem. In addition, there are various inter-cell connections (usually nickel strips), cables and connectors that can’t be ignored. Further, the various protective devices, current shunts, protection MOSFET on-resistance RDS(on) and fuses all contribute additional small but still significant losses. As these Li-Ion cells are being increasingly used in highercurrent situations, with charge currents of 5 A and more, and high-rate discharge currents of 10 A and above, these losses and their heat become more severe. Note that some applications now use cells which are rated at 10C to 20C (where 1C is the capacity rating of the cell in amp-hours). A cell discharging at 20C will only be able to do so for a short period, of course, but the current and associated heating will be significant during that time. 4) According to tests and data from Electrochem, the endothermic heat absorption while charging is a minor factor when weighed against heat sources, and has little mitigating effect. In contrast, the heat produced by the exothermic discharge reaction can be significant. As usual in chemical and thermal situations, there is hardly ever a “free lunch” that you can absorb what your produce. Even if this were the case, the heat

Table of Contents for the Digital Edition of Battery Power - March/April 2013

Battery Power - March/April 2013
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Battery Power - March/April 2013