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5 Saving Power for the Future | ADI Lithium-based chemistries are now state of the art for the batteries used in various markets, from automotive to industrial to healthcare. Different types of lithium batteries have different benefits to better suit the power requirements for a variety of applications and product designs. As an example, lithium cobalt oxide (LiCoO 2 ) has very high specific energy, and this makes it suitable for portable products; lithium manganese oxide (LiMn 2 O 4 ), with its very low internal resistance, enables fast charging and high current discharging, which means that it's a good choice for peak shaving energy storage applications. Lithium iron phosphate (LiFePO 4 ) is more tolerant to full charge conditions and can sustain being kept at high voltage for a prolonged time. This results in it being the best candidate for big energy storage systems that need to work during a power outage. The drawback is a higher self-discharge rate, but this is not relevant in the above-mentioned storage implementations. The differing needs of applications requires a variety of battery types. For example, automotive applications need high reliability and a good charging and discharging speed, while healthcare applications necessitate high peak current sustainability for efficiency and a long lifetime. However, the commonality among all these solutions is that the various lithium chemistries all have a very flat discharge curve at a nominal voltage range. As seen in Figure 2, while in standard batteries we see a voltage drop in the range of 500mV to 1V, in advanced lithium batteries, such as lithium iron phosphate (LiFePO 4 ) or lithium cobalt oxide (LiCoO 2 ), the discharge curve shows a plateau with a voltage drop in the range of 50mV to 200mV. The flatness of the voltage curve has tremendous benefits in the power management chain of ICs linked to the battery voltage rail: The DC-to-DC converters can be designed to operate at a maximum efficiency point in a small input voltage range. Converting from a known V IN to a very close V OUT , the power chain of the system can be designed to have an ideal duty cycle of the buck and boost converters to achieve >99 percent efficiency throughout all operating conditions. Moreover, the battery charger can perfectly target the charging voltage and the loads are dimensioned according to a stable operating voltage to increase the precision of the final applications, such as remote monitoring or patient in-body electronics. In case of old chemistries or non-flat discharge curves, the DC-to-DC conversion operated from the battery will work with lower efficiency, which results in a shorter battery duration (-20%), or, when linked to medical portable devices, the need to charge them more often because of the extra power dissipation. The main drawback of a flat discharge curve is that the state of charge (SOC) and state of health (SOH) ratings of the battery are much harder to determine. SOC must be calculated with a very high precision to ensure that the battery is properly charged and discharged. Overcharging can bring safety issues and generate chemistry degradation and short circuits that lead to fire and gas hazards. Over-discharging can damage the battery and shorten the battery lifetime by more than 50%. SOH gives information about the status of the battery to help prevent replacing good batteries and to monitor the state of bad batteries before an issue appears. The main microcontroller analyzes the SOC and SOH data in real time, adapts the charging algorithms, informs the user about the potential of the battery (for example, if the battery is ready for a high current deep discharge in case of power break), and makes sure that, in big energy storage systems, the balance between batteries in bad condition and batteries in good condition is optimal to increase the total battery lifetime. 1 2 Energy storage battery cells. (Source: tong patong/Shutterstock.com) Lithium battery discharge profile.