18650 li-ion battery

　　Why are the protection circuits of 18650 li-ion battery getting smaller and smaller? Improving performance is key!

　　With the development of science and technology, the size of portable devices is getting smaller and smaller, and with this trend, the requirements for the size of the protection circuit of 18650 li-ion battery are also getting smaller and smaller.

　　Many new technologies not only improve performance, but also increase system power consumption. For chemical companies that produce batteries, substantial progress in battery production technology is difficult, time-consuming, and costly. So one must look for ways to optimize power conservation. Smart battery systems (SBS) are the most promising technology to emerge and can significantly improve battery pack performance.

　　In the computer industry, 18650 li-ion battery are really loved and feared. Accidents that occurred in the early days of 18650 li-ion battery are still fresh in the minds of the companies involved. They learned an impressive lesson: Under no circumstances should the rated parameters of a lithium-ion battery be exceeded, otherwise it would definitely cause an explosion or fire. In addition to the chemical composition or electrode parameters of the battery, there are several certain parameters for 18650 li-ion battery. If exceeded, the battery will enter an out-of-control state. In the diagrams explaining these parameters (refer to the Lithium Ion Parameters Chart), any point outside the corresponding threshold curve is a runaway condition. As the battery voltage increases, the temperature threshold decreases. On the other hand, anything that causes the battery voltage to exceed its design value will cause the battery to overheat.

　　Beware of hazards caused by chargers

　　Battery pack manufacturers put in place several layers of cell and packaging protection to prevent dangerous overheating conditions. However, there is a component in the use of batteries that may cause these measures to fail and cause harm. This component is the charger.

　　There are three ways in which charging 18650 li-ion battery can cause harm: excessive battery voltage (the most dangerous situation); excessive charging current (excessive charging current causes lithium plating effects, thus causing heating); failure to terminate the charging process correctly, or Charging at too low a temperature.

　　Designers of lithium-ion battery chargers take extra precautions to avoid exceeding the allowable ranges of these parameters. To absolutely ensure that the relevant parameters of the system work within a safe range. For example, the smart battery charger specification allows a negative voltage deviation of -9%, but emphasizes that the positive deviation must not exceed 1%. Compliance with smart battery safety standards is guaranteed. Of course, in an actual design, the sign of the deviation is random. Therefore, designs that comply with this specification often set the charger's target voltage value near -4% of the rated value.

　　Due to the inaccuracy of the charging voltage (whether it is -4% or -9%), the battery is always undercharged. Fear of the potential dangers of 18650 li-ion battery results in low utilization of battery pack capacity. According to the experience of industry experts, even if the voltage after charging is only 0.05% lower than the rated value, the capacity drop is as high as 15%.

　　Battery built into computer

　　The principle of smart battery technology is very simple. A small computer is built into the battery to monitor and analyze all battery data to accurately predict the remaining battery capacity. The remaining battery capacity can be directly converted into the remaining operating time of the portable computer. Compared with the original capacity measurement method that only relies on voltage monitoring, the operating time can be immediately extended by 35%. Unfortunately, smart battery technology can only go so far. Unless they can communicate with the charger circuitry, they cannot determine their operating environment or control the charging process.

　　In the context of a smart battery system, the battery requests a smart charger to charge it under specific voltage and current conditions. The smart charger is then responsible for charging the battery based on the requested voltage and current parameters. The charger relies on its own internal voltage and current references to adjust its output to match the values requested by the smart battery. Since these benchmarks have inaccuracies of up to -9%, the charging process may end with the battery only partially charged.

　　A more detailed understanding of the charging environment can reveal more issues that affect the charging efficiency of 18650 li-ion battery. Even in the best-case scenario, assuming the charger is 100% accurate, the resistive elements in the charging path between the cells of the charger introduce additional voltage drops, especially during the constant current charging phase. These additional voltage drops cause the charging process to transition from constant current to constant voltage phase prematurely. Since the voltage drop introduced by the resistor gradually weakens as the current decreases, the charger will eventually complete the charging process. But the charging time will be extended. The energy transfer efficiency is higher during constant current charging.

　　Eliminate resistor voltage drop

　　The most ideal situation is that the charger's output accurately cancels the effect of resistor drop. One might propose a solution where a smart charger monitors and corrects its own output using smart in-battery monitoring circuit data at all stages of the charging process. This is feasible for a single battery system, but less so for a dual or multi-battery system.

　　In a dual-battery system, it is best to charge and discharge both batteries at the same time, if possible. Although battery charging is parallel, the typical charger with only one SMBUS port is not up to the job. Because if there is only one SMBUS port, the charger or other SMBUS device can only communicate with one battery at the same time. Therefore, an ideal system should provide two or more SMBUS ports so that both batteries can communicate with the charger at the same time.

　　Smart Battery System (SBS) Manager

　　In addition to providing multiple SMBUS ports, SBS manager technology can also significantly improve the performance of lithium-ion smart batteries. The SBS Manager is part of SBS and is defined by the SBS1.1 specification. It replaces the SmartSelector defined in the previous version.

　　On the one hand, the SBS manager provides an interface with the driver and vibration system, and on the other hand, it manages the smart battery and charger. The driver can read and request information about the battery, charger and the manager itself. The interfaces related to this information transfer are defined in the specification. In a multi-battery system, the SBS manager is responsible for selecting the system power source and deciding which battery to charge or discharge at a specific moment. In short, the SBS manager determines which battery to charge, which to discharge, and when.

　　A well-implemented SBS management has several major advantages: a more complete and faster charging process, efficient charging and discharging simultaneously, and the ability to detect and respond quickly to dangerous situations (such as potential voltage over-limits). An SBS manager that monitors the voltage of the battery itself can charge the battery to its true capacity. It can avoid insufficient charging caused by smart chargers due to inaccurate monitoring voltage (as mentioned above, generally -4% to -9%). Furthermore, this process does not require a particularly accurate voltage reference (accurate voltage references are expensive).

　　The strategy to avoid using a precise voltage reference is to use the measurement circuitry inside the smart battery to measure the battery voltage with an accuracy of up to 1%. In this way, the SBS manager can command the charger to increase the voltage appropriately until the monitored voltage reaches the appropriate value. A well implemented SBS manager can make the battery charging process faster than

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