Bq77910: The protector behind the lithium battery

Keywords: lithium battery protection circuit battery management

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With the advancement of technology and the improvement of environmental protection requirements, lithium batteries are gradually becoming the mainstream of rechargeable batteries. Especially in the field of mobile electronic products, lithium batteries rely on higher energy density and have occupied almost all markets. However, in some applications where power requirements are relatively high, lithium batteries do not have an advantage in terms of cost and discharge rate indicators, and their share is still not high. For example, in the field of electric bicycles, lead-acid batteries still occupy a high share. In the Chinese market, lithium batteries only have a market share of about 1%, while in developed countries in Europe and America, they account for about 50%. The main reason for such a low market share is the price, but the service life and safety of the lithium battery itself is also a shortcoming. These problems cannot be solved satisfactorily, which will greatly affect the market share of lithium batteries.

To solve the life and safety of lithium batteries, special monitoring and protection circuits are needed. This is a feature of lithium batteries compared to other batteries. It must have a protection circuit, otherwise it will explode because of heat. The mobile phone uses a lithium battery without a protection circuit, and as a result, the battery explosion causes a damage accident. This shows the importance of the protection circuit. In the past, many protection circuits were built with discrete components, which have no advantages in terms of cost and reliability. For this market situation, Texas Instruments has introduced the industry's first power supply. Tool and electric bike single-chip battery management device bq77910.

Why it's for power tools and electric bikes, starting with the number of supported battery packs. Because the device supports 4 to 10 strings, it can support up to 20 strings of batteries by cascading. The number of strings of lithium batteries determines its output voltage, and 10 strings are the output of 34V. In this range, the battery Applications include electric bicycles, electric scooters, portable gardening tools, power tools, and uninterruptible power supplies (UPS) that require certain voltage and current requirements, but are not very demanding. If you want to enter the field of electric vehicles, then this string can be more than 100 strings, which is not the area that the device can cover.

The main function of the battery protection circuit is to keep the battery pack safe under various conditions, to maximize the battery life, and to ensure full charge and utilization of the battery pack without exceeding the safety threshold. The bq77910 is an integrated battery management solution that provides all the necessary battery pack protection and integrated battery balance, including:

Battery unit overvoltage, battery unit undervoltage, charge short circuit, discharge short circuit overcurrent, battery pack temperature, open circuit short circuit thermistor detection and open circuit battery unit detection, power failure protection, to prevent power FET overheating and other functions. Its biggest feature is that it operates as a stand-alone device, eliminating the need for an external microprocessor and integrating the logic control part internally, which makes the overall solution low cost possible.

The high-power battery pack targeted by the device itself is subject to great challenges, such as extremely high currents, and requires a large discharge current, a small charging current, and a separate load and battery. A separate charge and discharge path is also required so that appropriate protection path elements (MOSFETs) can be selected according to the required size, thereby reducing cost. In some applications, such as electric vehicles and UPS, fast charge and discharge conversion is required. These have brought great technical challenges to the design of the battery pack and the protection circuit.

In order to meet the needs of high-power applications, the bq77910 has been carefully designed for the above requirements. For example, it can be configured to support battery pack operation with 4 to 10 strings of battery cells. In a 10-section configuration, the maximum input voltage is 50V. It presets safety thresholds and time delays (programmed in non-volatile memory) to match various types of Li-Ion battery systems and supports Li-C2, LiFePO4, and LiMn2O4 lithium battery types. The device can be programmed during battery pack manufacturing and can operate as a stand-alone protector without the need for an external microcontroller. It can directly drive low-side NMOS power FETs, and NMOS power control is less expensive and more efficient, especially for high-voltage, high-current applications. Its FET configuration is flexible, allowing you to choose different sized power FETs (serial or separate paths for charge and discharge currents) as needed, giving customers a cost advantage.

In general, competing devices of the same type do not integrate protective FET drive circuits, current sensing, and temperature sensing. This requires the use of separate discrete components to perform these functions. The bq77910's single-chip solution requires only external power FETs and passive components, integrated current sensing and temperature sensing, and internal FETs for battery balancing. There is no need for an external balanced FET, and an external FET can be used to achieve a higher current balance when necessary. The balance function of most other devices requires an external FET and an external microprocessor for battery balance management. Why is the battery balance function so important? Battery cells that do not match inside the battery pack cannot be detected by external contact, and the internal unit mismatch of the battery pack will cause a significant decrease in battery life, and the power imbalance will gradually increase over time, because one of the units fails. Will cause the entire battery pack to fail. Battery systems that use a large number of battery cells are more prone to imbalance due to temperature gradual changes and self-heating of the battery cells at high discharge rates. So this battery balance function is very important for all battery packs.

Lithium batteries have occupied more and more market share due to the increase of discharge rate in recent years, and the application of lithium manganate and lithium iron phosphate batteries in the field of electric vehicles has begun commercialization. BYD is in its own electric vehicle. Using the latter, it is clear that lithium batteries are the general trend of the entire power battery market, and the protection control chip necessary for lithium batteries is a market with great potential. I believe more manufacturers will enter this market and bring more mature products. product.

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