AAA NiMH batteries

release time:2024-06-13 Hits: Popular:AG11 battery

Why is it so important to correctly select and apply the AAA NiMH batteries technology in portable electronic equipment?

Preface

Today's AAA NiMH batteries technology in portable electronic equipment includes several aspects, such as power detection algorithm, AAA NiMH batteries charging algorithm and AAA NiMH batteries charging technology. As we all know, there are four types of rechargeable AAA NiMH batteries chemical reactions: nickel-cadmium, nickel-metal hydride, lithium-ion and lithium polymer. As for portable electronic devices, although these four AAA NiMH batteries types have their own characteristics, from the development and practice of energy density and safety, it can be seen that the advantages of lithium-ion batteries and lithium polymer batteries have become the ideal choice for small and long-running devices, such as laptops and hard-drive-based PMPs. For portable electronic equipment engineers, it is very important to correctly select and apply the AAA NiMH batteries technology in portable electronic equipment. This article will discuss this and analyze application examples.

1. AAA NiMH batteries charging algorithms for trickle charging, fast charging and stable charging

Depending on the energy requirements of the final application, a AAA NiMH batteries pack may contain up to 4 lithium-ion or lithium-polymer AAA NiMH batteries cells, and its configuration can be varied, with a mainstream power adapter: direct adapter, USB interface or car charger. Apart from differences in the number of cells, cell configuration or power adapter type, these AAA NiMH batteries packs have the same charging characteristics. Therefore, their charging algorithms are also the same. The best charging algorithm for lithium-ion and lithium-polymer batteries can be divided into three stages: trickle charge, fast charge and steady charge.

* Trickle charge. Used to charge deeply discharged cells. When the cell voltage is below about 2.8V, it is charged with a constant current of 0.1C.

* Fast charge. When the cell voltage exceeds the trickle charge threshold, the charge current is increased for fast charge. The fast charge current should be less than 1.0C.

* Steady voltage. During fast charge, once the cell voltage reaches 4.2V, the steady voltage phase begins. At this time, the charging can be interrupted by the minimum charge current or timer or a combination of the two. When the minimum current is less than about 0.07C, the charging can be interrupted. The timer relies on a preset timer to trigger the interrupt.

Advanced AAA NiMH batteries chargers usually have additional safety features. For example, if the cell temperature exceeds a given window, usually 0℃--45℃, the charging will be suspended. Except for some very low-end devices, the current lithium-ion/lithium polymer AAA NiMH batteries charging solutions on the market are integrated or have external components to charge according to the charging characteristics, which is not only for better charging effect, but also for safety.

2. Lithium-ion/polymer AAA NiMH batteries charging solution

The charging solution for lithium-ion/polymer batteries is different for different numbers of cells, cell configurations and power types. There are currently three main charging solutions: linear, Buck (step-down) switch and SEPIC (step-up and step-down) switch.

2.1 Linear solution

When the charger input voltage is greater than the open circuit voltage after a fully charged cell plus sufficient clearance, it is best to use a linear solution, especially when the 1.0C fast charging current is not much greater than 1A. For example, an MP3 player usually has only one cell, with a capacity ranging from 700 to 1500mAh, and a fully charged open circuit voltage of 4.2V. The power supply of the MP3 player is usually an AC/DC adapter or a USB interface, and its output is a regular 5V; at this time, a linear charger is the simplest and most efficient solution. Figure 2 shows a linear solution for charging lithium-ion/polymer batteries. The basic structure is the same as that of a linear voltage regulator.

* Example of charger application for linear solution - dual-input Li+ charger and smart power selector MAX8677A. MAX8677A is a dual-input USB/AC adapter linear charger with built-in SmartPowerSelector for portable devices powered by rechargeable single-cell Li+ batteries. The charger integrates all power switches required for charging and switching loads of batteries and external power, so no external MOSFET is required. MAX8677A is ideal for portable devices such as smart phones, PDAs, portable multimedia players, GPS navigation devices, digital cameras, and digital video cameras.

MAX8677A can operate from independent USB and AC adapter power inputs or from either input. When an external power source is connected, the smart power selector allows the system to be disconnected from the AAA NiMH batteries or can be connected to a deeply discharged AAA NiMH batteries. The smart power selector automatically switches the AAA NiMH batteries to the system load, using the unused input power portion of the system to charge the AAA NiMH batteries, making full use of the limited USB and adapter input power. All required current sensing circuits, including the integrated power switch, are integrated on-chip. The DC input current limit is adjustable up to 2A, while both the DC and USB inputs support 100mA, 500mA, and USB suspend mode. The charge current is adjustable up to 1.5A to support a wide range of AAA NiMH batteries capacities. Other features of the MAX8677A include thermal regulation, overvoltage protection, charge status and fault outputs, power-good monitoring, AAA NiMH batteries thermistor monitoring, and a charge timer. The MAX8677A is available in a space-saving, thermally enhanced, 4mm×4mm, 24-pin TQFN package and is specified over the extended temperature range (-40 to +85°C).

2.2Buck (step-down) switching solution

When the 1.0C charging current is greater than 1A, or the input voltage is much higher than the fully charged open-circuit voltage of the AAA NiMH batteries cell, a buck or step-down solution is a better choice. For example, in hard drive-based PMPs, single-cell lithium-ion batteries are commonly used, with a fully charged open-circuit voltage of 4.2V and capacities ranging from 1200 to 2400mAh. Today, PMPs are usually charged with car kits, which have an output voltage between 9V and 16V. The relatively high voltage difference between the input voltage and the AAA NiMH batteries voltage (minimum 4.8V) will reduce the efficiency of the linear solution. This inefficiency, coupled with a 1C fast charging current greater than 1.2A, will cause serious heat dissipation problems. To avoid this, the Buck solution must be used. Figure 3 is a schematic diagram of the Li-ion/Polymer AAA NiMH batteries Buck charger solution, and the basic structure is exactly the same as the Buck (step-down) switching voltage regulator.

2.3 SEPIC (boost and step-down) switching solution

In some devices that use 3 or even 4 Li-ion/Polymer cells in series, the charger input voltage is not always greater than the AAA NiMH batteries voltage. For example, a laptop uses a 3-cell Li-ion AAA NiMH batteries pack with a fully charged open circuit voltage of 12.6V (4.2Vx3) and a capacity ranging from 1800mAh to 3600mAh. The input power is either an AC/DC adapter with an output voltage of 16V or a car kit with an output voltage between 9V and 16V. Obviously, neither the linear nor the Buck solution can charge this AAA NiMH batteries pack. This requires the use of the SEPIC solution, which can work when the output voltage is higher than the AAA NiMH batteries voltage and when the output voltage is lower than the AAA NiMH batteries.

3. Power detection algorithm

Many portable products use voltage measurements to estimate the remaining AAA NiMH batteries power, but the relationship between AAA NiMH batteries voltage and remaining power changes with discharge rate, temperature and AAA NiMH batteries aging, making this method error rate up to 50%. The market demand for products with longer service life continues to increase, so system designers need more accurate solutions. Using a fuel gauge to measure the amount of power charged or consumed by the AAA NiMH batteries will provide more accurate AAA NiMH batteries power estimates over a wide range of application power levels.

3.1 Example of the application of the fuel detection algorithm: a fully functional single/dual AAA NiMH batteries portable application AAA NiMH batteries pack design

* Principle of fuel detection. A good fuel gauge should at least have AAA NiMH batteries voltage, AAA NiMH batteries pack temperature and current, measurement method; a microprocessor 9a; and a set of industry-proven fuel detection algorithms. The bq2650x and bq27x00 are full-featured fuel gauges that feature an analog-to-digital converter (ADC) for measuring voltage and temperature and an analog-to-digital converter for measuring current and charge sensing. These fuel gauges also feature a microprocessor that executes TI's fuel gauge algorithms. These algorithms compensate for self-discharge, aging, temperature, and discharge rate of lithium-ion batteries. The on-chip microprocessor offloads these computational burdens from the host system processor. The fuel gauge provides information such as the remaining charge status, and the bq27x00 family also provides the remaining run time (Run Time to Empty). The host can query the fuel gauge for this information at any time and then notify the user of the AAA NiMH batteries information through an LED indicator or screen display. The fuel gauge is very easy to use, and the system processor only needs to configure an I2C or HDQ communication driver.

*AAA NiMH batteries pack circuit description. Figure 4 (a) shows a typical AAA NiMH batteries pack application circuit that can be used with an identification IC. Depending on the fuel gauge IC used, the AAA NiMH batteries pack requires at least three to four external terminals. The VCC and BAT pins are connected to the AAA NiMH batteries voltage to power the C and measure the AAA NiMH batteries voltage. A small sense resistor is connected to the AAA NiMH batteries ground terminal, allowing the high impedance SRP and SRN inputs of the fuel gauge to monitor the voltage across the sense resistor. The current flowing through the sense resistor can be used to determine the amount of charge being charged or discharged from the AAA NiMH batteries. Designers must consider that the voltage across the resistor cannot exceed 100mV when selecting the sense resistor value. Too low a resistance value may cause errors at low currents. The board layout must ensure that the connections from SRP and SRN to the sense resistor are as close as possible to the sense resistor terminals; in other words, they should be Kelvin connections.

The HDQ pin requires an external pull-up resistor, which should be located on the host or main application side so that the fuel gauge can enable the sleep function when the AAA NiMH batteries pack is disconnected from the portable device. The recommended pull-up resistor value is 10kΩ.

* AAA NiMH batteries pack authentication. There is a growing problem of cheap counterfeit batteries that may not include the safety protection circuitry required by the OEM. Therefore, genuine AAA NiMH batteries packs can include the authentication circuit shown in Figure 4(a). When the AAA NiMH batteries is to be authenticated, the host sends a challenge to the AAA NiMH batteries pack containing the IC (bq26150, which functions as a cyclic redundancy check (CRC)). The CRC contained in the AAA NiMH batteries pack will calculate the CRC value based on the challenge value and the CRC polynomial built into the IC. The CRC is based on the host's query command and the CRC polynomial defined secretly in the IC. The host will also calculate the CRC value and compare it with the calculation result of the AAA NiMH batteries pack to determine whether the authentication is successful. Once the AAA NiMH batteries is authenticated, the bq26150 will issue a command to ensure that the data line communication between the host and the fuel gauge is normal. When the AAA NiMH batteries connection is disconnected or reconnected, the entire authentication process will be repeated.

3.2 Example 2 of the application of fuel detection algorithm, a new IC that can be applied to various general fuel gauges.

Many manufacturers today can provide a wide range of fuel gauge ICs, from which users can select the appropriate functional devices to optimize the cost performance of the product. Using the AAA NiMH batteries parameters measured by the fuel gauge, this split architecture allows users to customize the fuel measurement algorithm in the host. This saves the cost of the AAA NiMH batteries pack embedded processor. Here, we take the DS2762 chip of Dallasesemicconductor as an example for typical analysis. A new type of discrete fuel gauge IC, its structure is shown in Figure 5(a).

*Application features of DS2762

DS2762 is a single-cell lithium AAA NiMH batteries fuel gauge and protection circuit integrated in a tiny 2.46mm×2.74mm flip-chip package. Due to the internal integration of high-precision resistors for power detection, this device is very space-saving. Its small size and unparalleled high integration make it ideal for mobile phone AAA NiMH batteries packs and other similar handheld products such as PDAs. The integrated protection circuit continuously monitors the AAA NiMH batteries for overvoltage, undervoltage and overcurrent faults (during charging or discharging). Unlike independent protection ICs, DS2762 allows the host processor to monitor/control the conduction state of the protection FET, so that system power control can be achieved through the protection circuit of DS2762. DS2762 can also charge a deeply depleted AAA NiMH batteries, providing a current-limited recovery charging path when the AAA NiMH batteries voltage is less than 3V.

The DS2762 can accurately monitor AAA NiMH batteries current, voltage, and temperature, and its dynamic range and resolution meet the test standards of any popular mobile communication product. The measured current integrates the internally generated time base to achieve power measurement. The accuracy of power measurement is improved through real-time and continuous automatic offset correction. The built-in detection resistor eliminates resistance changes caused by manufacturing process and temperature, further improving the accuracy of the power meter. Important data is stored in a 32-byte, lockable EEPROM; 16 bytes of SRAM are used to store dynamic data. All communications with the DS2762 are carried out through the 1-Wire, multi-node communication interface, minimizing the connection between the AAA NiMH batteries pack and the host. Its main features are: single-cell lithium AAA NiMH batteries protector; high-precision current (fuel metering), voltage and temperature measurement; optional integrated 25mΩ sense resistor, each DS2762 is individually fine-tuned; 0V AAA NiMH batteries recovery charging; 32 bytes of lockable EEPROM, 16 bytes of SRAM, 64-bit ROM;

1-Wire, multi-node, digital communication interface; support for multi-AAA NiMH batteries power management, and system power control through protection FET; power supply current is only 2?A (maximum) in sleep mode; power supply current is 90?A (maximum) in working mode; 2.46mm×2.74mm flip chip package or 16-pin SSOP package, both can be selected with or without sense resistor; evaluation board is available.

4. Conclusion

Applying good AAA NiMH batteries technology for portable electronic equipment is the basis for selecting lithium-ion batteries and lithium polymer batteries and their chargers. As for how to make the right choice, it must also depend on the specific requirements of portable electronic equipment.

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