NiMH No. 7 battery

　　How to find the best balance point in NiMH No. 7 batteryand charging management

　　Portable electronic device designers have a wide variety of chemistries, charger topologies, and charge management solutions to choose from. Choosing the most suitable solution should be a simple task, but in most cases the process is complex. Designers need to find an optimal balance between performance, cost, form factor, and other key requirements. This article will provide designers and system engineers with some guidance and help to make this selection easier.

　　Start charging control with 3 “C”

　　All system designers using rechargeable batteries need to be aware of some basic design techniques to ensure that three key requirements are met:

　　1. NiMH No. 7 batterysafety: There is no doubt that end-user safety is the highest priority in all system designs. Most lithium-ion (Li-Ion) and lithium-polymer (Li-pol) NiMH No. 7 batterypacks contain protective electronic circuitry. However, there are some key factors that need to be considered in system design. These include, but are not limited to, ensuring a ±1% voltage regulation margin during the final stages of lithium-ion NiMH No. 7 batterycharging, a preconditioning mode to safely handle deeply discharged batteries, a safety timer, and NiMH No. 7 batterytemperature monitoring.

　　2. NiMH No. 7 batterycapacity: All NiMH No. 7 batterycharging solutions must ensure that the NiMH No. 7 batterycapacity is fully charged every time and every charging cycle. Terminating charging prematurely can result in reduced NiMH No. 7 batteryruntime, which is undesirable in today's power-hungry portable devices.

　　3. NiMH No. 7 batterylife: Following the recommended charging algorithm is an important step in ensuring the end user achieves the maximum number of charge cycles per NiMH No. 7 batterypack. Limiting each charge using NiMH No. 7 batterytemperature and voltage, preconditioning deeply discharged batteries, and avoiding late or abnormal charge termination are some of the steps necessary to maximize NiMH No. 7 batterylife.

　　NiMH No. 7 batteryChemistry Technology Selection

　　System designers can now choose from a variety of NiMH No. 7 batterychemistry technologies. Designers typically base their selection of NiMH No. 7 batterychemistry technology on a number of criteria, including:

　　*Energy Density

　　*Specifications and dimensions

　　*cost

　　*Usage patterns and service life

　　Although the trend toward using lithium-ion and lithium-polymer batteries has increased in recent years, Ni NiMH No. 7 batterychemistry remains a good option for many consumer applications.

　　Regardless of the NiMH No. 7 batterychemistry chosen, it is critical to follow the correct charge management techniques for each NiMH No. 7 batterychemistry. These technologies will ensure that batteries are charged to their maximum capacity every time and every charge cycle without compromising safety or shortening NiMH No. 7 batterylife.

　　NiCd/NIMH

　　Nickel-cadmium (NiCd) and nickel-metal hydride (NiMH) batteries must be inspected and conditioned before starting a charge cycle, and if possible before starting fast charging. If the NiMH No. 7 batteryvoltage or temperature exceeds the allowable limit, fast charging is not allowed. For safety reasons, charging of all "hot" batteries (generally above 45°C) will be temporarily stopped until the NiMH No. 7 batterycools down to the normal operating temperature range before it can be operated again. To deal with a "cold" NiMH No. 7 battery(typically below 10°C) or an over-discharged NiMH No. 7 battery(typically below 1V per cell), a gentle trickle of current needs to be applied.

　　Fast charging begins when the NiMH No. 7 batterytemperature and voltage are correct. NiMH batteries are usually charged with a constant current of 1C or less. Some NiCd batteries can be charged at rates up to 4C. Use proper charge termination to avoid harmful overcharging.

　　In the case of nickel-based rechargeable batteries, fast charge termination is based on voltage or temperature. As shown in Figure 1, a typical voltage termination method is peak voltage detection. At the peak, when the voltage of each NiMH No. 7 batteryis in the range of 0~-4mV, fast charging is terminated. The temperature-based fast charge termination method is to observe the NiMH No. 7 batterytemperature rise rate to detect full charge. A typical rate is 1°C/minute.

　　Lithium-ion/lithium-polymer battery

　　Similar to NiCd and NiMH batteries, lithium-ion batteries should be inspected and conditioned as much as possible before fast charging. The verification and processing methods are similar to those used above.

　　After verification and preconditioning, charge the lithium-ion NiMH No. 7 batterywith a current of 1C or less until the NiMH No. 7 batteryreaches its charging voltage limit. This charging phase typically replenishes up to 70% of NiMH No. 7 batterycapacity. The NiMH No. 7 batteryis then charged with a constant voltage, usually 4.2V. To combine safety and NiMH No. 7 batterycapacity, the charging voltage must be stabilized at at least ±1%. During this charging period, the charging current drawn by the NiMH No. 7 batterygradually decreases. For 1C charge rates, charging typically terminates once the current level drops below 10 to 15% of the initial charge current.

　　Comparison of switch mode and linear charging topologies

　　Traditionally, handheld devices have used linear charging topologies. This approach offers many advantages: low implementation cost, simplicity of design, and noiseless operation without high-frequency switching. However, linear topology increases system power consumption, especially when charging rates increase due to higher NiMH No. 7 batterycapacity. This can become a major disadvantage if the designer cannot manage the thermal issues of the design.

　　When the PCUSB port is used as a power source, some other disadvantages arise. USB charging options are available on many portable designs today, and all offer charging rates up to 500mA. In the case of linear solutions, due to their lower efficiency, the amount of "power" that can be transferred from pCUSB is greatly reduced, resulting in long charging times.

　　This is why switch-mode topology comes into play. The main advantage of switch-mode topology is the increase in efficiency. Unlike linear regulators, the power switch (or switches) operates in a saturated region, which greatly reduces overall losses. The main power losses in a buck converter include switching losses (in the power switch) and DC losses in the filter inductor. Depending on the design parameters, it is not surprising to see efficiencies well above 95% in these applications.

　　When people hear the term switch mode most people think of large ICs, large powerFETs, and very large inductors! In fact, while this is true for applications handling tens of amps of current, the new generation solutions for handheld devices The situation is different in terms of plans. The new generation of single lithium-ion switch mode chargers adopts the highest level of chip integration and operates at a frequency higher than 1MHz to minimize inductor size. Figure 1 illustrates such solutions that are already on the market today. The silicon chip measures less than 4mm2 and integrates high-side and low-side powerFETs. Due to the 3MHz switching frequency, this solution requires a small 1μH inductor with dimensions of only: 2×2.5×1.2mm (WxLxH).

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