Battery and charge management: choice and trade-off factors - Power Circuits - Circuit Diagram

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Keywords: battery management, charge management, power management, nickel-cadmium batteries, NiCd, NiMH, NiMH, PowerFET, Texas Instruments, TI

introduction

Portable electronics designers can choose from a wide range of chemistry technologies, charger topologies, and charge management solutions. Choosing the most appropriate solution should be a very simple task, but in most cases the process is quite complicated. Designers need to find the best balance between performance, cost, form factor and other key requirements. This article will provide some guidance and assistance to designers and system engineers 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 the following three key requirements are met:

1. Battery Safety: Undoubtedly, end-user security is the top priority in all system design. Most lithium-ion (Li-Ion) and lithium-ion (Li-Pol) battery packs contain protective electronic circuitry. However, there are some key factors that system design needs to consider. These include, but are not limited to, ensuring 1% regulation margin during the final phase of Li-Ion battery charging, safe handling of deep discharge battery pre-processing modes, safety timers, and battery temperature monitoring.

2, battery capacity: All battery charging solutions must ensure that the battery capacity can be fully charged every time and every charge cycle. Premature termination of charging can result in reduced battery run time, which is undesirable for today's high power portable devices.

3. Battery life: Following the recommended charging algorithm is an important step to ensure that the end user achieves the maximum charge cycle for each battery pack. Using battery temperature and voltage to define each charge, pre-treating a deep discharge battery, and avoiding late or abnormal charge termination are some of the steps necessary to maximize battery life.

Table 1 Summary of charging control

Choice of battery chemistry technology

System designers can now choose among a variety of battery chemistries. Designers often choose battery chemistry based on some of the following criteria, including:
• Energy density • Specifications and dimensions • Cost • Usage mode and service life

In recent years, despite the increasing trend of using lithium-ion batteries and lithium polymer batteries, Ni battery chemistry is still a good option for many consumer applications.

Regardless of the battery chemistry chosen, proper charge management techniques that follow every battery chemistry are critical. These technologies will ensure that the battery is fully charged at every and every charge cycle without compromising safety or reducing battery life.

NiCd / NIMH

Nickel-cadmium (NiCd) batteries and nickel-metal hydride (NiMH) batteries must be inspected and adjusted before the start of a charge cycle and as much as possible before the start of rapid charging. Fast charging is not allowed if the battery voltage or temperature exceeds the permissible limit. For safety reasons, charging for all "hot" batteries (typically above 45 ï‚°C) will be temporarily terminated until the battery is cooled to the normal operating temperature range. To handle a "cold" battery (typically below 10 ï‚°C) or an over-discharged battery (typically less than 1V per cell), a gentle drip current is required.

Fast charging begins when the battery temperature and voltage are correct. NiMH batteries are typically charged with a constant current of 1C or lower. 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, the typical voltage termination method is peak voltage detection. At the peak, that is, the voltage of each battery is in the range of 0 to -4 mV, fast charging is terminated. The temperature-based fast charge termination method is to observe the battery temperature rise rate T/t to detect full charge. A typical T/t rate is 1 C/min.

Figure 1 Charging curve of nickel battery chemistry