How to balance rechargeable batteries and power management

Portable electronic device designers can choose from a wide variety of chemistries, charger topologies, and charge management solutions. Selecting the most appropriate solution should be a simple task, but in most cases it is a complex process. Designers need to find an optimal balance between performance, cost, form factor, and other key requirements.

This article will provide some guidance and help to designers and system engineers to make the selection work easier.

Start with the 3 “Cs” to achieve charging control

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

1. Battery safety: It goes without saying that end-user safety is the top priority in all system designs. Most lithium-ion (Li-ION) battery packs and lithium-polymer (Li-Pol) battery packs contain protection electronic circuits. 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 tolerance during the final stage of lithium-ion battery charging, a pre-conditioning mode for safely handling deeply discharged batteries, a safety timer, and battery temperature monitoring.

2. Battery capacity: All battery charging solutions must ensure that the battery capacity is fully charged every time and every charging cycle. Premature termination of charging will result in a shortened battery operating time, which is undesirable in 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 number of charge cycles for each battery pack. Using battery temperature and voltage to limit each charge, pre-treating deeply discharged batteries 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

System designers can now choose from a variety of battery chemistries. Designers typically make their choice of battery chemistry based 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 battery chemistry remains a good option for many consumer applications.

Regardless of the battery chemistry chosen, it is critical to follow the correct charge management techniques for each battery chemistry. These techniques will ensure that the battery is charged to its maximum capacity every time and every charge cycle without compromising safety or shortening the battery life.

NiCd/NIMH

Nickel Cadmium (NiCd) and Nickel Metal Hydride (NiMH) batteries must be checked and conditioned before a charge cycle begins, and if possible before starting a fast charge. Fast charging is not allowed if the battery voltage or temperature exceeds the allowed limits. For safety reasons, charging of all "hot" batteries (generally above 45°C) will be temporarily terminated until the battery cools down to the normal operating temperature range. To handle a "cold" battery (generally below 10°C) or an over-discharged battery (usually less than 1V per cell), a gentle trickle current needs to be applied.

Fast charging begins when the battery temperature and voltage are correct. NiMH batteries are usually charged with a constant current at 1C or less. Some NiCd batteries can be charged at rates up to 4C. Use proper charge termination to avoid harmful overcharging.

For 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 -4mV, 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. The typical T/t rate is 1℃/minute.

Figure 1: Charging curves for nickel battery chemistry technology.

Lithium-ion/Lithium-polymer batteries

Similar to NiCd and NiMH batteries, Li-Ion batteries should be inspected and conditioned whenever possible before fast charging. The verification and processing methods are similar to those used above.

As shown in Figure 2, after verification and preconditioning, the Li-ion battery is first charged with a current of 1C or less until the battery reaches its charge voltage limit. This charging stage typically replenishes up to 70% of the battery capacity. The battery is then charged with a constant voltage, typically 4.2V. For safety and battery capacity, the charge voltage must be stabilized to at least 1%. During this charging period, the charge current drawn by the battery gradually decreases. For a 1C charge rate, charging is typically terminated once the current level drops below 10-15% of the initial charge current.

Figure 2: Lithium-ion battery chemistry charging curves.

Switch-Mode vs. Linear Charging Topologies

Traditionally, handheld devices have used a linear charging topology. This approach has many advantages: low implementation cost, design simplicity, and noise-free operation without high-frequency switching. However, the linear topology increases system power consumption, especially when the charging rate increases due to higher battery capacity. This can become a major disadvantage if the designer cannot manage the thermal aspects of the design.

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

This is where switch-mode topology comes in. The main benefit of switch-mode topology is the increased efficiency. Unlike linear regulators, the power switch (or switches) operates in the saturation region, which greatly reduces the overall losses. The main efficiency losses in a buck converter are switching losses (in the power switches) and DC losses in the filter inductor. Depending on the design parameters, it is not uncommon to see efficiencies well above 95% in these applications.

When most people hear the term switch mode they think of big ICs, big PowerFETs and really big inductors! While this is true for applications handling tens of amps, it is not the case for the new generation of solutions for handheld devices. The new generation of single-cell Li-ion switch mode chargers uses the highest levels of chip integration, using frequencies above 1MHz to minimize the inductor size. Figure 1 illustrates one such solution that is available on the market today. The silicon chip measures less than 4mm2 and integrates both the high-side and low-side PowerFETs. With a 3MHz switching frequency, the solution requires a small 1uH inductor with dimensions of only 2mmx2.5mmx1.2mm (WxLxH).

Charger selection

The Battery Charger Tool makes the process of selecting the correct charger easier for designers.