Designing a battery charger for tactical operations

An increase in military spending has provided a moderate boost for manufacturers of ruggedized or mil-spec battery packs. With the recent modernization of the military armament, soldiers are becoming more reliant on mobile power sources. The Land Warrior program provides a soldier system that improves situational awareness and communications for soldiers by connecting them via a wireless network and equipping them with head-mounted displays, computers, and sensors.

This next generation of mobile equipment carried by the "smart soldier" includes several pounds of battery packs. The current estimate is a soldier carries approximately 20 pounds of batteries. One of the objectives of the Land Warrior program is to consolidate multiple battery packs, powering numerous discrete devices down to a few common battery packs. In addition, many government and military procurement groups are leaning more toward rechargeable battery chemistries, driven by recent green initiatives and the total ownership cost of disposable batteries. As the percentage of disposable batteries grows, chargers for military batteries become more important to the survival and success of our military operations. Accordingly, battery capabilities and architectures are key considerations when designing battery chargers for modern military applications.

Current battery capabilities

Before a charger can be architected, one must consider and understand the battery chemistry and characteristics affiliated with the battery pack. Portable rechargeable cell chemistries include Alkaline, Nickel Cadmium (Ni-Cad), Nickel Metal Hydride (NiMH), and Lithium Ion (Li-ion). As presented in Figure 1, Li-ion provides the highest energy/density for portable or mobile applications. Lithium primary cells (called primary due to the one-time, disposable use model) are disposable and designed for single-use applications.

Figure 1 (click graphic to zoom by 1.4x)

Battery technology has kept pace as portable military devices have become more sophisticated and demanding. Today, most military OEMs are turning to smart Li-ion and Lithium Polymer (Li-polymer) battery pack solutions that have embedded intelligence to monitor their own state-of-charge and communicate via serial buses to the portable device. These chemistries offer the highest energy densities currently available and, in the case of Li-ion, a very competitive cost-per-watt-hour for their weight. With operating voltages ranging from 3.6 V to 3.8 V, only one rechargeable lithium chemistry cell is required for a 3 V operating system. In contrast, yesterday's nickel-based technology, which operated at 1.2 V, required three batteries for a 3 V operating system. Although NiMH systems can be configured with up to 10 cells in series to increase voltage, resulting in a maximum aggregate voltage of 12.5 V, Li-ion battery systems can be configured with up to 7 cells in series to increase voltage. This results in a maximum aggregate voltage of 25.2 V.

Multiple architectures for battery chargers

Demands on battery technology have required the use of more reactive materials; therefore, active safety circuits are required to ensure that certain battery chemistries are kept in a stable condition during charge and discharge. Even in less rugged environments, we have witnessed numerous battery failures in the past year that have prompted battery recalls from such vendors as Sony and Matsushita (Panasonic). With careful charger and battery pack design, incidents involving battery rupture or explosion are very rare. However, it should be recognized that under certain charging conditions more likely in battlefield conditions (such as extreme temperatures or punctured batteries), the battery pack integrity can be breached and subsequently expose the user to harmful chemicals or even flames.

Designing battery chargers or docking stations has its own unique set of electrical challenges. One of the first considerations in designing chargers is the trade-off between a dedicated charge control Integrated Circuit (IC) versus a microcontroller-based charging architecture.

Charging systems for military batteries typically have additional features over consumer chargers. An example of a military battery and charging bay is presented in Figure 2. Since military batteries support mission-critical applications, more intelligence is required to provide users with accurate state-of-charge information and expected battery performance. Examples of these features include: charging at high (more than 60 °C) or low (below -20 °C) temperatures, fast charging, state-of-charge indication on the charger (via LED or LCD), and monitoring and limiting the number of charge/discharge cycles for each battery.

Figure 2 (click graphic to zoom by 1.4x)

Architectures with dedicated charger ICs

Dedicated charger ICs are embedded in cell phones, portable digital music players, portable DVD players, PDAs, and many other high-volume consumer products. They are particularly popular for low-power devices that are powered by a single cell Li-ion battery. Almost without exception, one will find an available charge controller that comes fairly close to meeting the fundamental design requirements of a consumer application.

Charging circuits for single cell and 2 cells in series batteries (referred to as 2S) is less complex, and many off-the-shelf charger ICs exist for these configurations. However, when one moves to 3S and 4S military Li-ion packs with higher voltages, there are noticeably fewer controller components available and the implementation becomes more complex, as more overhead parts are required in the charge control circuit. Additionally, low-power alternatives (for example, charge currents of less than 2 A) permit the use of some very elegant dedicated charge ICs that contain power MOSFETs and the digital charger logic array on the same die. Lastly, a design that contains fewer parts has the potential for fewer manufacturing defects and higher MTBF.

However, the challenge with dedicated charging ICs is the minimal features for any given part and the subsequent inflexibility. Even with all the choices of components, finding one component that meets all of a product's requirements is not easy. This includes basic charge features such as waking up over-discharged Li-ion packs, precharging deeply discharged packs, supporting multiple charge termination thresholds for Li-ion batteries, and driving configurable LEDs for charge status indication. Support for advanced functions is more rare; these functions include battery drain/recharge, multi-bay charging with power management, and communicating accurate state-of-charge data with different variations of battery fuel gauge ICs.

Architectures with microcontroller control

An alternate approach introduced by charger manufacturers is a microcontroller architecture with software-controlled charging. Typically, every feature available in a dedicated charger IC can be implemented with a typical low-cost microcontroller. Many features offered via the microcontroller do not require additional hardware with a recurring part cost. Numerous communications protocols (SMBus, I2C, HDQ) are supported with microcontrollers, and the firmware can be tailored to almost any implementation of a battery fuel gauge. One microcontroller can be multiplexed to control more than one charge bay. This is dependent on the number of I/O pins and PWM controllers driven or supported by the microcontroller, but generally three or four bays can be managed with one microcontroller. For multibay chargers, this can result in significant cost savings over step-and-repeat implementations of a dedicated charge IC. Finally, microcontrollers with flash memory can also support field upgradeability for changing battery chemistries, supporting new styles of battery packs, or adding additional features after initial release.

In addition to the aforementioned features, other examples of custom, differentiated charging features enabled by microcontrollers include:

• High temperature charging ‚Äì Microcontrollers can vary the charge current to minimize temperature rise when the ambient temperature is already high.
• User notification through LED or LCD ‚Äì Any color and pattern can be implemented with LEDs. State-of-charge indication is easily and inexpensively conveyed to users with a four- or five-segment LED or displayed on an LCD. Although most customers choose multicolor LEDs, the LEDs can be flashed or combined to indicate various charge states and error conditions.
• Gas gauge management and recalibration ‚Äì Gas gauges can drift over time, and report inaccurate state-of-charge information. Microcontrollers can compare actual state-of-charge status (determined by progress in CC-CV charge cycle) to state-of-charge data (provided through the communications bus to battery).
• Wake up battery in sleep mode ‚Äì Li-ion batteries will go into sleep mode if they are forced into an under-voltage condition. A micro-controller can check battery voltage, and if under voltage, can properly recover an over-discharged battery by applying a zero-voltage charge current.
• Limit the number of cycles for a pack ‚Äì Military OEMs may wish to limit a battery's number of charge/discharge cycles so an overstressed battery does not go into the field. Once a battery exceeds a predetermined limit, the charger can refuse to charge the pack or alert the user.

Functionality and complexity drive architecture decision

As one considers which type of internal charger architecture to implement when designing a battery charger, one will conclude that dedicated charger ICs are best suited for fixed function, smaller, battery packs with stable temperature environments. When the battery pack configurations or charging environments expand beyond these optimal conditions, microcontroller architectures allow battery chargers to maximize operational readiness and battery performance, ensuring a refreshed battery provides maximum power and runtime when it is deployed in a harsh field environment. Vendors such as Micro Power are stepping up to the plate, providing a wealth of consulting, design, and manufacturing experience with battery chargers for use in rugged environments.

Jeffrey VanZwol is a senior marketing executive with more than 18 years of experience in all aspects of technical marketing, including product management, product marketing, marketing communications, and partner management. He has extensive presentation experience on a wide variety of technical topics and has presented at conferences such as Advancements in Battery Technology and Power Management conference, Power Systems World, and the Military Technology Conference. Jeffrey earned a Bachelor of Science degree in Computer Science from Saint Francis Xavier University and a Master's degree of Business Administration from McGill University. He can be contacted at jvanzwol@micro-power.com.