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Charging System Design: Custom Versus Off-The-Shelf

Mon, 11/01/2010 - 12:21pm
Marko Dimitrijevic, International Components Corporation, www.iccus.com

Advances in lithium-ion battery technology have created new opportunities for the development of portable devices. As these new devices demand more performance and longer battery life, newer and more innovative methods of battery management must be developed and adopted. While efficient charging is a key element of battery management, proper charging methods and charging management can be difficult to achieve.

A well designed charging solution must consider many technical factors. Charging location, cell cycle life, charge time, long term capacity maximization, development time, communication, and multi-bay requirements must all be taken into consideration. All of these must be balanced with the overall costs of the system, both up-front development costs and long-term production costs.

We believe that a custom designed charging solution, as opposed to an off-the-shelf charging circuit, is well worth the extra time and effort required for development. The benefits accrue in four key areas:

1. Battery Management: Charging Time, Accuracy, Cycle Life, and Cell Capacity
2. Communications / Authentication
3. Multi-Bay Chargers
4. Development Time

Custom Micro-Processor Versus Off-The-Shelf Integrated Circuits
Design engineers can opt to develop the charging system in-house using off-the-shelf charger integrated circuits (ICs).  Many commercially available lithium-ion chargers are readily available, along with detailed application notes and sample designs.

A standard charging solution may provide the quickest and most accessible development cycle, but it will not necessarily provide the greatest system capability or the most cost effective long-term production. In high production volumes, this type of solution tends to be quite costly. For example, if the battery needs to function as a sub-system by taking cues and communicating with the host device, the added IC feature set can increase the cost of the subsystem by a dollar or more as compared to a similarly featured custom design. At high production volumes typical of consumer and prosumer devices, this can significantly reduce a company’s profit margin over the product’s lifespan.

Design engineers could instead elect to partner with a company that specializes in custom microprocessor-based chargers, such as International Components Corporation. Not only will the custom solution provide better functionality, but the lack of unneeded features and elimination of the “one size fits all” design philosophy can provide significant cost savings over the product life cycle.

There are several technical benefits to a customized microprocessor-based charger. Even the most basic processor can be configured to operate as a switch-mode-regulator-based charger, either synchronous or asynchronous. A single device can support multiple charging circuits simultaneously, inherently simplifying the system and reducing component count. In addition to providing the charging task, microprocessor-based systems can be custom-configured to achieve a number of additional system interfaces and power management activities. Additional thermal management features can extend the performance of the battery pack, whereas normal packs are subject to degradation from elevated temperatures. The only negative aspect of this custom-tailored approach is the somewhat extended development time.

Here is a closer look at how these two design options fare in specific technological areas.

Charging Time, Accuracy, Cycle Life and Cell Capacity
Lithium-ion cycle life and long term cell capacity are influenced by cell charging and discharging dynamics. Proper charging will increase battery cycle life, allow for better capacity utilization, and enable safe use of lithium-ion cells. 

Maintaining precise charging voltage, for example, can enhance the overall battery pack’s long-term performance. Charging to 4.1 V per cell has been shown to increase the cycle life of lithium-ion cells. Most off-the-shelf charging regulators only provide a 50 mV output tolerance, where microprocessor-based solutions could effectively achieve 10mV accuracy. This ensures that a battery pack will be charged according to designer’s best intentions and will provide more reliable long-term performance.

Off-the-shelf solutions can be easily configured for lower voltage charging, but the general lack of output regulation accuracy will affect long term performance. Even best attempts to compensate the feedback network loop will in most cases yield marginal improvements to output tolerance, as comparator circuits and compensation are generally enclosed within the IC package. The net result is loss of charging accuracy.

Another benefit of microprocessor-based charging solutions is the potential provision for greater user interface. The end user can be provided with options to forego reduced-voltage charging and charge at the maximum permissible potential (per cell). In addition, the charger could be prompted to allow a maximum permissible charge rate. These options are not available in off-the-shelf components.

Custom charging systems can also adjust and optimize performance in response to variations in the available input power. This would be very beneficial in multi-bay or multi-pack chargers where input power is typically limited. For example, dynamic charging algorithms are especially useful in the field of portable medical devices which are often required to allow for quick or optimal charging.

It should be noted that the features described above might still be available through off-the-shelf solutions, but the costs of the relevant components would often become prohibitive for mass production.

Authentication
Battery packs are often required to communicate with the host system through protocols such as the System Management Bus (SMBus). In addition to standard parametric communication, authentication may be required. This is especially important in systems with on-board chargers and portable, point-of-load UPS devices.

The majority of off-the-shelf chargers are capable of standardized SMBus communication protocols.  However, more sophisticated authentication protocols, such as SHA-(X), often require auxiliary components which increase complexity, part count, and cost.

A simple microprocessor, however, can easily be configured to support both standard communication and authentication protocols as needed. Unifying these features, along with the charging power stages, yields greater versatility, reliability and lower overall system cost.

Multi Bay Chargers
Battery packs are often charged externally in multi-bay systems. This means that each bay must be controlled according to its own conditions and needs. Utilizing off-the-shelf solutions can mean allocating a separate charger IC per bay. If the bays require any level of interdependency or compensation for available power, auxiliary circuits must be added. Therefore, system complexity and cost go up dramatically.

A single microprocessor, however, can manage the entire charging platform, even in settings with six to eight charging bays. Furthermore, the system could be configured to accommodate simultaneous or sequential charging as well as complex bay management. All of these tasks can be effectively managed by a single microprocessor that costs less to produce than each one of the standard charger ICs in the off-the-shelf approach.

The following block diagrams illustrate two solutions for a four-bay charger with host system communication and authentication requirements. The discrete, off the shelf solution requires individual charge regulator and authentication components for every bay (Figure 1). A microprocessor solution is clearly simpler and provides a significant Bill of Material reduction (Figure 2).

Figure 1. Charging using an off-the-shelf IC.

A microprocessor-based system can achieve equivalent functionality with a single micro-controller (Figure 2):

Figure 2. Charging using a microprocessor.

Another technological challenge for a charging system is daisy-chaining. Manufacturers of industrial battery-operated products are increasingly requiring charging systems to be daisy-chained in order to accommodate system expansion. This allows end users to “grow” their systems as their business and financial situations change. In many instances, this expansion may be required without the additional expense of increased input power.

Manufacturers may want to leverage the input power overhead of smaller charger systems and creatively allow daisy-chaining without passing additional costs onto their customers.  Over the counter designs often rely on pre set parameters and are unable to compensate for variations in input power. If the input power is reduced beyond a certain threshold, the charger circuits could simply stop functioning.

A microprocessor-controlled charger platform, however, could effectively compensate for the input power challenge by dynamically altering the charging component of individual bays to accommodate fast charging of critical bays, First-In-First-Out Requirements, or even throttle down to enable parallel charging.

Development Time
The above sections have provided several examples of the technical superiority of a customized solution as compared to an off-the-shelf design. But lingering questions remain. How much up-front development time will all this require? And is it worthwhile in the long run?

Along with aggressive cost targets and complex system interface requirements, developers of battery-powered systems are increasingly demanding rapid development of battery packs. This often becomes a challenge for internal engineering departments that may not have the necessary battery management experience.

Partnering with a company that specializes in the development of rechargeable systems will provide the most ideal solution. A partner that specializes in both battery pack and charger development would work on the design in parallel with internal system design efforts, significantly reducing the duration of the development cycle. Partner companies can also provide economies of scale and established vendor relationships resulting in lower component costs. Experienced companies that utilize sophisticated technologies, such as microprocessor based designs, can provide a cost advantage and the most accelerated development cycle.

Conclusion
We have reviewed the aspects of two design approaches for lithium-ion battery charging systems, off-the-shelf integrated circuits versus a custom microprocessor-based system. Our experience indicates that a custom-designed solution is superior in all technical aspects. The technical advantages include more customizable and flexible charging profiles, ability to monitor and respond to ambient temperature changes, improved communication with the host system, and the ability to support multi-bay charging and daisy-chained systems. Rejection of the “one size fits all” philosophy of off-the-shelf components reduces part count, complexity, and most importantly, unit production cost. Finally, while the design of customized charging systems often requires significant development effort, this effort can be off-loaded to a partner company and performed in parallel with other design efforts, hence shortening the total product development cycle and getting your product to market much quicker.

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