The proper care and feeding of your battery

Counterfeit or aftermarket batteries not held to rigorous standards can cause systems to fail, adversely impacting the original suppliers' reputations, as well as their revenue streams. The highly publicized battery recalls that have cost suppliers hundreds of millions of dollars in recent years help illuminate the significance of the role of battery management in success or failure in the portable systems market today.

The proper care and feeding of your battery subsystem involves being cognizant of a host of battery-related complexities and options for addressing them in the system design process. Not only do system designers need to accommodate varying battery types, address cell imbalance and protect their integrity against counterfeiters, but the realities of the marketplace demand that they do so cost effectively and with the flexibility to respond to changing demands.

So many battery types, so little time...
Battery technology itself poses complications to be managed during the design cycle. The various cell technologies commonly used today, such as rechargeable Nickel Metal Hydride (NiMH) and Lithium-ion (Li-Ion), have different profiles, i.e. different cell voltages and different charging requirements. The multiplicity of profiles makes it difficult to design a single system compatible with different popular battery types.

Another issue related to battery type is the so-called memory effect that has plagued rechargeable batteries, such as Nickel-Cadmium (NiCad) and to a lesser extent NiMH. Memory effect describes a familiar phenomenon to users of portable devices in which a battery that has not been fully discharged prior to recharging fails to subsequently recharge to as high a level and also subsequently discharges more quickly. Memory effect has been a focus for systems designers and battery technologists for years. It is believed that the remedy requires a combination of battery technology as well as intelligence in controlling the charge cycle.

Charge termination also presents several challenges for different battery types. Depending upon the cell technology, termination may need to occur at certain times or under certain conditions. Some cells have timer-related requirements. Yet others can be prone to overheating, so temperature must factor into charge termination. Overcharge can significantly impact battery life and even safety, and for this reason also, termination is a particularly critical system design consideration.

Management of different battery types requires intelligent monitoring and control of the various factors that impact the efficiency and safety of battery operation in a given environment. For example, cell temperature needs to be monitored to avoid overheating. The current profile can be monitored to detect roll-off that occurs when charging is complete. Likewise, voltage can be monitored to detect the so-called "voltage bump" indicating a maximum voltage at which full charge has been attained. In most cases, a combination of these charge termination methods is appropriate.

Because of the many and varied complexities associated with different battery types, a default approach to managing this complexity has been to create a custom or semi-custom battery system for each battery technology of interest. While this approach provides a simple solution for each battery type, there are obvious inefficiencies associated with developing multiple solutions for various technologies, some of which may or may not even see much use in practice.

Instead, a flexible system design that accommodates multiple battery technologies is an important option to consider. A platform-based approach in which a single intelligent battery charging circuit or system, triggered or programmed by a small detection device that identifies battery type, manages charge cycles for multiple types of batteries and affords the most flexibility in system implementation. The platform-based approach gives the system designer the agility to rapidly deploy different battery technologies depending on either end application demands and/or market prices for specific battery types, without the need for redesign. Thus, design effort is leveraged across many potential applications and cell technologies, and return on engineering investment is maximized.

A balancing act
One of the most significant factors that contributes to battery overcharging and hence compromising battery life and risking overheating, is failure to manage cell imbalance. Cell imbalance refers to the different states of charge that exist between different cells in a system. An inevitable attribute of multi-cell systems, cell imbalance occurs as a result of innate differences in charge states, total cell capacities and even cleanliness and uniformity of cell contacts between different cells in a system.

In a system that performs a simple charge of all cells, imbalanced cells will not reach full charge simultaneously, and the unchecked imbalance can have significant negative consequences. Cells at a lower state of charge at the beginning of a charge cycle experience higher voltages during the charge that accelerate cell degradation. Overcharging of cells that achieve a full charge first can induce thermal runaway, introducing the potential for overheating or even ignition. If overcharge mechanisms are in place, imbalanced cells can prematurely terminate a charge. Finally, imbalanced cells can reduce total battery voltage and decrease the efficiency of battery usage.

So, some intelligence must be applied to cell charge management in multi-cell systems so that all batteries attain the same voltage level simultaneously. This involves monitoring, on a cell-by-cell basis, to ensure voltage is equalized across all cells. Once charge is attained on any given cell, that cell must be bypassed so that premature termination and thermal runaway can be avoided.

Several approaches have been applied to the problem of managing cell imbalance. An inductive transformer can be used to transfer charge from the battery to the cell with low charge to balance or even out cell charge. However, inductive transformers are expensive and add size to the overall solution, making them impractical for most consumer applications. Another approach is to deploy a pass transistor in parallel with each cell to enable cell bypass when it reaches full charge. Such current bypass transistors are easy to implement, but lack intelligence, thereby limiting their ability to assure optimal battery management.