Rechargeable batteries in general, and lithium-ion batteries in particular, are empowering the modern age of smart phones, wearable devices and portable applications, making battery management a critical element of good product design.
Although rare, it is not unknown for portable devices to suffer from overheating, which can often lead to total failure of the device and, at worst, potential threat to life. While manufacturers seek to avoid such issues, or to remedy them when they do occur via software or hardware updates, at times this is impossible and can lead to costly product recalls.
Although the cause of overheating may not be directly linked to battery management, it is obvious that the source of heat is fueled entirely by the battery’s capacity to deliver energy. For this reason, managing single-cell batteries in portable devices should be a high priority for OEMs.
Getting more from less
Devices are getting smaller, processors are getting faster and user expectations are getting higher. In many ways these trends are contradictory yet they prevail because OEMs are able to sustain them. But each one puts more pressure on battery manufacturers to package more power into ever-smaller dimensions, by pushing boundaries and encouraging innovation. This in turn is helping fuel demand in the end-markets, specifically smart phones and, more recently but no less aggressively, wearable devices.
The kind of innovation happening in battery technology includes new form factors, 3D printed materials and the exploration of new chemical compounds. Batteries are now more flexible, can be manufactured to conform to specific contours or volumes, while still delivering the power needed to keep our devices running for longer on a single charge.
All of this innovation cannot completely eliminate the fundamental requirement for good battery management, however. Any battery technology based on a chemical process will inevitably require a solution to managing its charge and discharge, with particular emphasis on the main threat to – and from – battery technology; thermal extremes.
The chemicals in battery cells do not react well to high temperatures, it accelerates the chemical process taking place inside the cell, leading to decreased performance and in the worst cases explosion or fire. Apart from external atmospheric conditions, several (avoidable) conditions can result in elevated battery temperatures. These include short-circuiting the terminals and unregulated charge/discharge current. Measures should be taken to mitigate the possibility of a short across terminals, at the very least by using thermal/electrical fuses, or diodes, but a better solution would involve using one of the application-specific battery management devices now entering the market, which are specifically designed to detect and prevent conditions that could damage single-cell rechargeable batteries.
Battery cells will have strict parameters for charge and discharge currents. Under discharge conditions, allowing the cell to drop below a minimum recommended voltage should also be avoided. Failure to take measures to avoid dropping below a low voltage limit could result in physical damage to the cell’s anode base metal. Similarly, over voltage protection should be included, as this too can lead to irrecoverable damage to the cell. During a charge cycle, the cell is at risk from damage if the manufacturer’s current and voltage recommendations are not observed.
Charge, deplete, repeat
Charging a lithium-based rechargeable cell is comparatively simple, with respect to other rechargeable battery types such as those based on nickel cadmium or nickel metal hydride. That’s because it essentially requires a constant current at a relatively low voltage at the beginning (assuming the cell is fully discharged) and a constant voltage/trickle current as it nears its maximum charge level.
Achieving this in practice requires the charge voltage/current to conform to the manufacturer’s parameters, however. This will impose quite tight tolerances on the voltage and current at the end of the cycle, as well as a minimum charge voltage during the main period of charging. The current will also be specified, often as a function of the cell’s capacity and the time needed to fully charge it under ideal conditions. This is known as the C-rate and is used to express the charge and discharge characteristics of a battery.
Although not directly relevant to a cell’s performance, the design of the charger will also have an influence on charge cycles and overall battery maintenance. It must consider how using a battery as a load will influence its behaviour, as well as how the charger’s characteristics will need to meet the battery’s requirements. This is why manufacturers will always recommend using the correct charger for a device.
Other design considerations exist for devices with multiple cells, such as cell balancing, which can further complicate design. Many of the earlier devices that used lithium ion batteries would have needed multiple cells, such as laptops. However, energy density and manufacturing techniques, coupled with lower overall system power requirements, means that the majority of modern portable and wearable devices can generally be powered from a single cell.
With a wealth of experience in managing the complexities of multi-cell designs, OEMs could easily overlook or underestimate the fact that single cell devices still require proactive cell management and protection from the numerous conditions that could manifest themselves.
Making safety inherent
While generating the charge voltage and current will be the remit of a power circuit (typically a dc/dc converter), monitoring the output of the charge circuit as it is applied to the battery terminals should be entrusted to a separate functional block.
This would include detecting an over-voltage and/or over-current condition during charging, as well as monitoring for possibly dangerous conditions during normal operation, such as short-circuits across the terminals. As stated earlier, over-discharge is also detrimental to a battery cell’s general performance and health, so any monitoring solution should also check for this condition. Adding this level of functionality into portable and wearable devices is challenging for several reasons. From an engineering point of view, it will require extra space and system power; from a commercial point of view it will add cost to the Bill of Materials. Any chosen solution needs to be small, low power and low cost.
There are now a number of devices on the market that meet these requirements. The AP9234L family from Diodes is a single-chip solution for single cell lithium ion management in smart phones, wearable devices and other portable applications. It integrates a highly accurate battery protection circuit with dual N-channel MOSFETs with ultra-low Rss(on). The device itself has been designed to operate at an extremely low quiescent current of 3.0 μA (typically), and even features a power-down mode that reduces quiescent current to 0.01 μA. It is available in a package measuring just 2.5 by 3.5 mm and 0.5-mm high.
As illustrated by the functional block diagram in Figure 1, the device is designed to control the charging and discharging of a cell. During normal conditions, the MOSFET allows normal charge/discharge operation, however it constantly monitors the system to detect possible fault conditions, at which time it will isolate the cell from the rest of the system, through the internal MOSFETs.
Fig. 1: Functional block diagram of Diodes’ AP9234 battery protection IC
Conditions that can be monitored and detected include over-charge voltage, over-discharge voltage, over-charge current and over-discharge current. If the conditions exist for more than a predetermined time, measured by the integrated Delay Timer Circuit (Fig. 1), the device enters a fault mode and takes the necessary action to prevent fault escalation.
The inexorable trends towards smaller devices, typified by the latest wearable devices, will ensure that OEMs must continue to push the boundaries of product development. As rechargeable cell technology strives to keep pace and enable these trends it will inevitably put pressure on developers to integrate more functionality in to ever-smaller form-factors. Through the development of highly integrated solutions, companies like Diodes are ensuring that battery management isn’t relegated to the ‘optional’ list, especially as it plays such an important role in overall product safety, reliability and integrity.