Innovations bring higher energy density to miniature electronics devices

Miniature portable devices, such as wireless Bluetooth headsets, are marvels of engineering in their own way. Through complex device specific optimisations and customised packages, engineers have built adequate energy capacity into form factors small enough – in the case of a headset – to be used in earphones, even when working within the limitations of conventional lithium ion battery technology. But these optimisation efforts require complex packages which must be assembled by hand, and which require their own protection circuit mounted on or alongside the battery pouch. In the kinds of applications which use these battery packs, any reduction in size and complexity is highly valued. But a new rechargeable battery design offers higher energy density in a more robust package which is easier to assemble. Available as a standard part, it may mount a challenge to highly specialised, custom engineered lithium battery packs. Miniature wireless devices typically implement rechargeable lithium technology – using a rechargeable, rather than a primary, battery means the device can remain permanently sealed; an essential feature for devices operating in regulated medical and security markets. Lithium cells offer higher energy density than other rechargeable chemistries), while providing a high enough output voltage – typically 3.7V – to support typical circuit requirements. In devices such as headsets, the battery is required to provide around 50mAhr while being small and light enough for the device to be worn in or near the ear. Previous generations of standard lithium coin cells did not offer sufficient energy density – a cell of the required energy capacity was too large. Consequently, headset manufacturers have used custom battery packs, so the maximum possible battery capacity is housed in the available space. This approach has enabled device manufacturers to meet minimum goals for energy capacity and therefore for operating time between charges. But it entails trade offs: in order to keep unit costs within a viable range, custom lithium batteries are typically packaged in a soft pouch or sleeve, because these are cheap to produce. But this material is vulnerable to damage during production and must be assembled manually. Custom lithium packs also require dedicated circuitry to regulate charging and to protect the cell from excessive currents, voltages and temperature. This circuitry is normally mounted on top of or alongside the pouch, adding to the space taken up by the battery. Designing the custom circuit adds cost and time to the product development process. Finally, a custom part carries a higher supply chain risk than a standard part, which is produced and stocked in high volume for multiple customers. So can standard cells be a viable design option for manufacturers of small devices? Coin cells, the electronics industry's favoured type of standard small form factor battery, have many advantages: * They have a hard metal casing which protects the cell during assembly process and in the end user equipment * They retain their shape and size under normal operating conditions. * They are easy to assemble using automated equipment * They have a circular footprint, useful in devices which have a naturally circular outline. In previous generations of rechargeable lithium coin cells, the internal structure of the cell – using either stacked or layered electrodes – restricted energy density. The footprint of the batteries is extended by a pronounced lip which, in conventional production processes, is required to seal the battery robustly. So the size, weight and footprint of standard lithium coin cells has been too large for manufacturers of small, portable devices requiring an energy capacity in the range from 40 to 100mAhr. Now, innovations in the construction of lithium coin cells have increased their energy density to the point where standard coin cells can replace custom battery packs. These innovations have been implemented in the Coin Power family from Varta Microbattery. The first two devices in the range have a size and capacity suited to applications such as Bluetooth headsets, access control tags and personal medical devices. The cells feature a coiled electrode construction, which makes more efficient use of the space inside the casing, while providing a greater surface area of electrode per cubic centimetre than conventional stacked- or layered electrode constructions (see fig 1). Varta has also invented and patented the ILoc casing construction in which precisely engineered upper and lower casings slide together securely, eliminating the requirement for a bulky rim and separate sealing material. This design also enables the cell to operate as a current interruption device: when the pressure inside the cell exceeds a certain level – for example, when the cell is charged at an excessive current or voltage – the upper and lower casings separate by a small, controlled amount sufficient to break the circuit and shut down the battery. While Coin Power's electrode design plays a crucial part in producing a cell with a high energy density, it is also important to note the supporting electronic circuit is also small. A Coin Power cell requires only a standard circuit protection ic, widely available at low cost, plus two miniature passives. One of a range of standard battery charger ics can be used to control the cell's charging process. The footprint of this circuitry is considerably smaller than the complex pcb generally implemented in custom battery packs. Since Coin Power is a standard part and the supporting circuitry can be implemented using standard components, device manufacturers can now avoid the design and production cost and risk associated with the use of custom engineered battery packs, while satisfying their requirement for high energy capacity in a small footprint. Meanwhile, they benefit from the use of a robust battery with a hard metal case which can be assembled using automated equipment, lowering production costs and eliminating the risk of manufacturing defects. The realisation by Varta of the Coin Power concept in the first devices in the range – the CP1254 and CP1654 – has proved that innovations such as coiled electrodes and ILoc can operate successfully in the field. The sweet spot for these first parts is expected to be in Bluetooth and other wireless headsets, as well as certain personal health and security devices. As the Coin Power family expands, it is expected the batteries will enable new wearable technologies, providing a power source that can be sewn into clothing or embedded in small electronic devices worn on the wrist or round the neck. In the past, the provision of an autonomous power supply has restricted the degree of miniaturisation possible in electronics device design. With the introduction of Coin Power, a rechargeable lithium power source for miniature devices is now available which is smaller, lighter, safer and more robust than ever before. Robert Hieber is senior product manager with Varta Microbattery.