Performance requirements drive changes in battery charging technology

4 min read

There are few products on the market today which rely on disposable alkaline batteries for their power. Instead, designers are specifying lithium-ion batteries in various formats. But the general trend is for products to get smaller and for users to not only expect longer times between charges, but also a faster charging process. These demands are pushing battery charging product developers to adopt more innovative approaches.

Patrick Heyer, battery charging product line manager for Texas Instruments, pointed to three charging approaches. "There's 'run of the mill', including USB and adaptors, which is still developing, with new USB standards and charging interfaces. Then there's wireless charging, with a charge management ic on the receiver side, a device which could also be a direct charger. Finally, there's anything related to energy harvesting. It's an emerging area that's not as visible as the others but which is clearly important." While TI is interested in all three areas, its recent focus has been on what Heyer called the 'run of the mill'. But the technologies being applied are far from mundane. "What we see happening is being driven by smartphones and tablets," said Heyer, "where there's a need for higher charge currents. There are also new charge sources, including USB PD, which allows higher power levels to be delivered over USB, although with a new connector." He added that USB will, in the future, support voltages of more than 5V and currents greater than 500mA. TI recently introduced two power management chipsets featuring MaxLife technology, which allows single cell Li-ion batteries to charged more quickly and to provide longer life. The bq27530 and bq27531 fuel gauge circuits, when coupled with the bq2416x and bq2419x chargers, optimise battery performance by using the highest possible charge rates with minimal battery degradation. "We want to take care of the battery," said Heyer. "Trying to put too much current into the battery isn't good and charging has to be controlled. That's where MaxLife comes in." MaxLife combines a fuel gauge, which gathers information about the battery's condition, with the power stage, which puts energy into the battery. "That's a new approach," Heyer claimed. "Before, these processes would be separate, regardless of the battery's state." Heyer said fuel gauges have a lot of information about batteries. "They can be used to determine warranty claims," he noted. "What's new is making more use of the data." By controlling charging based on the battery's condition helps to extend its life. Parameters being monitored include how old the battery is, the number of charge/discharge cycles it has endured, current in and out and its temperature. "The fuel gauge also measures battery impedance," Heyer pointed out. "If it sees problems, then the power stage won't use a full charge current. Instead, it'll use something more gentle." Using the information collected, the fuel gauge control charger modifies the charge profile. "It doesn't go with a constant current," Heyer explained. "Predefined charging profiles are adjusted in real time to take account of the battery's condition. This extends battery life, which is important in devices where the battery cannot be removed." The devices also feature new charging algorithms, which Heyer said have been developed 'over time'. "It's a variation of constant current and constant voltage charging adjusted to the state of the battery, improving overall system performance." Looking to the future, Heyer expects charge currents to increase, which will mean efficiency concerns will need to be addressed. "How can we make high current chargers more efficient and compatible with new charging sources?" he asked. In fig 1, the bq27530-G1 predicts the battery capacity and other operational characteristics of a lithium-ion rechargeable cell. The bq27530-G1 can control a bq2416x charger ic without the intervention from an application system processor. Using the bq27530-G1 and bq2416x chipset, batteries can be charged with the typical constant current, constant voltage profile or through a multilevel charging algorithm. The bq27530-G1's performance is underpinned by the proprietary Impedance Track algorithm. This uses cell measurements, characteristics and properties to create state of charge predictions that can achieve less than 1% error across a wide variety of operating conditions and over the lifetime of the battery. While the bq27530-G1and the bq2416x support charge currents of up to 2.5A, the bq27531 and bq24192 chipset provides for charge currents of up to 4.5A. Heyer also believes the fuel gauge/charger combo can provide better performance than can be obtained from a power management ic (pmic). "A pmic has dc/dc converters, a battery charger and so on. It gets hot and if pcb space is limited, it may be placed directly opposite an apps processor. That creates thermal issues and the system may have to power down so it doesn't overheat. "We suggest taking some of the battery management and high current charging devices and putting them closer to the battery. You get a better distributed system with information based charging." Pressure on process technology Supporting such devices places pressure on process technology. "We need to develop these parts for a process that can create FETs with a low Rds(on)," said Heyer, "as well as one which can combine digital. A charger isn't just analogue and some big power FETs any longer. There's a sizeable digital content to control the charging loop and to handle the communications interfaces. "We need higher digital density, but we can't compromise on power performance," he continued. "We also need higher voltage capability because input voltages are rising; you can't get away with a 5V process anymore and that demands silicon performance." Despite the development of such technology, Heyer said demand remains strong for simple linear battery charging solutions. "Expensive gadgets that used to work from alkaline batteries have moved to small capacity rechargeable Li-ion cells," he said. "But their price points haven't changed, which means expensive charging components can't be used and that benefits linear chipsets." This requires TI to work at both ends of the spectrum, he noted. "At the high end, we're working on efficient thermal performance. At the low end, we're looking to meet system cost requirements. We're now selling Li-ion charging solutions into the toys market," he concluded, "which is one of the most cost sensitive markets there is." Automatic enumeration for safe charging Maxim Integrated Products is sampling the MAX77301 Li+ battery charger, which integrates the intelligence required to enumerate with the host device, identify the adapter type automatically and determine the fastest rate to charge a battery. With temperature monitoring features, the MAX77301 modulates the charge current and battery regulation voltage automatically to maximise safety. To enhance battery safety, the MAX77301 solves sets charge parameters at a safe level automatically. It also provides full programmability via an i2c interface. "The MAX77301 is a perfect fit for devices that do not have application processors but which need to charge quickly," said Sam Toba, director of Maxim's mobility business management division. "This includes cameras, Bluetooth headsets and medical devices."