Managing batteries so they never fail

Battery management once was a pretty simple topic: Most battery management circuits were little more than diodes stuck in series with each cell of a battery pack to ensure strong cells didn’t discharge into weaker ones. Those days are long gone as high-powered handheld devices must last a long time and be energy efficient.

Complicating the issue is that the physical and chemical behavior of the simplest cells present mysteries, even today. The individual cells in a multiple-cell portable device are never “identical.” Variations in the manufacture of cells, even those in the same lot, bring significant differences in such important parameters as the overall ability to take a charge, the discharge rate, and various qualities that depend on age and temperature.

But though things have gotten more complicated, the overall goal for battery manager circuits has changed much: “You have to be careful to protect the battery,” says Texas Instruments Battery Management engineer Pat Hunter. “You’re looking at five or six batteries in series, and so the big problem is balancing them and protecting them.”

Indeed, the traditional lithium-ion cell acts touchy even with slight overcharging. One set of devices that address the issue is TI’s bq77908A and bq77910A balancing and protection ICs (four-to-eight, and four-to-ten cells in series, respectively). They can allow for differences in capacity and impedance between cells in the same pack and heat gradients within a lithium-ion/polymer pack.

“You don’t want to overcharge a lithium-ion cell,” adds Hunter. “If you’re putting 18 V across a stack of batteries and one of those batteries, for some reason, is at 2 V while the others are at 4 V, you’ll overcharge that cell.” TI is also about to introduce the bq34z100, a chip with multichemistry capabilities for balancing 1 to 14 cells. It’s suited for power tools.

Charging + a little more

Most battery charging ICs for lithium-ion batteries implement a few similar functions. They can provide an output to a battery that is voltage-regulated to on the order of a few tenths of a percent. They deliver a charging profile with trickle, constant-current and constant-voltage charging modes. They monitor battery voltage as a way to determine in which of these modes to operate. They also monitor charge current to note when the battery is reaching full charge so they can decide when to shut off.

Going back to 2008, Freescale Semiconductor had one of the more complete chips, its MC34674 1-A device for Li-ion/polymer batteries. As another example, STMicroelectronics also had its STw4102, a dual-input (wall adapter, USB port) linear battery charger with gas-gauge circuitry providing a read-out of remaining battery capacity and nominal fast-charge at up to 1 A, with external support for currents up to 2.5 A.

Today’s chargers provide more charging capability and are more integrated -- often in a smaller chip. A snapshot of some of the newest arrivals include Intersil’s ISL9230, a constant-voltage, constant-current charger for single-cell lithium-ion/polymers. It uses separate power paths to power the load from the get-go even if the battery is completely discharged. Intersil’s newest supporting chip, the ISL9211B, provides multiple-redundant safety protection. Whenever input voltage, battery voltage, or charge current exceeds a set limit or its own temperature exceeds 150C, the IC disengages the input source.

Other new arrivals include Analog Devices’ ADP5061 linear battery charger with power-path and USB mode compatibility in a 2.6×2-mm WLCSP package. It can deliver up to 2.1 A from an ac charger while in its low-dropout mode. It can handle input transients to 20 V (the evolved de facto spec), which can happen on the USB bus during various connect and disconnect situations.

Other new entries include an extension to Fairchild Semiconductor’s 5401x series of chargers, which tout the smallest switch-mode solution on the market, focusing on the thermal and time-to-charge aspects of the system. The company’s newest entry for USB-compliant on-the-go chargers for Li-ion batteries, its FAN54013, in a 1.96×1.87-mm CSP package, offers more stand-alone charging capability (1.45 A) and better USB-OTG (USB On the Go) capabilities---up to 500 mA.

The reason new battery charging chips sport more features is not hard to understand. The focus nowadays is on a lot more than just squeezing in the last words of a casual phone conversation before the cell phone goes dead. Battery backup has come to health care, along with the need for absolute confidence when batteries power hospital instruments. Portable outpatient monitors now perform must-not-fail reporting of critical body parameters. Beyond medical applications, small battery management is also becoming a selling point for portable industrial power tools.

Chip makers are responding with ever more efficient designs for the basic elements of the battery management system. Typical battery managers these days include gas-gauging ICs that monitor the amount of capacity left in the battery pack; multicell balancing and protection chips; and battery charging systems.

The last category includes a rather rapid expansion of ICs suitable for wireless chargers (i.e., inductively driven systems, for up to 5 W) and for batteries ganged with energy harvesting schemes (microwatts to milliwatts) that scavenge energy from such sources as thermal gradients or vibrations. So-called wireless charging and energy harvesting increasingly come into play for powering remote sensor networks and general trickle-cell charging for lower-power applications.

One issue with the setup for charging batteries in energy-harvesting schemes is the lack of system integration. Energy harvesting management functions tend to be separate from those needed to manage the battery charging and discharging. One of the more interesting energy-harvesting chip arrivals in that regard is Maxim Integrated Products Inc.’s MAX17710, which the company touts as the first IC to integrate all of the power management functions for harvesting, charging, and protecting the battery it charges. In this case, the chip works with a microenergy, rechargeable solid-state family of Thinergy cells from Infinite Power Solutions Inc.

The chip, in a low-profile 0.5-mm TQFN package, takes the input from a 1-µW to 100-mW source (e.g., photovoltaic, piezoelectric, or a thermoelectric generator providing at least 0.8 V) and charges the microcell. On nominal power-up where the potential of the energy-harvesting source is greater than the voltage on the battery, it initially trickle-charges the battery through an “ideal diode” circuit, during which time the outputs of the device’s linear regulator (on the load side) are disabled.

When the input from the energy-harvesting source voltage exceeds 4.15 V or so, the device goes into full operation. The device’s switching boost regulator limits the charging voltage to 4.125 V. At the same time the device’s output stage LDO (which has over-discharge protection) drives the application load (at a programmable 1.8, 2.3, or 3.3 V). If the harvesting source exceeds 5.3 V, an internal shunt kicks in to limit the input voltage and prevent battery overcharge. Maxim says that for an energy harvesting source at 0.8 V and a 4.1-V cell, the chip can deliver more than 20 mA as long as the energy harvesting source can support it.

TI’s latest chip for energy harvesting/RF harvesting applications, the bq25504, can accommodate a single solar cell at 300 mV to deliver 5 V up to 80 mA, which suits remote sensor networks.

The big picture

The single-function chips for basic “first generation” battery management today seem old hat. “People want system solutions, not a single function. Chips today provide multiple outputs, battery charging, and seamless control of input sources autonomously,” says Tony Armstrong, director of marketing for power products at Linear Technology Corp. “It’s all about simple straightforward solutions with high levels of integration that make the design task easier.”

The problem with such integration is that it may be necessary to compromise on the various functions combined into one device. For instance, it’s often difficult to find a sufficiently good gas-gauging IC, which sits at the heart of many battery management systems, integrated with other power management functions. So some designs still add a dedicated gas-gauging chip to a battery manager IC that has gas-gauging built in.

What’s called power-path management (known by several names, depending on the IC maker) continues to be a core architecture for efficient high-current charging from multiple input sources. It refers to a means of connecting the load to one of several inputs depending on the state of the battery, whether it is depleted or charged-up enough to power the load itself. The point of power-path management is to power up a critical load even if the battery is toast. This property, sometimes referred to as instant-on capability, works as long as there’s enough reserve power from the input source(s) to simultaneously run the load as well as charge the device’s battery.

So when used with a wall wart, a typical power-path management scheme uses all the energy to first power the system, then starts charging the battery once load requirements are met. It essentially manages a dynamically changing trickle-charge of the battery. And some power-path modes deliver energy from the battery to the system while supplying power from the adapter to the system (i.e., in a supplemental mode).

PowerPath today is widely used in battery-charger ICs. Some of the new products in this area include Linear Technology’s LTC4156, a dual-input, 3.5-A high-power manager and charger with I2C control. It’s suitable for charging Li-ion phosphate batteries, whose chemistry is fire-resistant (compared to the more prevalent Li-ion cobalt). The high efficiency (92%) device can handle charge currents at 3 to 4 A and 15 W in the charge mode. When powered by a USB port, it can deliver up to 2 A at 5 V. Another supporting device targeting high-powered handheld applications is Linear’s LTC4415, a monolithic dual 4-A ideal-power path switch.

ICs that monitor remaining battery capacity are at the hub of modern battery management systems. These “fuel monitors” continue to become more efficient in gauging the amount of energy still “in the tank.” “The focus now is on the end of curve,” says TI’s Pat Hunter. To make capacity readings more accurate near the battery’s end-of-charge point, fuel gauges now use algorithms that linearize the response. The ICs have always been good over the entire discharge profile, he notes, although improvements are still being made. But the main design thrust today is in coming up with better algorithms for accurately predicting how much power is still available.

The first high-performance fuel-gauges arrived in 2004 with Texas Instrument’s bq20z80 and its “Impedance Track” algorithm, which could predict remaining battery capacity (or, alternatively, how to calculate the time before loss of operating power) to within 1%. The performance was a tenfold improvement over the previous decade’s “battery monitors,” of which the best provided about 8% accuracy. The best of the modern gas gauges today take into account the user’s power-usage profile, the age of the battery, and the operating temperature. These devices usually incorporate a so-called coulomb counter or equivalent for measuring charge or current. (Earlier types relied on measuring voltage.) They also incorporate sensors for monitoring operating temperature and some means to track battery parameters.

Most modern gauges are compatible with the smart battery system (SBS). SBS is a specification for determining battery capacity more accurately and lets processors manage operations around the amount of juice remaining in the battery. The system also uses SBS to control energy charging the battery. Communication takes place over a two-wire SMBus. The specification originated with Duracell and Intel in the 1990s and is now a de facto standard. In principle, any battery operated product can use SBS, but in practice it’s found mainly in laptop computers.

The fuel-gauge chip in the battery pack monitors the battery and reports information to the SMBus. This information might include battery type, model number, manufacturer, discharge rate, and a prediction of remaining capacity. It might also generate an almost-discharged alarm to let the PC or other device shut down gracefully, and information about temperature and voltage to enable safe fast-charging.

However, Maxim Integrated Products Inc. developed the DS2780 for non-SBS systems at about the same time as TI’s device. Subsequent chips from Maxim have been suited to the firm’s 1-wire interface system. Microchip Technology Inc. also released its PS501 in the same timeframe, with a claimed accuracy of 1%. Intersil Americas LLC shortly thereafter produced the ISL6295 “high accuracy” gauge, which specified ± 0.5% current-measuring accuracy.

Supporting arrivals within the last two years include the bq78PL116 PowerLAN Master Gateway Controller from TI, which is part of a complete monitoring and cell-balancing solution for three to 16 Li-ion cells. A similar system comes from Atmel Corp.

Several new entries for basic single/dual cell applications offer parts-cutting alternatives. These chips perform better than traditional battery monitors and some approach the performance of one-percent gas-gauges. These include the LC709201F from Sanyo Semiconductor USA for single-cell lithium-ion batteries. Sanyo claims ±3% accuracy of remaining battery-operation time and the industry’s highest-accuracy device for chips having no sense resistor.

Maxim’s newer arrivals include its MAX17040/43 (one-cell) and MAX17041/44 (two-cell) compact fuel-gauges. Its ModelGauge algorithm eliminates the usual current-sense resistor and reportedly the coulomb counter as well to cut savings by 30 to 50%. TI’s latest devices include its bq27410 and bq27520 (system-side) gauges for various handheld devices, and the soon-to-arrive bq20z65 SBS-compliant gas gauge and protection IC.

Most recently, STMicroelectronics released its STC3105 which it characterizes as a battery monitoring IC using a basic gas gauge (coulomb counter circuitry to estimate battery capacity) for handheld applications. It’s housed in a 2×3×0.8-mm package (touted as the smallest in class).

The applications

After the cell phone and the tablet PC, health-oriented portable instrumentation is a key area for battery management systems. These uses include outpatient-monitoring systems, which are more of an interest in Europe but a growing niche in the U.S. The message is clear: Overall, both patients and health care providers favor remote patient monitors to deal with quality-of-life issues as well as to deliver economic benefits.

The U.S. medical roadmap admittedly has more bumps, with some vendors in an embryonic stage and others significantly further along. “Medical is a very big market for us now,” said TI’s Pat Hunter. “The problem with medical is there’s a lot of small customers, but you still have GE Healthcare, glucose meter people, and Abbott Labs. China alone is gong to be big in medical.”

Backup battery protection is perhaps the most critical concern for anything on wheels or in the field, where it’s often necessary to guarantee a 12-hour runtime. That includes wearable patient monitors. “Glucose meters and digital thermometers are disposable and will use a primary cell battery, which you just change out,” says Hunter. “With wearable patient monitoring, the concern is battery-backup. All the smarts go in the battery pack. So with fuel gauging, you’ll want to have all the utilities (e.g., protection ICs, etc.) with the cells at all times.”

Not all IC vendors include power tools on their battery management roadmaps because of cost-versus-performance concerns. But tools are becoming a bigger deal. Nickel-metal hydride (NiMH) batteries are still most often preferred by tool users because they have about the same energy density as Li-ion types. But at the same time they are also seen in a less favorable light because they have a high self-discharge rate and for a number of other reasons. Today’s trend favors Li-ion power sources. Industry analysts say they provide the highest energy density for the chemistry and will last longer.

But the problem with the Li-ion types in tools is the cell-balancing protection needed for the 18-V battery pack. Some now see lithium-ion phosphate technology as the best general solution for small battery management. Such packs have been in production for several years, notably by Sony, and are available from such major power-tool manufacturers such as DeWalt. These batteries have a more stable chemistry and don’t require all the protection of a traditional Li-ion pack. They’re said to be more environmentally safe as well.

Resources

Analog Devices Inc., Norwood, Mass., www.analog.com
Atmel Corp., San Jose, Calif., www.atmel.com
Infinite Power Solutions Inc., Littleton, Colo.,
http://www.infinitepowersolutions.com/
Intersil Americas LLC, Milpitas, Calif., www.intersil.com
Linear Technology Corp., Milpitas, Calif., www.linear.com
Farichild Semiconductor, San Jose Calif, www.fairchildsemi.com
Freescale Inc, Austin, Tex.,www.freescale.com
Maxim Integrated Products Inc., Sunnyvale, Calif,
www.maxim-ic.com
Microchip Technology Inc., Chandler, Ariz.,
www.microchip.com
Sanyo Semiconductor Co., Saddle Brook, NJ,
http://us.sanyo.com
STMicroelectronics, Coppell, Tex., www.st.com
Texas Instruments Inc., Dallas, www.ti.com

The original app for PowerPath

“We had several design requests more than a decade ago along about the same time, but none of them would tell us exactly what they were working on,” says Linear Technology Corp. power products director of marketing Tony Armstrong. “They said only that they had several rails including a battery charger, and they outlined the challenge: provide instant-on operation with a heavily depleted or missing battery. That was a big deal 10 or 15 years ago.” At the time, traditional battery fan systems were widely used. But if your battery was discharged the user couldn’t operate the device.

“To meet the challenge, we developed PowerPath management for watching current demand. (Linear has the patent on this technology, which in its various forms throughout the industry most often incorporates a state-machine architecture.) Because what you need to be able to do is accurately determine what power sources you’ve got, whether it’s the battery or an external source; determine which source to use; and then prioritize what goes to the system load, and what’s left over to charge the battery.” Thus, PowerPath management, which inherently incorporates instant-on capability, immediately powers the system load. If there is excess power available from the source, the battery charges once the load requirements are taken care of.