Battery Chargers and Monitors Conserve Power

Proper attention to battery-management circuits in medical devices can extend battery life, reduce power loss and ensure devices use every bit of available energy. But to obtain these benefits, engineers must think about battery management at the start of a product's design.

"Start with a plan that specifies the voltage rails and currents the circuits will need, along with the available input power sources," says Trevor Barcelo, bat­tery-management product line manager at Linear Technology. "If you need a 5-volt rail, one Lithium-ion cell and a boost converter might work well. You also might consider two Lithium-ion cells that nominally produce 7.4-volts to replace the boost converter with a more common buck converter. However, if the system must be charged from a 5-volt source, the stack of cells might create a challenge because you'll find few, if any, boost chargers on the market. Those are the types of things you must investigate."

Most engineers familiar with Lithium-ion batteries know when they charge a 4.2V battery to 4.2V, they maximize its short-term capacity - the amount of energy a battery can deliver spec­ified as amp hours, or Coulombs. "But after a year in storage, a Lithium-ion battery charged to 4.2 volts will have less capacity than a battery charged to only 4.1 volts," explains Barcelo. "At 4.1 volts a battery reaches between 80 and 85 percent of maximum capacity, but it retains more of its capacity over time than a battery charged to 4.2 volts, so you can extend battery life and reduce operating cost." Many charger ICs provide the option to charge Li-ion batteries to 4.2 or 4.1V.

Engineers also can extend Li-ion battery life by avoiding overcharging and high temperatures. To help engi­neers avoid these conditions, the Japan Electronics and Information Technology Industries Assn. (JEITA) and the Battery Assn. of Japan created guidelines and charging profiles for Li-ion batteries. High temperatures decrease battery life and can cause battery failure.

The LTC4099 from Linear Technology, for example, has an over-temperature battery-conditioner option. "When the charger detects a temperature above 60C and a battery voltage close to 4.2 volts, it actively discharges the battery through an internal load that draws 180 mA," says Barcelo. "When the battery voltage drops to 3.9 volts, or the thermistor reading drops below 58C, the IC turns off the internal load."

"Designers might not know a charger circuit draws some bias current all the time," continues Barcelo. "If you use an energy-harvesting technique to charge a battery, say, with a piezo-electric generator, you want a charger that draws only a few microamps. Otherwise the ‘free' harvested energy powers the charger and doesn't charge the battery."
Li-ion batteries aren't the sole choice for engineers. Nickel-metal-hydride, or NiMH, batteries look attractive, too.

"Designers will choose NiMH because they like the low cost," says Len Sherman, senior scientist at Maxim Inte­grated Products, a manufacturer of many lithium-battery and other types of charger ICs. "But then they realize it's more expen­sive than expected to design a charger that detects a NiMH battery's negative-delta-V or zero-delta-V termination point. As you charge a NiMH battery, the voltage across it rises. But as it gets close to full, the cell volt­age falls slightly. So the charger must detect a 10- to 20-mV drop as it adds charge. A good ADC can measure that drop, but you won't get useful values if you charge at less than C/4 or even C/2." For a double-A 2900 milliamp-hour battery, C/2 represents a charge current of 1.45A. The higher the charge current, the greater the delta-V at the 100-percent-charged point.

"Engineers don't want to spend money on a high current adapter," says Sher­man. "Instead, they might rely on a timer and run at a low charge current from a basic wall-plug adapter. But if someone unplugs it before the end of a cycle and plugs it in again, the timer will reset and run for another full-charge cycle. The first generation of NiMH batteries got a bad reputation, mainly because the chargers were so poor."

 

Also, standard NiMH batteries self discharge to about 65 percent of capacity in a year, whereas Sanyo's new Eneloop NiMH batteries, for example, retain about 85 percent capacity over the same period. "Newer technologies in NiMH batteries offer longer shelf life and can save energy by elimi­nating charge cycles needed to maintain battery capacity," explains Sherman. "The longer shelf life, however, comes at the expense of a bit less battery ca­pacity. Nonetheless, new types of NiMH batteries might prove attractive in medical devices."

According to Rich Del­Rossi, product marketing manager for TI's battery-management solutions group, by using a "fuel gauge," product designers can get the most energy out of a battery. "When you monitor a battery's voltage, you see a fairly flat voltage for most of the discharge and then the voltage drops to a point at which a medical device can no longer operate. The discharge curve changes shape at different discharge rates and temperatures, so it's not some­thing you can confidently predict."

"If a medical device has data in RAM, you will lose it if you suddenly remove power when the voltage drops," continues DelRossi. "But a battery fuel gauge can tell the equipment when the battery will reach that threshold voltage so the system knows when to copy information and settings into Flash memory. Without a fuel gauge, the equipment would have to guess when to make that transfer. More complex equipment could use a fuel gauge to control power. In a handheld instrument, for example, it might dim an LCD or tell a processor to run slower to save power."

Texas Instruments uses an imped­ance-track technique to account for the different discharge rates and battery conditions. "We store a model of the open-circuit voltage of the battery, from 100 to zero percent without a load across it," explains DelRossi. "Then we store resistance as a function of delivered battery capacity. So for every voltage value, we also have a resistance value. A fuel gauge such as the bq27541 for single-cell battery packs measures voltage and current, and computes the cell's resistance. We use that resistance throughout the entire discharge cycle to derate the open-circuit curve into a curve that accurately describes the actual battery conditions. The curve also accounts for resistance changes at different temperatures."

"Then a host controller can query registers to determine remaining capacity, time to empty, and other characteristics based on measured values," says Del­Rossi. "The controller then determines the time at which battery voltage can no longer power a device and thus, when to back up data and settings. Essentially, the fuel gauge acts like a Coulomb counter and it monitors electron flow to and from a battery."