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, batÂtery-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
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 specÂified 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 engiÂneers 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
âfree' 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 InteÂgrated
Products, a manufacturer of many lithium-battery and other types of charger ICs. "But then they realize it's more
expenÂsive 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
voltÂage 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 SherÂman. "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 elimiÂnating charge cycles needed to
maintain battery capacity," explains Sherman. "The longer shelf life, however,
comes at the expense of a bit less battery caÂpacity. Nonetheless, new types of
NiMH batteries might prove attractive in medical devices."
According to Rich DelÂRossi,
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 someÂthing you can confidently
"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
Texas Instruments uses an
impedÂance-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 DelÂRossi. "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."
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