Choosing
the right battery for portable computing
(BU38)
Laptops are known to be tough hosts on their
batteries. The host demands a stream of uninterrupted power but
offers a poor working environment in return. As a result, the battery
cannot provide the promised runtime and the service cuts short,
often with little notice. In this paper we address the unhappy marriage
between the host and battery, and examine what causes a battery
to deteriorate faster than in other portable devices.
What's
the best battery for laptops?
Batteries for laptops have a unique
challenge - they must be small and lightweight. In fact, the laptop battery should
be invisible to the user and deliver enough power to endure a five-hour flight
from Toronto to Vancouver. In reality, a typical laptop battery provides only
about 90 minutes of service. Many users complain of much shorter runtimes.
Computer manufacturers are hesitant to add a larger battery because of increased
size and weight. A recent survey indicated that, given the option of larger size
and more weight for longer runtimes, most users would settle for what is being
offered today. For better or worse, we have learned to accept the short runtime
of a laptop.
The energy density of modern batteries improves by about
10% per year. However, the benefit of better battery performance is eaten up by
higher power requirements of laptops. This results in the same runtime but more
powerful laptops.
 | Figure
1: Net runtime.
The energy density of modern batteries increases by about
10% per year. This gain is compensated by the demand for better laptop performance.
The runtime remains the same. |
During the last few years, batteries have improved in terms of
energy density. But any benefit in better battery performance
is being eaten up by the higher power requirements of the laptops.
This trend is continuing and the net effect will be the same runtimes
but more powerful laptops.
Most laptops are powered by lithium-ion. This chemistry has a
high energy density and is lightweight. There is no immediate
breakthrough on the horizon of a miracle battery that would provide
more power than the current electro-chemical battery.
Fuel cells, when available, will offer a continued stream of power
by allowing the exchange of fuel cartridges when empty. Unfortunately,
commercial fuel cells for laptops and other portable devices are
still several years away. Power handling, size and cost remain
the biggest hurdles. The early fuel cells will function more like
a portable charger than a battery replacement. The fuel cells
currently in use have the difficulty in providing spontaneous
high power on demand.
The runtime of a laptop battery is based on the activity of the
computer. The basic housekeeping, which the computer needs to
stay alive, draws less power than, for example, reading, writing,
computations and searching for files. Manufacturers prefer using
idle time when specifying runtime.
A battery in a laptop ages more quickly than in other applications
because of heat. During use, the inside temperature of a laptop
rises to 45°C (113°F). The combination of high temperature
and full state-of-charge promotes cell oxidation, a condition
that cannot be reversedonce present. The battery's life expectancy
when operating at high temperature is half compared to running
at a more moderate 20°C (68°F) or lower. Leaving the laptop
in a parked car under the hot sun will also aggravate the situation.
All batteries suffer permanent capacity loss as part of elevated
temperatures but lithium-ion is affected more than other batteries.
Some Japanese computer manufacturers have introduced a number
of sub-notebooks in which the battery is mounted externally, forming
part of the housing. This design improves battery life because
the battery is kept at room temperature. Some models carry several
different battery sizes to accommodate a range of user demands.
Lithium-ion is well suited for laptop users who continually switch
from fixed power to battery use. This user pattern is typical
for those in the sales, service and medical field. Here is the
reason why:
With nickel-based batteries, the charger applies a full charge
each time the portable device is connected to fixed power. The
battery is put on charge until a signal is received indicating
that the battery is full. This signal is in form of a voltage
change or rising temperature. Because of the sluggish response,
permanent capacity loss occurs caused by overcharge and elevated
temperature. Lithium-ion only receives charge if the voltage is
low.
How
to calibrate the battery
Most laptop batteries are 'smart'; meaning that they know how
much energy is left. Such a feature has definite benefits but
the readings are often inaccurate. A laptop may indicate 30 minutes
of remaining runtime when suddenly the screen goes dark. Here
is the reason why:
With use and time, a tracking error occurs between the chemical
battery and the digital sensing circuit. The most ideal use of
the 'smart' battery, as far as fuel-gauge accuracy is concerned,
is a full charge followed by a full discharge at a constant current.
In such a case, the tracking error would be less than 1% per cycle.
In real life, however, a battery may be discharged for only a
few minutes and the load may vary widely. Long storage also contributes
to errors because the circuit cannot accurately compensate for
self-discharge. Eventually, the true capacity of the battery no
longer synchronizes with the fuel gauge and a full charge and
deliberate full discharge will be needed to 're-learn' or calibrate
the battery.
There are no standards to tell what constitutes a fully charged
and fully discharged battery. Lithium-ion packs are considered
fully charged when the limiting voltage (4.20V/cell) is reached
and the saturation current has decreased to 3% of the nominal
value (50mA on a 1700mAh cell). Some chargers use 5% and 8% as
'ready' criteria.) A full discharge occurs when the cell reaches
3V/cell or lower. At this voltage level, the battery has a remaining
capacity of 3 to 10%. Modern batteries adjust to a lower cut-off
voltage on high load currents and include temperature compensation.
To calibrate a battery, a full charge and discharge is necessary.
One without the other does not constitute a calibration. A problem
arises if the battery is recharged after a brief use without providing
the opportunity of a full discharge. A forced discharge to "Low
Battery" may be needed from time to time.
What happens if no battery calibration is done? Can such a battery
be used in confidence? Most 'smart' battery chargers obey the
dictates of the chemical cells rather than that of the electronic
circuit. In this case, the battery will fully charge regardless
of the fuel gauge setting and function normally but the digital
readout will become increasingly more inaccurate. If not corrected,
the fuel gauge simply becomes a nuisance. Cadex Electronics manufactures
'smart' chargers and battery analyzers that are capable of calibrating
a 'smart' battery.
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Created: February 2003, Last updated: September
2006
