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Portable Power Solutions: Designing the Optimal Portable Power System Battery packs free electronic devices from the tethers of stationary power sources. Good battery pack design
results in smaller, lighter, and longer-running portable devices. It also ensures that packs meet the special
needs of particular applications and operate safely in the process.
Manufacturers of portable devices quickly discover that it is a difficult task to equip a portable device with
the right battery pack. For one thing, they must choose from among a wide variety of options, from simple
Sealed lead acid battery packs to complex Lithium-ion packs that include electronic safety, monitoring, and
charge-control circuitry. They must also deal with critical matters such as packaging, eco-environmental
factors, and regulations that affect battery-pack design and construction.
This paper examines the many issues involved in the design and manufacture of a custom battery pack.
Topics covered include battery cell selection, critical battery pack safety measures, chargers, packaging,
inspection, and testing.
Portable Power Solutions:
Designing the
Optimal Portable
Power System
Chris Turner, Director of Battery Technology, and
Lon Schneider, Director of Product Development DESIGN CONSIDERATIONS
The process of designing a battery pack for a portable
device begins with application specific information
such as:
Budget constraints for the power system;
Space and weight that can be allocated to the
power source;
Load consumption of the device during operation
or standby, if typical applications include long
periods of inactivity between uses;
Voltage range of the device: minimum
and maximum values for supply voltage;
Acceptable run time of the device: how long
the device must run between full charges
1
;
(Footnote below)
Storage capacity: how much energy the battery
pack must hold;
Rate of discharge: the pack output needed to meet the
products peak load or continuous current requirements.
Different cell chemistries that store the same amount of
energy may not be able to deliver it at the same rate;
Charge rate: how fast the pack should be charged;
Cycle life: acceptable battery life in the typical usage
scenario;
Operating environment: factors such as temperature,
humidity, and vibration have an impact on a battery
pack. For example, battery capacity is affected by
internal resistance, which usually increases as
temperature drops;
Shelf life: batteries discharge while in storage.
Self-discharge rates for popular cell chemistries range
from 5-30 percent per month.
MAJOR RECHARGEABLE
CELL CHEMISTRIES
Application information guides the designer in the
selection of a battery cell, a decision that will have a
major impact on the functionality, size, cost, and success
of a portable device. Today, the choice is typically one of
three rechargeable cell types: Nickel-metal hydride
(NiMH), Lithium-ion (Li-ion), and Lithium-polymer
(Li-polymer). (See Figure 1.)
Figure 1. Comparison of energy densities for various
primary and re-chargeable battery chemistries.
1
Charge and discharge rates are often expressed as C-rates, where 1C is 1x the capacity of the battery cell.
This means that a 1000mAh battery being discharged at 1000mA is being discharged at a 1C rate. The selection of a battery cell will have a
major impact on the functionality, size, cost,
and success of a portable device.
Nickel Metal Hydride
NiMH batteries offer capacities up to twice the
equivalent-size nickel cadmium (NiCd) cells. In addition,
NiMH is lighter and more environmentally friendly
ionthan NiCd cells. Unlike widely used lithium chemistries,
NiMH does not require safety electronics, making it a less
expensive option for battery packs.
NiMH is replacing Lead-acid in many applications as
the cost effective intermediate step between Lead-acid
and Li-ion.
On the downside, there are only a few major NiMH
suppliers. Additionally, the self discharge rate of these cells
requires that they be recharged at least once per year when
in storage. Although to a lesser degree than NiCd, NiMH
also suffers from voltage depression, causing memory effect.
Lithium Ion
Featuring the highest energy density of the widely available
rechargeable chemistries, Li-ion cells offer 1.5 times the
capacity by weight and up to twice the energy by volume of
NiMH cells. In addition, they have no memory effect and
their self-discharge rate is one third that of nickel-based
chemistries.
Unlike nickel-based cells, however, Li-ion cells require pro-
tection circuitry to keep voltage and current within safe lim-
its. This is one reason why Li-ion is a more expensive option
than NiMH and NiCd.
Until recently, Li-ion has not been suitable for power tools
and other devices that require a high discharge rate.
However, several battery suppliers now offer Li-ion cells that
can provide high discharge rates. As a result, Li-ion is rapidly
becoming the standard chemistry for high-end power tools,
medical electronics, and other high rate applications.
Nanoscale electrode technology has been utilized to provide
a next-generation high rate cylindrical Li-ion cell.
A123 Systems has developed a new Li-ion cell that provides
unprecedented power and cycle life compared to conven-
tional Lithium technology. Licensed from the Massachusetts
Institute of Technology (MIT), this new cell delivers up to
5X power gains, 10X longer life, and the ability to recharge
to 90% of its capacity within five minutes, according to
independent testing. The A123 cell is an extremely
stable electrochemical system that can withstand abusive
conditions such as overcharge or overdischarge without
going into thermal runaway, making it a safer and more
robust alternative to traditional lithium chemistries.
(See Figure 2 below).
Figure 2. Continuous discharge rate of high-power
A123Systems Li-ion cells at 25� C. Figure 3. Discharge capacity of a Li-polymer
cell from 60�C down to minus 20�C
Figure 4. Swelling rate of Li-polymer and Li-ion cells.
Lithium Polymer
Li-polymers advantages are higher energy density by
weight than Li-ion and higher volumetric energy density
in thin formats using less than 5mm cell thickness. Unlike
other chemistries that are typically available in limited
standard sizes, Li-polymer is available in virtually any
footprint, providing much greater flexibility to device
designers. Moreover, Li-polymers superior stability in
over-voltage and high temperature conditions gives
designers a wider margin of safety than with Li-ion.
Because Li-polymers electrolyte is a high viscosity gel or
solid polymer, and not a liquid like Li-ion, a common
misconception about these cells is that its conductivity
and rate capability is poor. Figure 3 shows the discharge
capacity of a Li-polymer cell from 60�C down
to minus 20�C.
In many ways, Li-polymer chemistry is very similar to that
of Li-ion. However, one significant difference is that the
Li-ion electrolytes typically have lower boiling points,
causing the liquid to turn to gas at high temperatures.
As the electrolyte turns to gas, the metal can will swell.
Designers in applications that may see above normal
temperatures must account for this cell swelling in the
mechanical design of the host device. By contrast, the
Li-polymer electrolyte is a gel or solid typically with a
higher boiling point, making it more stable and less prone
to swelling at high temperatures, as shown in Figure 4.
Historically Li-polymer cells have cost significantly more
than Li-ion cells. Recently, however, the cost of Li-ion and
Li-polymer has converged to a similar level in high
volumes. Like their Li-ion counterparts, Li-polymer cells
are found in consumer items and handheld instruments
that require a small and light power source. In most cases,
just one Li-polymer cell is used to power a device. The designer must thoroughly understand
each cell suppliers technology and be able
to match it to a particular application.
OTHER RECHARGEABLES
In addition to the three chemistries discussed above,
several other types of rechargeable cells are widely
used in battery pack design. These include:
Nickel Cadmium
Less expensive than NiMH, NiCd cells are a cost-
effective option for portable applications. Another NiCd
attraction is long cycle life, making the cells suitable for
devices such as cordless phones. The cells also provide
the high discharge rate needed by power tools and
mobile printers.
However, NiCd includes toxic cadmium, which has now
been banned in European batteries. In addition, NiCd
energy density is lower than that of NiMH and only about a
third that of Li-ion.
Sealed Lead Acid
Sealed lead acid (SLA) batteries have long been found in
products that do