Download Li-Ion Cells Build Better Batteries for Power Tools

ANALOG FEEDBACK
Li-Ion Cells Build Better Batteries for Power Tools
By David Morrison, Editor, Power Electronics Technology
A
substantial, vendors are using the technology to market their
new products. For example, Milwaukee Electric refers to its
battery pack as the V28 Lithium-Ion cordless technology and
its end products as the V28 line of tools. Cells based on the
V28 technology employ manganese-oxide cathodes rather
than the cobalt-oxide cathodes commonly used in low-power
applications. To create a 28-V pack requires seven of these
4-V, 3-Ah Li-ion cells.
To serve the power tool applications, Li-ion batteries have
been modified to operate at high discharge rates. So, while
the standard Li-ion batteries specify their capacity at a 1C
(or even lower) discharge rate, high-power Li-ion batteries
typically specify performance at a 10C or higher rate.
Low-power and high-power Li-ion cells differ with
respect to rated capacity and the variation in capacity as a
function of discharge rate. In general, the cells developed
for high discharge rates sacrifice some of the energy density
achievable at lower discharge rates to obtain lower impedance in the cell. For example, in the 18650 cylindrical format,
Sanyo now offers a high-power cell with 1500-mAh capacity at 10C, while one of its low-power cells currently offers
2600 mAh at a 0.2C rate in the same package.[1]
Typically, conventional Li-ion cells have limited ability to
deliver high power. In contrast, the high-power Li-ion cells
offer nearly the same capacity at high and low discharge
rates.
fter becoming the dominant rechargeable
chemistry in cell phones, PDAs and a host
of other mobile applications, Li-ion batteries
are just starting to prove their metal in highpower portable applications such as power
tools. Just as in the lower-power applications, high-power
Li-ion cells offer higher energy density than the NiCd and
NiMH cells they replace. Recently developed Li-ion cells
offer two to three times the gravimetric energy density of
the nickel-based chemistries—an advantage that can be
exploited either to build lighter battery packs or to extend
battery run times.
One equipment manufacturer, Milwaukee Electric Tool,
collaborated with an established battery manufacturer
to develop a 28-V Li-ion battery pack that offers greater
power and up to twice the run time of conventional 18-V
NiCd models. Despite its higher performance, the 28-V
pack weighs slightly less than the 18-V pack. As a result, the
company is able to create some of the first cordless versions
of professional-grade power tools (see the figure). One
example is a cordless bandsaw that is described as “powerful
enough for everyday use.”
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Sanyo (www.sanyobatteries) continues to develop cells
with a high discharge rate/higher power capability. A recent
example is Sanyo’s UR18650W, a cell that specifies a typical
capacity of 1600 mAh and a minimum capacity of 1500 mAh.
The cell achieves this minimum capacity at a 10C (15-A)
discharge rate (see the table). The cell is constructed with a
manganese-oxide cathode to improve safety over what would
be obtained with the cobalt-oxide cathode typically used in
standard Li-ion cells.
Despite its safer cathode, a combination of risk factors
such as the possibility of overcharge and an internal short
made it necessary to incorporate overcharge protection
into the UR18650W. This circuitry protects against voltage
imbalance during normal cell use and abnormal charging
conditions. The latter can occur when the battery encounters
a broken charger or the wrong charger, or when a battery is
charged manually.
The UR18650W follows two power cells previously
In its V28 line of power tools, Milwaukee Electric Tool is replacing 18-V
NiCd battery packs with slightly lighter 28-V Li-ion packs that deliver
higher power and up to twice the run time of the NiCd packs.
Along with offering greater power and run time, Li-ion
cells offer better performance at temperature extremes. This
advantage includes greater cycle life than NiCd batteries
under extreme conditions.
The impact of Li-ion batteries on power equipment is so
Power Electronics Technology February 2006
52
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Vendor/Model
Capacity at
1C or Less
(mAh)
Capacity
at 10C
(mAh)
Internal
Impedance
(m) at
1 kHz
Charge
Voltage
(V)
Discharge
Voltage
(V)
Cycle Life
Sanyo
UR18650W
1600 at 1C
1500
—
4.2
3.7
700 cycles to 75%
initial capacity
Sony 18650VT
1080 avg.
1000 (rated
capacity) at 1C
1030 avg.
25
4.1
3.76
500 cycles to 90%
of initial capacity,
2-A discharge
Sony 26650VT
2500 avg.
2400 (rated
capacity), at
0.2C
2400 avg.
13
4.1
3.75
Same as above, but
at 5-A discharge
Valence
Technology
Saphion
IFR18650p
1100 at 1C
(rated
capacity)
1050
<20
3.65
3.2 at 1C
>600 cycles to 70%
capacity at 10 A
Table. High-power Li-ion cells.
pacities at 10C, 4C and 1C discharge rates (see the table). As
these specifications reveal, the capacities of the high-power
cells vary only slighty across a range of discharge rates.
For example, the 18650VT specifies an average capacity
of 1030 mAh at 10C versus an average capacity of 1080 mAh
at a 1C rate. Similarly, the 26650VT specifies a 2400-mAh
developed by Sanyo. The UR18650H/V offer 2000 mAh of
capacity, but at a lower discharge rate (5C).
Another cell manufacturer, Sony (www.sony.com/energy)
describes its VT series of Li-ion cells as the first developed for
power tool use.[2] The 18650VT and 26650VT, which were
introduced early last year, are cylindrical cells that specify ca-
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APPLICA
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RFI/EMI Suppression
MLP Capacitors @ 500KHz
Max I rms
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Max RMS Current vs. Capacitance
20.0
18.0
16.0
14.0
12.0
10.0
8.0
6.0
4.0
2.0
0.0
1
5
10
15
20
Cap Value (microfarads)
V
Max RMS Current vs. Capacitance Value
P.O. Box 4539 • 1205 McConville Road • Lynchburg, VA 24502
434-239-4730
TEL 434-239-6941 • FAX
F
[email protected]
Available through these distributors:
• Atlantic Components (1-800-433-6600) • Future (1-800-388-8731)
• Gopher (1-800-592-9519) • Cornell Dubilier/Mallory (1-508-996-8561)
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Power Electronics Technology February 2006
ANALOG FEEDBACK
average capacity at 10C versus a 2500-mAh average capacity
battery in its Vitz, a minivan sold in Japan, to provide power
PETech
at a 0.2C rate.
within a stop-start system.
Although these cells were originally developed using
cobalt-oxide cathodes, newer cells incorporate manganese
References
and nickel, which is said to eliminate the need for an over1. “New Concept Lithium Ion Batteries in Sanyo” by Masacharge protection circuit.
toshi Takahashi, Sanyo Electric Co., Power Solutions Group,
Although these cells exhibit about half the enerpresented at Portable Power Conference, Sept. 20, 2005.
gy density of the convention Li-ion cells for low2. “R&D Activities & Results for Sony Batteries,” by Kenji
power applications, their performance is still signifiOgisu, R&D Division, Energy Group, Sony, presented at
cantly better than the chemistries they are replacing.
Portable Power Conference, Sept. 20, 2005.
The VT series cells offer 94 Wh/kg gravimetric en3. Valence Technology Datasheet, available online at www.
ergy density versus 47 Wh/kg for NiMH and 35 Wh/kg
valence.com/solutions.asp.
for NiCd.
Last year, Valence Technology (www.
valence.com) targeted power tool applications when it introduced its Power Cell in an
18650 cylindrical format. Like other Li-ion
batteries offered by the company, the Power
Cell employs the Saphion chemistry, which
incorporates phosphate-based cathodes
rather than cobalt-oxide. The phosphatebased cathodes offer greater safety and
less susceptibility to thermal runaway than
cobalt oxide.
The Power Cell offers nearly a 1.1 Ah of
capacity at a 10C rate but can deliver higher
current pulses.[3] For instance, the cell can
supply a 30-A pulse for 30 sec. Valence is
working on the next revision of the Saphion
power cell, improving the product’s cycle life
and power density. The company expects
the Saphion power cell revision to be in
production within six months.
Although Li-ion cells developed for
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power-tool applications are relatively new,
we can expect that their development will
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follow some of the same trends as their
low-power counterparts. With consumers
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always wanting longer run times for their
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batteries, cell vendors will likely strive to
offer higher energy densities at the higher
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discharge rates.
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Vendors will strive to improve cell safety
and lower material costs to enable high-power
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Li-ion cells to be adopted in more applications. To that end, cell makers are already
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developing Li-ion cells for even higher
discharge rates.
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Hybrid electric vehicles (HEVs) rep����������������
resent one of the most promising target
applications for such batteries, and several
battery suppliers and at least one automaker
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(Toyota) are developing Li-ion cells to power
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HEVs. Although most of these batteries
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may be years away from use in commercial
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vehicles, Toyota is already using a Li-ion
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Power Electronics Technology February 2006
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