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Managing A123 Cells with FMA Cell Balancing Technologies


Managing A123 Cells with
FMA Cell Balancing Technologies


By
FMA Staff


























Latest revision: March 23, 2007

Document provided by
FMA, Inc.
5716A Industry Lane
Frederick, MD 21704 U.S.A.
Phone: (800) 343-2934
Fax: (301) 668-7619
Web:
www.fmadirect.com


Contents

Executive summary ................................................................................. 3

A123 cell characteristics ......................................................................... 3

Cell testing equipment and conditions .................................................. 4

Test data and analysis............................................................................. 4

A123 charge algorithm ............................................................................ 8

Notes ......................................................................................................... 9

Suggested reading................................................................................. 10

From the e-mail blast ............................................................................. 11




3
Executive summary

FMA Incorporated has independently tested A123 Lithium Ion nanotechnology
cells and found them to be very good high energy density cells. A123 discharge
rates that approach and exceed 17C continuous rival contemporary LiPo cells.

FMA charge and control technology improves A123 pack service life.



A123 cell characteristics

A123 cells have an abrupt drop in voltage at the end of discharge that can lead to
pack unbalance. That characteristic can be countered effectively using FMAs
Discharge Protection Module (DPM). A123 cells are not damaged by under
voltage at 2.5V, but do suffer shortened life if voltage is allowed to drop to 0V (as
would happen if left with the receiver/servos connected for a period long enough
to drain the pack).

A123 cells can withstand short bursts at discharge rates up to 33C (33 times
capacity). Despite outlandish marketing claims of 20C-30C capability, LiPo cells
struggle to deliver much greater than 15C, and that is accompanied by very short
life in the range of 50 cycles. Capacity for A123 cells at this time is limited to a
nominal 2AH.

While A123 energy density is superior to LiPos, cell weight is 56% heavier than
LiPo for a given WH delivery. Applications in RC for the A123 will be limited to
those aircraft that are not sensitive to the weight disadvantage (e.g., helicopters)
or that demand a very high discharge rate (e.g., hotliners).

FMA, Inc. has developed and has available optimized charge and control
equipment that matches the charge and discharge regime unique to A123 cells.
FMA has performed the research and development needed to create products that
allow for totally safe charging of Lithium Ion and Lithium Polymer packs in any
voltage and capacity range. Our balanced discharge control products, when
combined with balanced charging, maximize the life and safety of these cells.

A123 cells are not as sensitive to under voltage as LiPos, but still do suffer
damage if subjected to low undervoltage. An A123 cell has almost zero capacity
left at a 2.5V cut-off. FMAs CellPro Discharge Protection Module (DPM) has
an adjustable low voltage cut off that protects A123 cells overdischarge.

The charge characteristics of A123 cells tested by FMA, Inc. show a definite
need for balance charging of the cells, particularly in higher voltage packs.
Failure to balance charge may not result in fire, but will certainly lead to
damaged cells in a pack.

4
Cell testing equipment and conditions

FMA operates intelligent Li Ion / LiPo testing equipment that provides 1%
measurement accuracy, and can discharge at up to 500A. This equipment was
programmed for A123 charge and discharge cutoff voltages.

This equipment can subject cells or packs to a variety of scenarios, including:
A constant current load.
A stepped constant current load.
A duty cycle simulating operation in some application.
Repetitive charge/discharge to determine cycle life in some application.

Sample cells were non-destructively tested to produce the data cited in this
report. End of life was defined as a decline to 80% of original capacity.



Test data and analysis

The voltage curves below were obtained by discharging A123 cells at constant
current at room temperature. Cycle life and cell operating temperatures are noted
in the legend.

One shortcoming of A123 cells is that maintaining acceptable voltage under high
discharge rates may require adding cells in series.

FMA previously determined that LiPo cycle life is directly related to cell
operating temperature. We recommend 140° F as the maximum LiPo operating
temperature. Operating temperature is such a key factor that we define the
nominal LiPo discharge as the rate at which cell temperature reaches 140° F.

FMA introduced and promoted cell temperature as a cell rating factor. A 2s LiPo
pack tested at 140° F exceeded 450 cycles. Operating cells above 140° F has a
devastating effect. An identical 2s pack reached end of life at 38 cycles when
operated at 175° F.

In general, cycle life is about flat up to an operating temperature of 140° F.
Above that life deteriorates at about 12 cycles per degree F.

The discharge curves below indicate that the 140° F guideline also applies to
A123 cells. For the cells tested, a cycle life of greater than 500 is possible if
constant current discharge is kept below 12.7C (which keeps operating
temperature below 140-145° F.
5



In most cyclic load applications, including power tools and RC aircraft, average
load is never as high as the constant current load, but load peaks are often well
above the average load. This is significant, because at high discharge rates,
temperature rises as the square of the current.

As the voltage depression graph below shows, a 10s application would require at
least one extra A123 cell to accommodate the 0.6V/cell drop under a 45A load.



6
When rated like a LiPo, an A123 cell would be conservatively rated at 17C, and
would tolerate 33C in bursts. Keep in mind that peak and continuous ratings
depend on acceptable cycle life. At 16.5C, A123 cells deliver 230 cycles.


Here is a comparison of an Enerland 3300 LiPo cell with an A123 cell:

Enerland
A123
Energy at 2C
12.25 Wh
6.83 Wh
Energy at 8C
11.48 Wh
6.41 Wh (8.7C)
Efficiency 93.7%
93.8%
Energy density
142.8 Wh/kg
91.6 Wh/kg
1Wh/7g
1Wh/10.9g


Here is a further analysis with cost factored in:


Enerland 3s pack
A123 3s pack
Power
152.46 W
103.4 W
Energy
10.56 Wh
5.87 Wh
Retail price
$125
$75
Power density
1955 W/g
1477 W/kg
Energy density
135.4 Wh/kg
84 Wh/kg
Cost $3.95/Wh $4.25/Wh




Although the A123 technology has a slightly higher cost per watt-hour, it is
attractive for its lower initial cost, safety and longevity. The A123 also offers
good cycle life if the temperature is kept below 140° F.
7
Here are some other comparison factors:


Enerland 3s pack
A123 3s pack
Voltage 9.9V
6.47V
Power
9.9V x 54.45 A = 539 W
6.47V x 54.45A = 352 W
Capacity at 16.5C
98%
99%
Energy
32.Ah x 3.3V = 10.56 Wh
2.158 Ah x 2.72V = 5.87 Wh

The Enerland 9.9V versus the A123 5.87V is key. Adding a cell brings the A123
pack up to 8.59V and produces similar power and energy. However, the A123s
cost per watt-hour will further exceed that of the LiPo, plus the (now) 4s pack
gains another 70 grams.

FMA uses a method for accelerated life cycle testing that has proven reasonably
accurate in hundreds of runs over the past four years. Cells are run through
multiple constant current discharge sequences consisting of a 4.2A baseline
followed by a high current (for example, 16A). Capacity measured during the
second step subtracted from capacity measured during the first step is a measure
of damage incurred from the high current run. The discharge rate is increased by
small increments after each set, so that testing is accelerated without destroying
cells.

The follow