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The A r t of Battery Charging
Richard C. CopeaandYury Podrazhanskyb
"Advanced Charger Technology, 680 Engineering Drive, Suite 180, Norcross, GA 30092, rcc@actcharge.com
bAdvanced Charger Technology, 680 Engineering Drive, Suite 180, Norcross, GA
30092,
ymp@actcharge.com
ABSTRACT
The demand for portable products is show ing exponential growth with n o end
in
immediate sight. A long with the overall
growth in volume h as com e increased demand for greater features and functions. This combination has brought the issue of
power manage ment to the forefront of engineering design considerations. The overall success of a portable product will not
only be dictated by its features and functions, it will also be influenced by how long it can perform before running out of
power, the time it takes to return the batteries to full capacity and the life e xpec tancy of the battery. Soun d engineering design
begins with a good working knowledge of batteries and battery charg ing techniques .
Rechargeable Battery Bas ics
The lead acid battery was the first on the scene in the
mid-1800's. It was almost a century before Nickel Cad-
mium batteries followed. These two battery types still
dominate the rechargeable battery m arket today. Recently,
new chemistries have been developed for comm ercial use
which are making significant headway into the market-
place. At the vanguard of these new chemistries is Nickel
Metal Hydride, Lithium Ion, Rechargeable Alkaline Man-
ganese and Zinc Air. All of these commercial battery
types operate on the same basic type of electrochemical
process. As a battery is discharged, its internal electro-
chemical process results
in
the transfer of ions from one
electrode to the other through the electrolyte. When the
battery is charged, the process is reversed and the ions
travel in the opposite direction. During this electrochemi-
cal process, e ach electrode goes through a chem ical reac-
tion which generates these ions at o ne electrode and con-
sumes the ions at the opposite electrode. How well this
process is carried out has a significant impact on the over-
all performance of the battery.
A battery consists of
tw o
electrodes, a negative anode
an d a positive cathode, with a porous separator in be-
tween. If the electrodes come into contact with one an-
other, the battery would be shorted and of no use. The
electrodes and sep arator are placed in
an
electrolyte
solu-
tion which has
an
initial concentration of ions to support
the chemical reaction and provides a medium for subse-
quent ion transport. The rate and uniformity by which the
ions move from one electrode to the other significantly
impacts the performance of the battery. The chem ical re-
action rate at the ele ctrode which consu mes ions is limited
by the con centration of the ions at its surface. This con-
centration is related to how well the ions are able to move
through the electrolyte and separator. If the ion concen-
tration across the surface of an electrode is uneven, the
chemical reaction rate will not be uniform, leading to the
development of den drite s-ou tgrow ths of material from
the electrode. If not addressed dendrites can eventually
grow through the separator and cause the two
electrodes
to co me into co ntact and short out the ba ttery.
Another factor in the performance of a battery is cen-
tered around the metallic structure of it's electrodes. A
finer grain structure reduces internal resistance and in-
creases surface area. Under extended low current condi-
tions, the slower chemical reactions rates ca n lead to the
development of relatively larger metallic crystals. These
larger metallic crystals reduce the surface area, causing a
potential drop in overall battery capacity and an increase
in internal resistance. The increase in internal resistance
will result in a lower battery voltage for a g iven discharge
current.
To maximize the performance of a rechargeable battery,
the ch arging regime should work with the electrochem ical
process to en sure a high uniform ion concentration at the
electrode which is consuming ions. In addition to these
issues with the basic electrochemical process, Nickel
Cadmium has a characteristic which manifests itself as a
voltage depression, often referred to as memory effect.
Memory effect occurs when portions of the nickel elec-
trode are left in a charged state for long periods of times.
The charged portion of the nickel electrode will change
it's metallic structure over time into one w hich requires
the cell voltage to be dropped below normal for it to re-
turn to it's normal electrode configuration.
Most
elec-
tronic equipment will stop operating before the battery
can reach
a
low enough per-cell voltage to recover the
voltage depression. In other words, the capacity lost to
voltage depression in normal operations may not be re-
cove red without performing special proced ures.
Conventional Charging
There are several techniques used in the conventional
approach to charging a battery. The first and the most
common in consumer products is the constant current
trickle charge. These chargers provide a very low, con-
stant current rate to the battery and rely on user interven-
tion to stop the charge when the battery has returned to
0-7803-4967-9/99/$10.00 @ 1999 IEEE 233
full capacity. These slow , “overnight charge rs” are gener-
ally designed to fully charge a battery in approximately
ten hours. They are very econom ical and simple to design
but do nothing to optim ize the performan ce of the battery.
Their low charge rate allows the chemical reactions to be
localized on the electrode surface leading to potential
dendrite growth. Their dependence on the user to m anage
the charging process makes the battery susceptible to
overcharg ing and, in the case o f Nickel Cadmium , voltage
The next step up in technology is to increase the con-
stant charging current to achieve faster charge times. The
increased charge current requires the addition of rudi-
mentary charge control circuitry which will determine
when the battery is fully charged and terminate charging.
The adv antage of this method is that an equivalen t charge
is achieved
in
only
two
o three hours. However, this ap-
proach also igno res the e lectrochemical process within the
battery, resulting in significant long-term negative effects.
The high constant current will cause significant deviation
in ion concentrations between the electrodes. Charging at
a high constant current rate can overdrive the chemical
reactions with regard to the supporting ion concentration
available at the electrodes. This results in the generation
of heat, along with dendrites and poor electrode crystal-
line formation. All these factors lead to reduced capacity
and sho rtened cycle life of the battery.
A deviation on the constant current charge approach is
the constant current/constant voltage charge profile. Un-
der this arrangement, a constant current is applied until
battery voltage rises to a predetermined value, at which
point the charging voltage is held constant and the current
is
reduced. When current has reached a minimum value,
the charging stops. This approach drops current in the
final phase of charging when less electrode surface is
available to react and the overall concentration of ions
may be lower. This approach suffers from all the same
problems to a slightly lesser degree
as he constant charge
regime.
The direct result of these lower-cost, simply designed
conven tional battery. charg ers is a p otential reduction of
battery capacity and a shorter life span.
Pulse Charging
In spite of these deficiencies, technological improve-
ment of battery charging has been slow to emerge. Re-
search into more effective means for charging batteries
has been in progress since the early 1900’s, much of it
driven by the military and space agencies until recently.
In the 1970’s, pulse charging arrived on the com mercial
scene. This approach to charging was the first to increase
the efficiency of the charging process by addressing the
chemical processes occurring in the battery. The tech-
nique relies on p roviding a pulse current to the ba ttery for
up to one sec followed by a rest period of n o charge last-
ing for milliseconds.
As
in the constant current charge
method, ions are generated at one electrode during the
charging period and must move to the other electrode. If
the constant current is applied for a significant period of
time, an ion concentration gradient builds up due to m ass
transport lim itations within th e battery. This leads to poor
charge efficiency which results in heat generation , poorer
battery capacity and shorter life span. Periodically inter-
rupting the charge allows the ions to diffuse and distribute
more evenly throughout the battery. By allowing the ion
concentration to return to norm al levels on a routine basis,
the negative effects seen with a constant current charge
are minimized.
In the late 1970’s, a variation was added to the pulse
charging regime. This involves adding a discharge pulse
into the rest period. Following the pulse charge period
there is a short rest period followed by a very short dura-
tion discharge pulse, approximately
2.5
times the magni-
tude o f the charge pulse. This is followed by an other rest
period and the process is repeated. The addition of th e
single negative discharge pulse accelerates the balancing
of the ion concentration and addresses some of the nega-
tive effects caused by peripheral chemical reactions. The
increased speed at which the battery returns to balanced
conditions allows ever greater charge efficiency and im-
proved battery performan ce.
Since the ad vent of p ulse charging, little research work
within the commercial community was focused on im-
proving charging methods until the late 1980’s. Much of
the research work has been focused on determining when
a battery is fully charged and on addressing new chemis-
tries.
State-of-the-Art
In the late 1980’s and early 1990’s, research into im-
proving the charging method for all battery chemistries
was pick ed up once again by a Ru ssian immigran t to the
US named Yury Podrazhansky. Currently, Podrazhansky
is the V P of Research at Advanced Charger Technology,
Inc (ACT), where his innovative research has resulted
in
ground-b reaking product design.
Podrazhansky began working with pulse charging with
a sing le negative pulse, and h as significantly advanced the
technology from there. The limitation with the single
negative pulse is that if it
is
applied for too long of a du-
ration, negative effects can occur in the reverse direction.
These include excessive discharge of the battery, which
extends the charg e time and causes ion transport problems
in the discharge direction. Through research and analysis
Podrazhan sky found that applying m ultiple, short duration
negative pulses with a much greater magnitude circum-
vents the potential negative effects of an extended single
pulse and brings significant benefits to all battery chem-
234
istry types. The larg er magnitude dischar ge pulses are
inherently focused in the a rea of dendrites serving to re-
move them; the momentary high currents rapidly balance
the ion concentration and improve the metallic crystalline
structure of the electrodes. The improved balancing of ion
concentration leads to a highly efficient charge process
which supports a much higher charge current. This high
charge current yields the shortest charge times possible.
In addition, an added benefit is found with Nickel
Cadmium batteries. The multiple, short high magnitude
discharge pulses momentarily pull the battery voltage
down to below 0.8 volts per cell resulting in the reversal
of the effects of voltage depression. Through this method,
‘ Nickel Cadmium batteries are conditioned as they are
charged, eliminating the need to discharge the battery
before recharging. This advancement in battery charging
technology provide s an elegan t solution with many
benefits.
The research and dev elopmen t work at ACT has raised
-
charge
0
charge 0
Fault
0
complete
-
monitors and responds dynam ically to th e electrochemical
state of the battery. The “Dynamic Electrochemical
Waveform” technology has taken the work of pulse
charging and moved it to a new level. Following these
advancements in the method for battery charging, ACT
focused on methods to accurately monitor the electro-
chemical state of the battery and dynamically adjust the
charging wav eform to obtain an even greater charge effi-
ciency. As a result
of
this research, three patents have
been issued, four more are pending, and new products
have been brough t to m arket which will redefine battery
recharging for all chemistry types. Future work a t ACT is
focused on adding automatic battery chemistry recogni-
tion, automatic battery capacity determination, methods to
improve ion mass transport, addressing new battery
chemistries as they enter the market and continually
looking for new and innovative ways to move the state-
of-the-art of battery charging ahead. ACT as a company is
committed to being the leader in the advancement
of
bat-
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