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

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