Electronic Engineering (EE) Times
July 14, 1997
"Pulse charging augments memory loss"
By Joe Pendergrass, Vice President of Engineering, Advanced Charger Technology Inc., Norcross, Ga.
Research into more effective ways to charge batteries has been in progress since the
early 1900s, much of it until recently driven by the military and aerospace industries. In
the 1970s, pulse charging arrived on the scene as the first approach to increase charging
efficiency by addressing the chemical processes occurring in the battery. Now an advance
in pulse-charging technology promises to provide a number of benefits, including
eliminating the so-called "memory effect" that bedevils the nickel-cadmium
batteries used in many portable electronic products.
Pulse charging relies on providing a pulse current to the battery for up to one second, followed by a rest period (no charge) lasting for milliseconds. As in the constant-current charge method, ions generated at one electrode during charging 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 mass-transport limitations within the battery. This leads to poor charge efficiency, which results in heat generation, poorer battery capacity and shorter life span.
Periodically interrupting the charge allows the ions to diffuse and distribute more evenly throughout the battery. Letting the ion concentration return to normal levels on a routine basis minimizes the negative effects seen with a constant-current charge.
In the late 1970s, a variation was added to the pulse-charging regime involving adding a discharge pulse to the rest period. Following the pulse-charge period, comes a short rest period, topped by a discharge pulse of very short duration approximately 2.5 times the magnitude of the charge pulse. This is followed by another rest period, and the process repeats.
The addition of the single negative-discharge pulse accelerates the balancing of the ion concentration and addresses some of the negative effects caused by peripheral chemical reactions. The increased speed at which the battery returns to balanced conditions allows ever greater charge efficiency and improved battery performance.
Since the advent of pulse charging, little commercial research took place to improve charging methods until recently. Much of the work that did occur focused on determining when a battery is fully charged and on addressing new chemistries. In the late 1980s and early 1930s, research into improving the charging method for all battery chemistries was picked up once again by a Russian immigrant named Yury Podranzhansky, the vice president of research at Advanced Charger Technology (ACT). Podranzhansky began working with pulse charging with a single negative pulse, and has significantly advanced the technology from there.
If a single negative pulse is applied for too long, negative effects can occur in the reverse direction. These include excessive discharge of the battery, which extends the charge time and causes ion-transport problems in the discharge direction. Podranzhansky found that applying multiple negative pulses of short duration-but with a much greater magnitude-circumvents these potential negative effects, and, brings significant benefits to all battery chemistries. The larger-magnitude discharge are charged, eliminating the need to discharge the battery before recharging.
The lead-acid battery was the first on the scene in the mid-1800s.
It was almost a century before NiCd batteries followed. These two battery types still
dominate the rechargeable-battery market today, although new chemistries developed for
commercial use have recently begun making headway. At the vanguard of these new
chemistries are nickel metal hydride, lithium ion, rechargeable alkaline manganese and
All of them operate on the same basic type of electrochemical process. As a battery is discharged, its internal electrochemical process results in the transfer of ions from one electrode to the other through the electrolyte. When the battery is charged, the process it reversed and the ions travel in the opposite direction. During this electrochemical process, each electrode goes through a chemical reaction that generates these ions at one electrode and consumes the ions at the opposite electrode. How well this process is carried out has a significant impact on the overall performance of the battery.
A battery consists of two electrodes-a negative anode and a positive cathode-with a porous separator in between. The electrodes and separator are placed in an electrolyte solution that has an initial concentration of ions to support the chemical reaction and provides a medium for subsequent ion transport. The rate and uniformity by which the ions move from one electrode to the other significantly affects the performance of the battery.
The chemical-reaction rate at the electrode that consumes ions is limited by the concentration of the ions at its surface. This concentration is related to how well the ions are able to move through the electrolyte and the separator. If the ion concentration across the surface of an electrode is uneven, the chemical-reaction rate will not be uniform, leading to the development of dendrites.
Another factor in the performance of a battery is centered around
the metallic structure of its electrodes. A finer-grain structure reduces internal
resistance and increases surface area. Under extended low-current conditions, the slower
chemical-reaction rates can lead to the development of relatively larger metallic crystals
that 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 given discharge current. To maximize the performance of a
rechargeable battery, the charging regime should work with the electrochemical process to
ensure a high, uniform ion concentration at the electrode that is consuming ions.
In addition to these issues with the basic electrochemical process, NiCd has a characteristic that manifests itself as a voltage depression, often referred to as memory effect. Memory effect occurs when portions of the nickel electrode are left in a charged state for long periods of time. The charged portion of the nickel electrode will change its metallic structure over time into one that requires the cell voltage to drop below normal before it can return to its normal electrode configuration. Most electronic 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 recovered without performing special procedures.
Several techniques are used in the conventional approach to charging a battery. The first and most common in consumer products is the constant-current trickle charge. These chargers provide a very low, constant-current rate to the battery and rely on user intervention to stop the charge when the battery has returned to full capacity. These slow, "overnight chargers" are generally designed to fully charge a battery in approximately 10 hours. They are very economical and simple to design, but do nothing to optimize the performance of the battery.
Moreover, 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 manage the charging process makes the battery susceptible to overcharging and-in the case of NiCd-to voltage depression.
The next step up in technology is to increase the constant-charging current to achieve faster charge times. The increased charge current requires the addition of rudimentary charge-control circuitry, which will determine when the battery is fully charged and terminate charging. The advantage of this method is that an equivalent charge is achieved in only two to three hours. However, this approach also ignores the electrochemical 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 crystalline formation. All these factors lead to reduced capacity and shortened cycle life of the battery.
A deviation on the constant-current approach is the
constant-current/constant-voltage charge profile. Under 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 the constant-charge regime.
Overall, technological improvements in battery charging have been slow to emerge. Research-and-development work at ACT has raised the level of battery-charging technology to one that monitors and responds dynamically to the electrochemical state of the battery. The "dynamic electrochemical waveform" technology has taken the work of pulse charging and moved it to a new level. ACT has also focused on methods to accurately monitor the electrochemical state of the battery and dynamically adjust the charging waveform to obtain an even greater charge efficiency.
As a result of this research, three patents have been issued, four more are pending and new products have been brought to market which will redefine battery recharging for all chemistry types.
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