Research Project
Pulse Charging Algorithms
for Nickel Metal Hydride
Battery Chemistries.
By Chris Harrison
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Introduction
(History and background Information about project)
The history of batteries dates back to around 1800 when Alessandro Volta
experimented with the generation of electrical current from chemical reactions
between dissimilar metals. The original voltaic pile, or stack of alternating metals
(cathode and anode), used zinc and silver disks separated by a porous nonconducting material saturated with seawater. When stacked together a voltage
could be measured across each silver and zinc disk. Johann Ritter first
demonstrated the elements of a rechargeable battery in 1802. However,
rechargeable batteries remained impractical until the development, much later in
the century, of steam-driven dynamos to recharge them. George Leclanche of
France developed a form of the carbon-zinc battery in the 1860’s. The original
version was a wet cell, a battery with a liquid electrolyte. It became popular
because it was tough, easy to produce, and had a good shelf life. The original
design was improved with the advent of a paste-like electrolyte, which yielded
better results. Over the next 60 years, people experimented with different
combinations of metals and electrolytes.
Carl Gassner is credited with constructing the first commercially
successful "dry" cell, or battery that doesn’t require a liquid or paste electrolyte.
This was a important step towards modern batteries. Batteries could now be
completely sealed and didn’t leak electrolytes, and therefore didn’t need “topping
up”. Also, these new batteries were less sensitive to temperature and weather
conditions. Another major advance towards modern batteries was made during
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World War One. Batteries needed large-scale production for use in powering
lights and field radios. This demand accelerated research and greatly improved
battery performance by the end of the war. Development over the last century
has been rapid, and has seen the evolution of many new types of batteries, many
created for specific applications due to desirable qualities. Modern batteries use
a variety of chemicals to power their reactions. Typical battery chemistries
include:
• Zinc-carbon batteries - Also known as standard carbon batteries. Zinc-carbon
chemistry is used in all inexpensive AA, C and D dry cell batteries. The
electrodes are zinc and carbon, with an acidic paste between them as the
electrolyte.
• Alkaline batteries - Used in common Duracell and Energizer batteries. The
electrodes are zinc and manganese oxide with an alkaline electrolyte.
• Lithium photo batteries - Used in cameras because of their ability to supply
power surges.
• Lead-acid batteries - Used in automobiles (car batteries). The electrodes are
made of lead and lead oxide with a strong acidic electrolyte and have
rechargeable capability.
• Nickel-cadmium battery - Uses nickel hydroxide and cadmium electrodes with
potassium hydroxide as the electrolyte. Rechargeable.
• Lithium-ion - Very good power-to-weight ratio and found in high-end laptop
computers and cell phones. Rechargeable.
• Zinc-air – Lightweight, rechargeable.
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• Zinc-mercury oxide - Most commonly used in hearing aids.
• Silver-zinc - Used in aeronautical applications because the power-to-weight
ratio is good.
• Metal Chloride - Used in electric vehicles.
• Nickel-metal hydride (NiMH) - Rechargeable. Rapidly replacing nickel-cadmium
because it does not suffer from the memory effect that nickel-cadmiums do.
Overall, Nickel-based cells are excellent for jobs requiring the ability to use
rechargeable batteries that take very little time to recharge and survive under
rough conditions. This is the type of battery discussed in this research paper.
In order to make batteries economically viable, they must be reusable.
Charging batteries was seriously developed with the introduction of the steampowered dynamos in the 1870s. The electricity generated by the dynamos could
be fed into a battery in the reverse way (positive to negative). This, in turn, would
reverse the chemical reaction in the battery and return it to the state the battery
was in before it was discharged. This process of charging and discharging is
called a “cycle”.
A common human dislike is that of wasting time, and because of this fact
researchers are developing new charging techniques to recharge batteries faster.
Researchers have developed a multitude of charging “profiles” that force energy
back into the battery in different styles. These different styles are responsible for
several important factors.
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• Battery life – If the charging is too intense, or heats the battery too much, the
battery life will decrease because of the strain on the delicate materials inside the
battery.
• Charging time – The more efficient the charging profile, the faster it will charge,
and the less heat it will release. The heat generated from a battery is wasted
energy. Normally you have to put 110% back into a battery to compensate for
wasted energy like heat loss.
• Battery Capacity – If the charging process is inefficient or doesn’t charge it to it
maximum capacity, the battery starts to “memorize” the place where it is charged
to, e.g. 80% full. If this process is done several times, the battery has a “memory
effect” that will set the new battery capacity at that 80% full. This is very serious
because if the process continues the capacity will keep decreasing and be render
the battery useless. If a battery is correctly charged, the factors discussed above
can remain similar to their optimal state.
There are presently hundreds of charging profiles. These profiles can be
displayed as graphs showing current going through the battery and the battery’s
internal voltage over time. Two standard profiles and another type of profile
relating to this project will be discussed.
• Float charging at first “opens the flood gates” and rapidly charges the battery to
almost full (e.g. 85%). Then, as the battery gets fuller, the battery becomes more
resistant to the current. This starts to heat the battery. The charger has sensors
to detect this change and modifies the charging current, reducing it slowly. This
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forces the remaining energy in more slowly, which will prevent the battery from
overheating.
• Taper charging does something similar. Instead of rapid charging and then
slowly charging due to the heat constraints, Taper charging slowly decreases the
charge current from the beginning to keep the heat relatively constant. (See fig.
1 for graphs for a comparison of Float and Taper charges.)
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(Fig. 1)
• Pulse charging is another form of charging. Instead of a constant current there
is a pulsing one. There are two popular types, rectangular and cosine (See Fig.
1). When pulse charging is used the battery is charged with a high intensity
pulse, followed by a space where the battery can absorb the energy forced into it,
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and to cool down before the next pulse. Pulse charging methods can be
modified in a multitude of ways. The height, also referred to as amplitude, can
be modified to increase the amount of current during the pulse. The width of the
pulse can be adapted to have longer periods of charging per pulse. The space
between pulses can also be changed to regulate the time the battery receives no
charging. Different combinations of amplitude, width of pulses and the spaces
between the pulses can result in a perfect ratio where charging becomes very
efficient. This project will investigate the efficiency of pulse charging NiMH
batteries.
Review of The Literature
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Forward Drive “The Race to Build Clean Cars for the Future”, Jim
Montavalli, Sierra Book Club, San Francisco, 2000.
Helped me develop a better understanding of electric car, batteries,
and the history and the future of both.
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ACT Obtains European Distribution, Atlanta Tech, Atlanta, Nov 11, 1998.
Gave me a reference for Advanced Battery Technologies. I
explored this and found a wealth of information regarding pulse
charging technologies, specifically charging algorithms.
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Maintenance-Free Batteries, Seconds Edition, D. Berndt, Research
Studies Press Ltd., Somerset, 1997.
Provided me with background information about DC and pulse
charging. It also covers the techniques employed for charging
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NiMH batteries, including a little on pulse charging. Allowed me to
start thinking about the connection in my experiment between the
NiMH batteries and pulse charging them.
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The Fifteenth Annual Battery Conference on Applications and Advances
(Proceedings), California State University, Long Beach, Jan 11-14, 2000.
Improved Charge Algorithms for Valve Regulated Lead-Acid Batteries, E
Sexton Et Al. Page 211.
Included a very scientific paper about pulse charging Valve
Regulated Lead-Acid Batteries (VRLA). The paper gave me a
better understanding of how an pulse charging experiment was
performed and also showed some simple modification of the pulse.
This pushed me to a higher level of understanding of how my
experiment could run.
* Wilfredo Chaluisant, Applications Engineer, Curtis Instruments, Inc. Mount
Kisco, NY.
Mr. Chaluisant’s generosity with his knowledge and time has played
an influential part in my development as a research student. My
understanding of this research topic would be deficient without his
guidance.
Hypothesis
Pulse charging will provide a more efficient charge due to the less
strenuous nature of pulse charging on the battery, and therefore extend battery
life, increase charge efficiency, and decrease charge time.
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Major Materials
• NiMH Batteries
• Modifiable charging apparatus
• Computer + software to monitor charging and battery statistics
Methods
The experiment will be to compare several pulse charging methods with
an industry standard constant current charging method. Trials will consist of 2 or
more batteries running the same profile so as to obtain the most reliable results
(as opposed to using just one battery, where results may be more unreliable).
The trials would be run as follows:
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Completely discharge battery to a fixed level - this must be the same for
all trials.
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Modify charger to desired settings. These are the options mentioned in
the introduction. “Pulse charging methods can be modified in a multitude of
ways. The height, also referred to as amplitude, can be modified to increase the
amount of current during the pulse. The width of the pulse can be adapted to
have longer periods of charging per pulse. The space between pulses can also
be changed to regulate the time the battery receives no charging.”
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Attach battery to monitoring equipment and charger.
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Repeat steps 1-3 for subsequent batteries
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Start monitoring batteries, charge currents and chargers.
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Start charging process for all batteries.
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When batteries are fully charged, the charging will stop and results will be
recorded.
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The results will be compared to previous trials to see which is the fastest
and/or most efficient.
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Plan next trial based on all data collected.
Expected Results
Experimenting with pulse charging will produce a great variety of charge
profiles. Most will be more inefficient than current industry standards. However ,
some trials will produce efficient profiles with serious benefits. Some will be
trade-offs: Fast charging but reduces battery performance, slow charging but
great battery performance. It is anticipated that one may prove to be close to the
“perfect ratio”.
Significance
With a more efficient charging profile, less energy will be wasted during
charging and batteries can be properly recharged so they can have extended life.
This is beneficial economically because batteries will charge more efficiently and
save energy as well as not have to be purchased as often. Subsequently, less
will end up in landfills that can damage the environment. This charging profile
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will could also prove to be faster than conventional chargers and could be used
to rapidly charge certain applications with time constraints, for instance, electric
cars.
Conclusion
Because the topic is new and with no formal experimentation completed, stating
any conclusions would be rash. However, as mentioned in the expected results,
a relatively good charging profile is expected. The data collected could be used
by other individuals in an attempt to find the “perfect ratio” by completing more
trials. The more trials that are conducted, relationships concerning pulse
charging will be able to be more easily identified.
Acknowledgements
Wilfredo Chaluisant – Instrumental in the development of my research project
and my understanding of the accompanying material.
Terry and Terri Harrison – Helped and encouraged me throughout my 1 1/2
years in this research class.
Nancy Williams – Encourages me to do my best and strive for excellence.
(last name alphabetical)
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