MICHIGAN STATE UNIVERSITY COLLEGE OF ENGINEERING
RECHARGEABLE BATTERY APPLICATIONS
Danny MacBeth
11-5-2013
Introduction
More and more of our daily lives are becoming dependent on electricity as a source of
energy. Applications that require wireless mobility or stored charges often utilize various styles
of batteries to accomplish this. Batteries can be divided into two main categories, rechargeable
and non-rechargeable, and for both styles there exists many variances based on the materials that
are used when designing the battery. This is why different applications require different types of
batteries. As with most engineering matters, there are going to be trade-offs between the
available choices. One of the difficulties of design is choosing what characteristics to give away
in order to gain other features that the design must have. To truly balance the trade-offs in a
design it is essential to understand the components in consideration. This document will outline
characteristics of common rechargeable batteries and how to choose the correct battery for a
given application.
General Overview
Rechargeable batteries come in a variety of
shapes and sizes, from button sized cells to industrial
megawatt systems. The main advantages that
rechargeable batteries have when compared to their
disposable equivalents is a lower total cost of use and
much lower environmental impact. Though they are
initially more expensive than disposable batteries of the
same size, their ability to be recharged many times
makes them cost effective over time.
Since all rechargeable batteries accomplish the
same basic objective, they all also follow a similar
design. Each battery is made up of a number of cells to
Diagram of Secondary Cell
reach an appropriate voltage. Within each cell lie two
electrodes in an electrolyte solution. The electrolyte
may simply act as a buffer for electron flow between electrodes, but can also be a participant in
the electrochemical reaction. It is important to note that in rechargeable cells (secondary cells),
the electrodes act in reverse in the charging phase than in the discharging phase; the anode
becomes the positive electrode and the cathode becomes the negative electrode in the charging
phase. There is also a permeable membrane placed between the anode and cathode called the
separator. The main purpose of the separator is to prevent any short circuits between the
electrodes while allowing charge carriers (electrons) to flow through the electrolyte.
The chemicals that make up the positive and negative materials, along with the
electrolyte, are known as the active components of a secondary cell. The positive and negative
parts are made up of different materials. The positive component must exhibit reduction
potential, “the tendency of a chemical species to acquire electrons and thereby be reduced”1,
while the negative component must oppositely possess an oxidation potential. By varying the
chemical constituents used during the construction of a battery, the overall characteristics of a
battery can change drastically.
Battery Ratings
The characteristics between different kinds of batteries can be compared through a
variety of ratings. The most obvious rating when looking at batteries is capacity. Battery capacity
refers to the total amount of energy that can be stored within a battery. There are five factors that
dictate the capacity of a given battery: size, temperature, cut off voltage, discharge rate, and past
usage. Most of these considerations are determined by the manufacturer, but keeping in mind the
planned usage of the battery in question (operating temperature, expected usage/maintenance)
should be an important factor in any design.
The two most common ratings for capacity are Amp Hours and CCA (Cold Cranking
Amps). Amp Hours is a rough estimate of how much can current can be supplied for a given
amount of time, though it is not a linear progression. If a significantly high current is expected to
be drawn continuously from a battery, there will be additional losses not completely accounted
for in this rating. This rating is typically applied to batteries that provide a constant current, such
as deep cycle batteries. CCA refers to the instantaneous current a battery can provide at 0 F for
30 seconds. This rating is nearly exclusively used for vehicle batteries in regards to starting
currents during the winter months.
Typically, batteries are optimized to have either high energy density or high power
density, making these the most referenced ratings. Normally you will want the highest density
available, but which density to reference will vary with the application. Batteries with a high
energy density rating can store large amounts of energy, and release it reliably over long periods
of time. Power is the rate at which energy is expended; therefore batteries with a high power
density can release large amounts of energy quickly. This rate depends on the internal resistance
of the battery.
Another important rating to look at when choosing a battery is the charge/discharge
efficiency. This calculation is made by measuring the amount of power used from the battery
while discharging divided by the amount of power delivered to the battery while charging and
then multiplying this by 100. The largest factor that negatively affects this efficiency is the
power lost through heat, therefore the lower the efficiency is, the more likely it is that the battery
will be warmer than one of better efficiency. Separate to the charge/discharge efficiency, but
similarly listed is a batteries self-discharge per month. This is simply the percentage you can
expect the battery to self-discharge in a month’s time being isolated from any other circuitry.
The last major assessment when choosing between batteries is the number of cycles you
can expect in the lifetime of the battery, known as cycle durability. The number of cycles for a
rechargeable battery indicates how many times it can undergo the process of complete charging
and discharging until it loses capacity. The number of cycles in this rating is assuming proper
usage and maintenance during its lifetime (no overcharges, short circuits, ect).
Charging & Discharging
Rechargeable batteries consist of one or more electrochemical cells. The various types of
rechargeable batteries operate on the basis that the chemistry within each respective type is
electrically reversible. When a battery goes “dead”, by reversing the current flow you reverse the
chemical process within the battery and effectively recharge it.
During a charge, the positively
charged ions (holes) travel from the
cathode through the electrolyte. They
will continue to land on the anode until
no more charge can be stored.
Conversely during a discharge the
charges flow in reverse until the cathode
can hold no more charge. One complete
revolution of this charge trade is called
a cycle.
While the specific voltages and
currents to charge each style of battery
are different, they all follow the same
method to charging. The first stage is
the bulk charge. This is fastest part of
Phases of Battery Charging
the charge, where current is essentially
dumped into the cells to speed up charging. The voltage of the cells will rise at the end of this
stage and will then transfer into the second stage, known as the topping charge. As the battery
approaches a full charge the current will gradually decrease to avoid overcharging the cells.
Once the battery is fully charged (around 98%), the trickle charge stage begins, which essentially
just provides a very small amount of current to prevent the battery from self-discharging.
Discharging rates depend on the type of battery being used and the load being applied. As
stated earlier, battery discharge ratings are not linear based on the load, and are also affected by
temperature, age of the battery, and overall condition of the cells. There are acceptable levels of
discharges for each battery. Even for batteries made of the same active components, these levels
can vary greatly. For example, lead acid automotive batteries can only be discharged to about
80% full capacity without harming the battery, whereas lead acid deep cycle batteries can be
drained to 20% full capacity regularly. The acceptable discharge rate should be double checked
before implementing a battery into a design.
Common Rechargeable Batteries
Nickel Cadmium (NiCd)
Nickel Cadmium batteries are a very old and well understood type of battery. They are
very long lasting batteries, allowing for over 1000 charge/discharge cycles. These batteries are
most well known for being the most rugged style of rechargeable battery available, performing
well at very low temperatures and forgiving to various abuses, including overcharging.
Interestingly enough, they maintain charges very well if fast-charged, even if previously stored
for a long amount of time.
There are many tradeoffs to having such a rugged battery. These batteries have a very
poor energy density compared to more recent technology such as lithium-ion, and exhibit the
memory effect as well (if not periodically fully discharged, the battery will lose capacity). Since
these batteries contain toxic metals, they are among the worst when considering environmental
factors. Many countries are limiting the use of these metals, mainly cadmium, making them hard
to implement in modern technology.
Even with the disadvantages of nickel-cadmium, they are still used for rugged
applications such as two-way radios, emergency medical equipment and some power tools. This
is nearly completely due to their long life and ability to be abused and be able to bounce back.
This should not be the choice for most consumer designs as it is terrible for the environment, and
many countries now make the companies remove the same amount of cadmium from their
country as they bring in with their products, creating huge disposal costs.
Nickel-Metal Hydride (NiMH)
NiMH batteries are best described as a bridge between old and new technology. They
improved on older NiCd technology in a few areas. NiMH has a 30-40% higher capacity over a
standard NiCd battery, and is far less prone to the memory effect found in NiCd. The best
advantage over its predecessor is that it is environmentally friendly.
The number of disadvantages with this type of battery will usually outweigh other
choices. While it can withstand some deep-cycling, they will eventually be damaged from
practicing this. They are also prone to damage when used for high discharges, and compared to
newer technologies has a very high self-discharge. Like the Ni-Cd batteries, they require full
discharge regularly to prevent crystalline formations on its electrodes, plus are more expensive
than NiCd.
Nickel-Metal Hydride batteries are good for applications which require both attributes of
NiCd and Li-ion batteries. They are more rugged than Li-ion, but have far less environmental
hazards than NiCd batteries. They are much cheaper than Li-ion batteries, but require
maintenance. This is the middle of the road for smaller mobile applications. They can be used in
applications such as cell phones and laptop computers, but seem to be losing the market to Li-ion
batteries, despite being lower priced.
Lithium Ion (Li-ion)
In comparison to aqueous batteries (lead-acid, nickel-metal hydride, nickel-cadmium),
lithium-ion batteries have a higher open-circuit voltage. Lithium-ion cells however will typically
have a higher internal resistance of many other comparable technologies. Over time the internal
resistance of the cells rises with increased cycling, making this technology subject to aging.
Eventually the increase of internal resistance will make the battery terribly inefficient and it will
no longer be able to operate for an adequate amount of time. This effect is most easily alluded to
with laptop computers; after a few years of use it is not uncommon to have to replace the
lithium-ion cells to ensure the laptop can hold a charge for an appropriate amount of time.
Other concerns to keep in mind regarding Li-ion is that they are regarded as the most
expensive type of battery to purchase as they are the newest form of battery technology, and
have yet to be perfected. They are considered fragile, especially compared to older NiCd
batteries, as they require a protection circuit to limit voltage and current. However, less circuitry
is required with respect to their charging circuit due to their extremely low self-discharge rate;
there is no need for a trickle charging stage like other rechargeable batteries.
Lithium ion is now the most widely used battery for mobile applications where highenergy density and lightweight is of prime importance. They are anywhere from mp3 players and
power tools to electric automobiles. This is due to many reasons, but mostly for their impressive
energy density and charge/discharge efficiency. Lithium-ion batteries contain no toxic metals
unlike batteries containing cadmium or lead. The majority of lithium-ion batteries are considered
safe for both incinerators and landfills.
Lead Acid
Lead-Acid batteries are by far the most economical for larger power applications, as long
as weight is of little concern. These batteries have been around since the mid-1800s because they
are durable and provide dependable service. The only maintenance requirements are keeping
them from discharging too far and there is no memory effect to deal with.
There are two styles of lead-acid batteries, starting batteries (automotive) and deep cycle
(marine). The main differences in the design of these batteries are in the electrodes. To provide
higher instantaneous currents the starting batteries maximize the surface area of its electrodes to
maximize current flow. This provides high currents at the cost of not being able to be deeply
cycle (the plates become too corroded to conduct). Deep-cycle batteries have thicker electrodes,
which make them able to be more deeply discharged and provide constant current, but cannot
provide the large starting currents needed to start motors.
Like every battery, there are concerns associated with lead-acid batteries as well. Being
discharged past a certain point will ruin the capacity of the battery. This is important not to let
this happen since they are very environmentally unfriendly as they contain lead. The limiting
factor of these batteries for most applications though is purely the size and weight of these units.
By far the most common application of these batteries are with vehicles for starting
engines, as they are heavy enough that the weight is not of much importance. They are also
popular in larger energy applications that require constant current, such as golf carts and solar
power storage. They are very cheap and available nearly everywhere.
Choosing the Right Battery
When determining which battery should be used for a specific application, the most
important thing to be aware of is the differences between the options available. Once you know
what characteristics are present with each type of battery, a series of questions can be posed to
help narrow down your search:
1. Should be a primary (non-rechargeable) or secondary (rechargeable) cell be used?
Since primary cells can only be used once and then have to be discarded, any application that
requires a long lifetime of use should use rechargeable batteries. Primary cells do cost less per
battery, and have a higher energy density, but over time will be the more expensive option.
2. What temperature range will the battery be expected to perform in? Not all batteries
perform well in temperature extremes, and the ones that typically do are not usually
environmentally friendly.
3. What kind of discharge rate will the battery need to meet to supply the correct power?
There can be extreme differences in performance even within the same type of batteries, such as
the lead-acid case of starting and deep-cycle batteries.
4. Is the battery expected to be used right away, or to be stored most of its life? Though it
is never a good idea to store batteries in a device that is seldom used, if a device is meant to be
used intermittently, choose a battery with low self-discharge.
After posing these questions, it is usually much easier to narrow down your search to a
specific battery. Most of the other choices to make concern ratings such as capacity, cycle
durability, and charge/discharge efficiencies. Environmental issues can also be a large factor
especially in consumer products due to issues with safe disposal. By choosing the right battery
characteristics for an application you can ensure maximum performance of your design.
References
Buchmann, I. (2013). All about chargers. Retrieved from
http://batteryuniversity.com/learn/article/all_about_chargers
Buchmann, I. (2013). Whats the best battery?. Retrieved from
http://batteryuniversity.com/learn/article/whats_the_best_battery