Choosing the Right Battery Pack
When designing a battery pack many considerations
need to be taken into account to ensure the battery performs
to its specified requirements. A precise specification leads
to a correct design being developed.
One of the most important and often most
difficult design decision is the type of cells used for the
battery pack. Data sheets accompany all available cells providing
information on the operational characteristics of the cell
and detailing their rating. These data sheets, although useful,
cannot be the only source of reference when deciding which
if a given cell matches up to the requirements of the pack
to be assembled. The stated cell ratings are derived from
controlled experiments in controlled environments with cells
running at room temperature with low discharge rates. In real
use, this is often not the case and so the actual performance
of the cell could be quite different to that specified on
the data sheet.
In order to determine the most suitable
cell technology and size for an application, it is important
to evaluate the cells based on a 'real world usage profile";
this profile is based on a number of important factors, these
are outlined below:
1. Voltage Requirements
- Firstly the voltage required, or Nominal
Voltage is important to begin the design process. More detailed
information including voltage ranges is also useful so better
decisions can be made on the type of cells to be used in
the pack. Cells themselves do not deliver their stated voltage
constantly; therefore if the application has specific requirements
such as operational cut-off voltages, then the cells used
must not fall below this voltage during normal operation.
2. Capacity Requirements
- In order to work out the capacity the
pack needs to have, the amount of current drawn from the
battery during operation over a given time period needs
to be known. The type of this discharge
current also needs to be known, i.e. if it is drawn
continuously or in pulses. The rated capacity of cells is
generally the "best case" performance under laboratory
conditions at specific temperature and current drain; if
your application is different to these values, then the
battery may not perform as expected. For this reason, it
is important to specify the average and maximum discharge
currents so the correct cells can be selected. One factor
that is often overlooked is start-up or surge current.
3. Service Requirements
- The application of your battery will
determine the specifications of the cells used in your battery
pack. For both primary/non-rechargeable and secondary rechargeable
cells; the required run-time of the battery will affect
the battery type, size and chemistry of the cells used in
your pack. The environment in which the battery is operating
will also need to be specified, these include information
about; operating temperature's, storage, weight and dimension
- Obviously cost is a factor as with any
custom built product; the limitation in battery pack design
is that a technically ideal battery may be available, but
cost restrictions may limit you to a reduced performance
cell. It is however important to note that more expensive
cells more often than not pay for themselves with the increased
performance they deliver.
5. Primary or Secondary
- Primary cells are not intended to be
recharged and are to be used only once. Secondary cells
are intended to be re-charged and re-used a number of times
but require the use of an appropriate charger. Primary cells
generally have higher capacity but
lower self-discharge rates than secondary cells.
6. Storage and Self Discharge
- Batteries self-discharge over a period
of time when not in use, this means that the battery loses
energy whilst in storage. It is important to know how long
the battery will spend in storage and what environments
conditions it will be stored under. This table shows average
self-discharge rates of various battery chemistries:
|Alkaline (Zinc Manganese
||Less then 2% per year.
Approx shelf life of 4 years.
||Around 6% per month. Approx
shelf life of 18 months.
||Approx 4% per year. Approx
shelf life of 2 years.
|Zinc Silver Oxide
||Less then 2% per year.
Approx shelf life of 4 years.
|Lithium Manganese Dioxide
||Less then 1% per year.
Approx shelf life of 10 years.
|Lithium Poly Carbon-monofluoride
||Less then 1% per year.
Approx shelf life of 10 years.
|Nickel Metal Hydride (NiMh)
||Approx 1% per day if unused.
|Nickel Cadmium (NiCd)
||Approx 15-20% per month
||Approx 10% per month
- Note: These values are for storage in
a dry, well ventilated area at room temperature, storage
in damp, high temperature environments can greatly increase
the discharge rates.
- The temperature in which the battery
is both stored and operates (both charging and discharging
for secondary cells) is very important. Temperature can
have a great effect on the operating characteristics of
the cells making up the battery pack. Generally low temperatures
compromise performance reducing power output; high temperatures
greatly reduce battery life, e.g. increasing self-discharge
8. Weight and Dimensions
- Cells are produced in a wide range of
different sizes; custom packs designs can utilise the different
cell shapes and sizes in order to produce a pack that meets
the weight and dimension restrictions of various applications.
It is often the case that packs of different sizes can produce
the same output characteristics; in this case it is generally
the smaller, lighter version which is the most expensive.
There is however always a limit on the size and weight a
pack can be regardless of cost.
9. Charging and Cycle Life
- If the battery pack is to be re-chargeable
(which is common), then the way in which the pack is charged
and the number of charge/discharge cycles required over
its life need to be considered. Even rechargeable cells
have a limited life, generally expressed as the number of
cycles (one cycle means one discharge followed by a single
charge). In order to maximise the performance and life span
of the cell, it is important to choose the right charging
method. The application may restrict the charging methods
available and choosing the right chemistry is vital for
maximising pack performance.
- If the pack is to be used in a safety
critical application then it is important that any safety
requirements are known. Standard safety circuitry is added
to custom packs where it is necessary, however, before any
further safety features will need additional information
regarding the appropriate safety requirements.
11. Technical Information
- Depending on the application of the
battery pack, more technical information may be required
before finalising pack design. Battery
Termination is one such consideration, operation
environment and even orientation of the pack during discharge may need to be considered.
The importance of such factors will generally depend on
the criticalness of the packs application. For safety critical
applications it is important that specific details are communicated
and designs are approved before production begins to ensure
the best design choices can be made.
A common mistake when selecting a battery
for an application is using the Ampere-Hour (Ah) rating stated
on a battery to calculate how long a battery will last. For
Say a battery has a 20Ah rating, you may think that if you
took 20A from this battery that it would last 1 hour, this
is not the case. The rating given on a battery is generally
the best possible capacity of the battery at a specified current
under a defined temperature and featuring a specified cut-off
The actual capacity of a battery depends upon a number of
factors; including operating temperature, discharge current,
battery age and cut-off voltage.
A more accurate rating of a battery is
at a 10th of the Ampere-hour rating given, e.g. 3Amps for
a 30Ah battery. However, if you require your battery to power
a critical application then it is important that you consult
the manufacturers' data sheets to get exact values regarding
battery ratings under various conditions.
or start-up current
When a device is first switched on, it
is possible that very high currents may flow for a very short
time until the internal circuitry of the device reaches its
steady operating state. This is a particular problem for circuits
powering electric motors. It may be necessary to program a
delay into fast acting protection circuits to avoid false
triggering during start-up. It may also be possible to reduce
the problem by applying the load progressively rather than
Capacity (Ampere-hour) is the amount of
electrical power that can be withdrawn continuously from a
battery over a period of time. The value actually stated on
the battery is the best possible capacity at a specified current
drain under a defined temperature and featuring a specified
cut-off voltage. An example of this can be seen in the figure
It can be seen that the graph makes use
of a value C (or C rate); this is the rated capacity of the
cell. The discharge current is then expressed as a fraction
of this value. E.g. If C = 50Ah and a discharge current of
5A is applied, then this can be expressed as 0.1C (C/10);
if a current of 10A is applied, this will be expressed as
0.2C (C/5). From this it can be seen that, the larger the
fraction of C, the greater the discharge current.
An important point to make is what discharge
current the rated capacity is based upon. Say it is based
on a discharge of 5 hours, any discharge faster than 5 hours
causes' loss of efficiency and the power (Voltage delivered)
the cell can give will be reduced. This affect can be seen
from the figure above.
The cut-off voltage is the minimum voltage
level when a battery is considered no longer useable in a
After every cycle, the cell is loosing
some of its initial capacity. To begin with, this loss of
capacity is linear in its nature; however, as the cell gets
closer to the end of its life, this process becomes more rapid
until it gets to a point where the loss of capacity is so
great that the cell becomes technically dead. The cycle life
of the cell will depend on the technology, application and
charge methods used; the technical end-of-life also depends
on the technology used, a rough guide to when this is reached
is given below:
- Ni-Cd: 60% of initial capacity
- Ni-MH: 75% of initial capacity
- Li-Ion: 80% of initial capacity
The length of time it takes for the cell
to reach this point can be down to a number of factors, a
summary of these are listed below:
- The number of cells per charge string
and the number of cells in discharge (the more strings in
parallel the greater the affect).
- The depth of discharge per cycle, the
deeper the discharge, the shorter the cycle life providing
the same discharge current.
- Charging method, the decision on which
method is the best depends on the technology, configuration
- Temperatures the pack is exposed to
both during operation and storage.
- The number of internal connections;
the more connections, the greater the impedance of the battery.
- The application of the battery, whether
it be cycling or standby function and whether the current
drawn is constant or pulse.
There are a number of different ways in
which a battery can be terminated, various tag, pin and wire
arrangements are available. The type of termination very much
depends on the packs application. Follow the following link
to view various pin and tag options available Custom Tag and
Environment and Orientation
The physical environment the battery is
stored and operated in is important and can have major effects
on performance. When looking at operating environment, important
factors are temperature, humidity, altitude/pressure, vibration
and magnetic properties; these affect the type of cell used
and the way in which it is packed, i.e. the type of sleeving
The orientation of the pack relates to
the position of the positive and negative contacts of the
battery during discharge. The effects of orientation depend
on the mechanical cell design and system properties and cause
certain dependencies of available capacity on cell orientation.
The electrolyte inside the cell has a tendency to move towards
the void and inactive space of the battery if the orientation
deviates from the preferred direction. The capillary effect
of the cathode and separator pores acts against this tendency.
The orientation effect is smaller for thin cathodes than it
is for thick ones. The general effect on cell capacity from
orientation is summarised below:
- Throughout nominal discharge current
range, available capacity is practically unaffected if batteries
are discharged upright or horizontally.
- The same applies at low discharge currents
or infrequent, short, high current discharge pulses.
- With small and flat cells (AA, 2/3AA,
1/2AA, 1/6D, 1/10D, BEL), the effects of orientation are
minimal even with high discharge currents.
- Bigger cells (C, D, and DD) are affected
when discharged upside down with high discharge currents,
this orientation should be avoided.
- If cells are moved occasionally during
discharge, available capacities of all cell sizes are not
affected by orientation.