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 restrictions.

4. Cost

• 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:

• 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.

7. Temperature

• 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 rates.

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.

10. Safety

• 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.

Additional Information

Discharge Current

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 example:

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 voltage.
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.

Surge 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 instantaneously.

Capacity

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 below:

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.

Cut-off voltage

The cut-off voltage is the minimum voltage level when a battery is considered no longer useable in a given application.

Cycle Life

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 and application.
• 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.

Battery Termination

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 Pin Options.

Operation 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 used.

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.