The overall system specification generally does not specify the power source. After system requirements are well defined, the power source is specified. If a battery is desired, a general battery specification is written around a particular electrochemical system on the basis of various trade-off factors.
Any battery design effort starts with a clear definition of the system requirements. Topics to be defined include:
- Discharge Loads
- Voltage (including maximum, average, and minimum)
- Discharge Time
- Operation Environment (temperature, shock, vibration, and humidity)
- Transportation and Storage Environment
- Flammable and Hazardous Materials
The interrelationships of these requirements will not always be apparent to the user. It is important to define the actual requirements in detail. Batteries are electrochemical systems which produce less power when cold. Rechargeable cells often need to be cycled when new or after having been stored to obtain the best performance and suffer from a decline in performance with use. Performance is also affected by their environment and method of charging. They may have memory, a condition which occurs when they are subjected to repetitive small discharges.
A successful acquisition program will consider the battery and its space allocation and location from conception to the completion of system design. As the system design evolves, electrical demand on the battery often grows; battery size and space requirements grow as well. Frequently, the battery and its location in a system are an afterthought and the battery ends up in whatever space is left available. This space is often small and irregular in shape, promoting the use of non-standard battery shapes or it may be subject to undesirable environmental conditions.
The system designer will recognize that requirements for one aspect of the battery will often restrict options in other areas. Unnecessary limits on weight and volume are to be avoided. Requirements for a closely controlled voltage may eliminate cell types with sloping discharge profiles or initial voltage spikes. If the design environmental requirements understate the real world, the battery may present reliability and safety risks during use, but demanding unnecessary environmental specifications may negatively affect other parameters such as weight, volume, and cost.
Table 4-1 is a sample application checklist which lists some of the considerations in selecting a battery system.
After stating battery loads (power or current) and time required, define the minimum and maximum acceptable voltages. End of life is defined as failure to meet minimum performance levels.
Space and Weight
Does the maximum battery envelope include connectors, sensors, etc? If a rechargeable battery is used, remember to allow space for the battery charger. Specify also the desirable weight.
Specify operating temperature and electrical requirements at extreme temperatures, non-operating temperatures, altitude, acceleration, and environment the battery has to withstand.
The size, shape, and location of mechanical parts, sensors, connectors, and other items are indicated in the drawing package. Connectors and sensors are specified to mate with the rest of the system.
Small positioning and dimensional tolerances are to be avoided to simplify manufacturing and aid in keeping cost down. Temperature variations can cause materials to expand or contract more than the tolerance.
The time scale for the project will dictate whether an existing battery must be identified. Several years of development maybe required if no existing battery is adequate.
An expensive battery with a high cycle life can be a more economical selection than an inexpensive lower cycle life battery when the number and depth of cycles per day as well as expected life of the battery are considered.
There is economic and logistic justification for standardization of batteries for various applications. Take into consideration the trade-offs involved. However, the need for standardization should not jeopardize the function and efficiency of the selected battery, and thereby, the mission.