Reliability is the probability that an item can perform its
intended function for a specified interval under stated conditions. This is
only possible if the battery is designed with an adequate margin of
performance when it is new. All batteries, being electrochemical devices,
deteriorate with time. This deterioration results in a reduced battery
capacity. The reduced capacity must be taken into account during development
by overdesigning the battery to compensate for end-of-life requirements. Rate
of deterioration is very difficult to determine for some batteries.
Deterioration may be very slow in batteries that are properly manufactured,
but may be very rapid if manufacturing conditions are not properly maintained.
Reliability data may be gathered during Test and Evaluation, but some modes of
deterioration may take many years to appear. Unfortunately, accelerated
testing has not proven effective in such situations because of the difficulty
in controlling the effects of overstressing one parameter without affecting
Battery system reliability is a function of all the internal and
external variables which directly impact the battery performance. All
batteries are exposed to external environmental, functional, and mechanical
variables that can damage electrical and mechanical components and alter
electrochemical components. Substandard battery performance can result from
exposure to these external variables. Workmanship and quality of components
are critical internal variables that affect battery performance. A battery
system may consist of a single cell or complex multicell packs containing one
or more electronic or mechanical components, i.e., thermal switches, sensors,
heaters, gas generators, squibs, diodes, and venting systems. The more complex
battery systems, which have more internal variables to malfunction, should
only be used where the operating environment and the functional requirements
demand a more sophisticated technology. Ultimately good battery system
reliability is the responsibility of the system manager. The system manager's
knowledge of the full operating environment and a realistic view of the
functional requirements of the battery system are imperative if the correct
battery is to be chosen.
Data base information is available for some battery systems and
can be used to specify expected reliability. However, in some applications
sufficient data are not available.
To determine projected reliability:
Determine battery use profile and environment as accurately
Plan for real world maintenance, if applicable.
Compare to similar applications.
Develop and test to realistic cycle life regime.
Consider system integration.
Determine and base predicted reliability on an appropriate
use factor such as cycles (depth of discharge, frequency of discharge),
operating hours, sorties or engine starts.
Maintainability is the measure of the ability of an item to be
retained in, or restored to, a specified condition. Maintainability is a
function of many variables. Most non-rechargeable batteries are manufactured
to require little or no maintenance while they are installed in the system.
Maintenance may consist of replacement when their service life has been
exceeded. Rechargeable batteries require regular recharging after each use,
and periodic charging when in the standby mode. This requires a resident
charging system that is designed for that particular battery type, or removal
and off-line recharging in a shop with special equipment and personnel.
In addition to recharging, many rechargeable batteries require
water addition or other maintenance activities. Some systems may be maintained
in place by water addition. Other batteries must be removed for water
addition, cell equalization, deep cycle maintenance, or cleaning. Sealed cells
may require periodic deep cycle maintenance.
Water addition to rechargeable vented cells is the primary
reason for battery maintenance. Charging systems which minimize gassing can
appreciably lengthen the period between scheduled maintenances. Unscheduled
maintenance is typically the result of abnormal use or system failure. When
designing battery installations, consider maintenance requirements. Many
maintenance hours are spent in discharging and charging batteries and adding
In designing for maintainability, consider the following
Coordinated system design
Design to standard maintenance requirements
Limitations of battery systems
Trade-off considerations such as increased electrolyte
reservoir versus increased weight
Charger/controller versus unregulated bus
Sealed cell versus vented cell technology
Adequate maintenance instructions.
Availability is a measure of the degree to which an item is in
an operable and committable state at the start of a mission. All batteries
require a finite amount of time to become active if they are not already on
line when the system is called upon to perform its assigned task. If a
rechargeable battery is always on-line, then it must be recharged periodically
to maintain a full state of charge. This requires a charging system designed
for the particular battery being used in the