|
Is the runtime of a portable device directly related to the
size of the battery and the energy it can hold? In most cases,
the answer is yes. But with digital equipment, the length
of time a battery can operate is not necessarily linear to
the amount of energy stored in the battery.
In this chapter we examine why the specified
runtime of a portable device cannot always be achieved, especially
after the battery has aged. We address the four renegades
that are affecting the performance of the battery. They are:
declining capacity, increasing internal resistance, elevated
self-discharge, and premature voltage cut-off on discharge.
The amount of charge a battery can hold gradually
decreases due to usage, aging and, with some chemistries,
lack of maintenance. Specified to deliver about 100 percent
capacity when new, the battery eventually requires replacement
when the capacity drops to the 70 or 60 percent
level. The warranty threshold is typically 80 percent.
The energy storage of a battery can be divided
into three imaginary sections consisting of available energy,
the empty zone that can be refilled and the rock content that
has become unusable. Figure 6-1 illustrates these
three sections of a battery.
In nickel-based batteries, the rock content may
be in the form of crystalline formation, also known as memory.
Deep cycling can often restore the capacity to full service.
Also known as ‘exercise’, a typical cycle consists of one
or several discharges to 1V/cell with subsequent discharges.
|
|
Figure 6-1:
Battery charge capacity.
Three imaginary sections of
a battery consisting of available energy, empty zone
and rock content.
With usage and age, the rock content grows. Without
regular maintenance, the user may end up carrying rocks
instead of batteries.
|
The loss of charge acceptance of the Li-ion/polymer
batteries is due to cell oxidation, which occurs naturally
during use and as part of aging. Li-ion batteries cannot
be restored with cycling or any other external means. The
capacity loss is permanent because the metals used in the
cells are designated to run for a specific time only and are
being consumed during their service life.
Performance degradation of the lead acid battery
is often caused by sulfation, a thin layer that forms on the
negative cell plates, which inhibits current flow. In addition,
there is grid corrosion that sets in on the positive plate.
With sealed lead acid batteries, the issue of water permeation,
or loss of electrolyte, also comes into play. Sulfation can
be reversed to a certain point with cycling and/or topping
charge but corrosion and permeation are permanent. Adding
water to a sealed lead acid battery may help to restore operation
but the long-term results are unpredictable.
|
