 Profile Ni-MH
BatteryStorage
Characterictics |
|
|
STORAGE
CHARACTERISTICS |
|
 Ni-MH
BATTERY |
|
 Specification
tables |
Essentially
all rechargeable battery cells gradually discharge over time whether
they are
used or not. This capacity loss is typically due to slow parasitic
reactions occurring
within
the cell. As such, the loss rate(self-discharge rate) is a function
of the
cell chemistry and the
temperature environment experienced by the cell. Due to the
temperature sensitivity of the
self-discharge reactions, relatively small differences in
storage temperature may result in large
differences in self-discharging rate. Extended
storage with a load connected not only
speeds the discharge process, but may
also cause chemical changes after the cell is discharged,
which may be difficult or
impossible to reverse.Cell
and battery storage issues of concern to most application designers
relate either to
the speed with which the cells lose their capacity after being charged
or the ability
of
the cells to
charge and discharge "normally" after storage for some
period of time. In
both situations,
general guidelines developed for nickel-cadmium cells will work
acceptably
for
nickel-metal hydride cells.
1. Retained
CapacityFigure 24 illustrates the amount of capacity available from nickel-metal hydride cells after standing for a given number of days in four different thermal environments. The common rule of thumb for nickel-cadmium cells that a 10 Celsius increase in storage temperature halves the time required for a cell to self-discharge to a given level remains approximately correct for nickel-metal hydride cells.  |
| Overview |
|
| Features |
|
| Comparison
of Ni-MH and Ni-Cd Cells |
|
| Major
applications |
|
| Structural
designs |
|
| Electrochemical
processes |
|
| Discharge
characteristics |
|
| Charge
characteristics |
|
| Charging
methods |
|
| Cycle
life characteristics |
|
| Storage
characteristics |
|
| Safety
characteristics |
|
| Designing
for Ni-MH cells |
|
| Battey
pack designs |
|
| Battery
pack configurations designation system |
|
| Precautions
for using Ni-MH batteries |
|
| Battery
selection |
|
Figure
24.Self-Discharge Characteristics for Nickel-Metal Hydride Cells |
|
| 2. Recommended Storage Conditions | |
Storage
recommendations for nickel-metal hydride cells parallel those for
nickel-cadmium
cells: |
|
>Store
at the lowest feasible temperatures(0 to 30 celsuis being the generally
recommended
storage temperatures). |
|
| >Store cells/batteries open-circuit to eliminate loaded storage effects(see next page). | |
>Storage
in a clean, dry, protected environment to minimize physical damage
to batteries. |
|
| >Use good inventory practices(first in, first out)to reduce time cells spend in storage. | |
| 3. Capacity Recovery After Storage | |
In
normal practice, stored cells will provide full capacity on the
first discharge after removal
from storage and charging with standard methods. Cells stored for
an extended
period or at elevated temperatures may require more than one cycle
to attain
pre-storage capacities. Consultation
with the manufacturer is recommended if prolonged storage and rapidrestoration
of capacity is planned. |
|
| 4. Loaded Storage | |
Cells
and batteries intended for storage for extended periods of time(pass
the point where
they are fully discharged)should be removed from their load. In
particular, many
portable electronic devices place a very low-level drain requirement
on their batteries
even when in the "off" position. These micro-current loads
may be sustaining
volatile memory, powering sense circuits or even maintaining switch
positions.
Such loads should be eliminated when storing devices for protracted
periods.When
nickel-metal hydride cells are stored under load, small quantities
of electrolyte can
ultimately begin to seep around the seals or through the vent. This
creepleakagemay
result in the formation of crystals of potassium carbonate, which
detract
cosmetically from the appearance of the cell. In extreme cases,
creep leakage can result
in corrosion of cells, batteries, or the adjoining componetry. Although
such occurrences
are rare, positive methods of electrically isolating the cell, such
as an
insulating tape over the positive terminal or removal from the product,
are suggested for
applications requiring extended storage of cells. |
|
