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Chapter 14: Non-Correctable Battery Problems
Non-correctable battery problems are those that cannot be improved
through external means such as giving the battery a full charge
or by applying repeated charge/discharge cycles. Deficiencies
that denote the non-correctable status are high internal resistance,
elevated self-discharge, electrical short of one or several
cells, lack of electrolyte, oxidation, corrosion and general
chemical breakdown. These degenerative effects are not only
caused by normal usage and aging, but they include less than
ideal field conditions and an element of neglect. The user may
have poor charging equipment, may operate and store the battery
in adverse temperatures and, in the case of nickel-based batteries,
may not maintain the battery properly.
New battery packs are not exempt from deficiency
syndromes and early failure. Some batteries may be kept in
storage too long and sustain age-related damage, others are
returned by the customer because of incorrect user preparation.
In this section we examine the cause of non-correctable
battery problems and explore why they occur. We also look
at ways to minimize premature failure.High Self-discharge
Self-discharge is a natural phenomenon of any
battery. It is not a manufacturing defect per se, although
poor manufacturing practices and improper maintenance and
storage by the consumer enhance the problem.
The level of self-discharge differs with each
chemistry and cell design. High-performance nickel-based batteries
with enhanced electrode surface area and super conductive
electrolyte are subject to higher self-discharge than the
standard version cell with lower energy densities. Self-discharge
is non linear and is highest right after charge when the battery
holds full capacity.
NiCd and NiMH battery chemistries exhibit a high
level of self-discharge. If left on the shelf, a new NiCd
loses about 10 percent of its capacity in the first 24 hours
after being removed from the charger. The rate of self-discharge
settles to about 10 percent per month afterwards. At
a higher temperature, the self-discharge rate increases substantially.
As a rule, the rate of self-discharge doubles with every 10°C
(18°F) increase in temperature. The self-discharge of the
NiMH is about 30 percent higher than that of the NiCd.
A major contributor to high self-discharge on
nickel and lead-based batteries is a high cycle count and/or
old age. With increased cycles, the battery plates tend to
swell. Once enlarged, the plates press more firmly against
the delicate separator, resulting in increased self-discharge.
This is common in aging NiCd and NiMH batteries but can also
be seen in lead acid systems.
Loading less active materials on the plates can
reduce the plate swelling on nickel-based batteries. This
improves expansion and contraction while charging and discharging.
In addition, the load characteristic is enhanced and the cycle
life prolonged. The downside is lower capacity.
Metallic dendrites penetrating into the separator
are another cause of high self-discharge. The dendrites are
the result of crystalline formation, also known as memory.
Once marred, the damage is permanent. Poorly designed chargers
that ‘cook’ the batteries also increase the self-discharge.
High cell temperature causes irreversible damage to the separator.
While the nickel-based systems can withstand
some abuse and tolerate innovative or crude charge methods,
the Li-ion demands tight charging and discharging regimes.
Keeping the voltage and current within firm boundaries prevents
the growth of dendrites. The presence of dendrites in lithium-based
batteries has more serious implications than just an increase
in self-discharge — dendrites can cause an electrical short,
which could lead to venting with flame.
The self-discharge of the Li-ion battery
is five percent in the first 24 hours after charge
and averages 1 to 2 percent per month thereafter. In
addition to the natural self-discharge through the chemical
cell, the safety circuit draws as much as 3 percent per
month. High cycle count and aging has little effect on self-discharge
on lithium-based batteries.
An SLA self-discharges at a rate of only five percent
per month or 50 percent per year. Repeated deep cycling
increases the self-discharge. When deep cycling, the electrolyte
is drawn into the separator, resulting in a crystalline formation
similar to that of a NiCd battery.
The self-discharge of a battery is best measured
with a battery analyzer. The procedure starts by charging
the battery. The capacity is read by applying a controlled
discharge. The battery is then recharged and put on a shelf
for 24 hours, after which the capacity is measured again.
The discrepancy between the capacity readings reveals the
level of self-discharge.
More accurate self-discharge measurements can
be obtained by allowing the battery to rest for at least 72 hours
before taking the reading. The longer rest period compensates
for the relatively high self-discharge immediately after charge.
At 72 hours, the self-discharge should be between 15
and 20 percent. The most uniform self-discharge readings
are obtained after seven days. On some battery analyzers,
the user may choose to adjust the desired rest periods in
which the self-discharge is measured.
Research is being conducted to find a way to
measure the self-discharge of a battery in minutes, if not
seconds. The accuracy and repeatability of such technology
is still unknown. The challenge is finding a formula that
applies to all major batteries and includes the common chemistries.
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