Discharging at high and
low temperature
Batteries function best at room temperature. Operating batteries at an
elevated temperature dramatically shortens their life. Although a
lead-acid battery may deliver the highest capacity at temperatures above
30ºC, prolonged use under such conditions decreases the life of
the battery. Similarly, a lithium-ion performs better at high
temperatures. Elevated temperatures temporarily counteract the battery's
internal resistance, which may have advanced as a result of aging. The
energy gain is short-lived because elevated temperature promotes aging
by further increasing the internal resistance.
There is one exception to
running a battery at high temperature - it is the lithium-polymer with
dry solid polymer electrolyte, the true 'plastic battery'. While the
commercial lithium-ion polymer uses some moist electrolyte to enhance
conductivity, the dry solid polymer version depends on heat to enable
sufficient ion flow. This requires that the battery core be kept at an
operation temperature of 60ºC to 100ºC .
The dry solid polymer
battery has found a niche market as backup power in warm climates. The
battery is kept at the operating temperature with built-in heating
elements that is fed by the utility grid during normal operation. On a
power outage, the battery would need to provide its own power to
maintain the temperature. Although said to be long lasting, price is an
obstacle.
Nickel-metal-hydride
degrades rapidly if cycled at higher ambient temperatures. For example,
if operated at 30ºC, the cycle life is reduced by 20%. At 40ºC,
the loss jumps to a whopping 40%. If charged and discharged at
45ºC, the cycle life is only half of what can be expected if
used at moderate room temperature. The nickel-cadmium is also affected
by high temperature operation, but to a lesser degree.
At low temperatures, the
performance of all battery chemistries drops drastically. While -20ºC
is threshold at which the nickel-metal-hydride, sealed lead-acid
and lithium-ion battery cease to function, the nickel-cadmium can go
down to -40ºC. At that frigid temperature, the nickel-cadmium is
limited to a discharge rate of 0.2C (5 hour rate). There are new types
of Li?ion batteries that are said to operate down to -40ºC.
It is important to remember
that although a battery may be capable of operating at cold
temperatures, this does not automatically allow charging under those
conditions. The charge acceptance for most batteries at very low
temperatures is extremely confined. Most batteries need to be brought up
to temperatures above the freezing point for charging. Nickel-cadmium
can be recharged at below freezing provided the charge rate is reduced
to 0.1C.
Pulse discharge
Battery chemistries react
differently to specific loading requirements. Discharge loads range from
a low and steady current used in a flashlight, to sharp current pulses
for digital communications equipment, to intermittent high current
bursts in a power tool and to a prolonged high current load for an
electric vehicle traveling at highway speed. Because batteries are
chemical devices that must convert higher-level active materials into an
alternate state during discharge, the speed of such transaction
determines the load characteristics of a battery. Also referred to as
concentration polarization, the nickel and lithium-based batteries are
superior to lead-based batteries in reaction speed.
The lead-acid battery
performs best at a slow 20-hour discharge. A pulse discharge also works
well because the rest periods between the pulses help to disperse the
depleted acid concentrations back into the electrode plate. A discharge
at 1C of the rated capacity yields the poorest efficiency. The lower
level of conversion, or increased polarization, manifests itself in a
momentary higher internal resistance due to the depletion of active
material in the reaction.
Different discharge
methods, notably pulse discharging, affect the longevity of some battery
chemistries. While nickel-cadmium and lithium-ion are robust and show
minimal deterioration when pulse discharged, the nickel-metal-hydride
exhibits a reduced cycle life when powering a digital load.
In a recent study, the
longevity of nickel-meal-hydride was observed by discharging with analog
and digital loads to 1.04V/cell. The analog discharge current was 500mA;
the digital mode simulated the load requirements of the Global System
for Mobile Communications (GSM) protocol and applied 1.65-ampere peak
current for 12 ms every 100 ms and a standby current of 270mA. (Note
that the GSM pulse for voice is about 550 ms every 4.5 ms).
With the analog discharge,
the nickel-metal-hydride provided an above average service life. At 700
cycles, the battery still provided 80% capacity. By contrast, the cells
faded more rapidly with a digital discharge. The 80% capacity threshold
was reached after only 300 cycles. This phenomenon indicates that the
kinetic characteristics for the nickel-metal-hydride deteriorate more
rapidly with a digital rather than an analog load. lithium and lead-acid
systems are less sensitive to pulsed discharge than
nickel-metal-hydride.
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