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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 30°C (86°F), prolonged use
under such conditions decreases the life of the battery.
Similarly, a Li-ion performs better at high temperatures.
Elevated temperatures temporarily counteracts the battery’s
internal resistance, which is 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 Li-ion polymer
uses some moist electrolyte to enhance conductivity, the dry
solid polymer version depends on heat to enable ion flow.
This requires that the battery core be kept at an operation
temperature of 60°C to 100°C.
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. During
normal operation, the core is kept warm with power derived
from the utility grid. Only on a power outage would the battery
need to provide power to maintain its own heat. To minimize
heat loss, the battery is insulated.
The Li-ion polymer as standby battery is said to outperform
VRLA batteries in terms of size and longevity, especially
in shelters in which the temperature cannot be controlled.
The high price of the Li-ion polymer battery remains an obstacle.
The NiMH chemistry degrades rapidly if cycled at higher
ambient temperatures. Optimum battery life and cycle count
are achieved at 20°C (68°F). Repeated charging and discharging
at higher temperatures will cause irreversible capacity loss.
For example, if operated at 30°C (86°F), the cycle life is
reduced by 20 percent. At 40°C (104°F), the loss jumps
to a whopping 40 percent. If charged and discharged at
45°C (113°F), the cycle life is only half of what can be expected
if used at moderate room temperature. The NiCd is also affected
by high temperature operation, but to a lesser degree.
At low temperatures, the performance of all battery chemistries
drops drastically. While -20°C (-4°F) is threshold at which
the NiMH, SLA and Li-ion battery stop functioning, the NiCd
can go down to -40°C (-40°F). At that frigid temperature,
the NiCd 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 -40°C.
It is important to remember that although a battery may
be capable of operating at cold temperatures, this does not
automatically mean it can also be charged 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. The
NiCd can be recharged at below freezing provided the charge
rate is reduced to 0.1C.
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