|
The charge process of a Li-Polymer is similar
to that of the Li-ion. Li-Polymer uses dry electrolyte
and takes 3 to 5 hours to charge. Li-ion polymer
with gelled electrolyte, on the other hand, is almost identical
to that of Li-ion. In fact, the same charge algorithm
can be applied. With most chargers, the user does not need
to know whether the battery being charged is Li-ion
or Li-ion polymer.
Almost all commercial batteries sold under the
so-called Polymer category are a variety of the
Li-ion polymer using some sort of gelled electrolyte.
A low-cost dry polymer battery operating at ambient temperatures
is still some years away.
Rechargeable batteries can be used under a reasonably
wide temperature range. This, however, does not automatically
mean that the batteries can also be charged at these temperature
conditions. While the use of batteries under hot or cold conditions
cannot always be avoided, recharging time is controlled by
the user. Efforts should be made to charge the batteries only
at room temperatures.
In general, older battery technologies such as
the NiCd are more tolerant to charging at low and high temperatures
than the more advanced systems. Figure 4-6
indicates the permissible slow and fast charge temperatures
of the NiCd, NiMH, SLA and Li-ion.
|
|
Slow
Charge (0.1)
|
Fast
Charge (0.5-1C)
|
|
Nickel Cadmium
|
0°C
to 45°C (32°F to 113°F)
|
5°C
to 45°C (41°F to 113°F)
|
|
Nickel-Metal Hydride
|
0°C
to 45°C (32°F to 113°F)
|
10C°
to 45°C (50°F to 113°F)
|
|
Lead Acid
|
0°C
to 45°C (32°F to 113°F)
|
5C°
to 45°C (41°F to 113°F)
|
|
Lithium Ion
|
0°C
to 45°C (32°F to 113°F)
|
5C°
to 45°C (41°F to 113°F)
|
Figure 4-6: Permissible temperature
limits for various batteries.
Older battery technologies are more
tolerant to charging at extreme temperatures than newer, more
advanced systems.
NiCd batteries can be fast-charged in an hour
or so, however, such a fast charge can only be applied within
temperatures of 5°C and 45°C (41°F and 113°F). More moderate
temperatures of 10°C to 30°C (50°F to 86°F) produce better
results. When charging a NiCd below 5°C (41°F), the ability
to recombine oxygen and hydrogen is greatly reduced and pressure
build up occurs as a result. In some cases, the cells vent,
releasing oxygen and hydrogen. Not only do the escaping gases
deplete the electrolyte, hydrogen is highly flammable!
Chargers featuring NDV to terminate full-charge
provide some level of protection when fast-charging at low
temperatures. Because of the batterys poor charge acceptance
at low temperatures, the charge energy is turned into oxygen
and to a lesser amount hydrogen. This reaction causes cell
voltage drop, terminating the charge through NDV detection.
When this occurs, the battery may not be fully charged, but
venting is avoided or minimized.
To compensate for the slower reaction at temperatures
below 5°C, a low charge rate of 0.1C must be applied. Special
charge methods are available for charging at cold temperatures.
Industrial batteries that need to be fast-charged at low temperatures
include a thermal blanket that heats the battery to an acceptable
temperature. Among commercial batteries, the NiCd is the only
battery that can accept charge at extremely low temperatures.
Charging at high temperatures reduces the oxygen
generation. This reduces the NDV effect and accurate full-charge
detection using this method becomes difficult. To avoid overcharge,
charge termination by temperature measurement becomes more
practical.
The charge acceptance of a NiCd at higher temperatures
is drastically reduced. A battery that provides a capacity
of 100 percent if charged at moderate room temperature
can only accept 70 percent if charged at 45°C (113°F),
and 45 percent if charged at 60°C (140°F) (see Figure 4-7).
Similar conditions apply to the NiMH battery. This demonstrates
the typically poor summer performance of vehicular mounted
chargers using nickel-based batteries.
Another reason for poor battery performance,
especially if charged at high ambient temperatures, is premature
charge cutoff. This is common with chargers that use absolute
temperature to terminate the fast charge. These chargers read
the SoC on battery temperature alone and are fooled when the
room temperature is high. The battery may not be fully charged,
but a timely charge cut-off protects the battery from damage
due to excess heat.
The NiMH is less forgiving than the NiCd if charged
under high and low temperatures. The NiMH cannot be fast charged
below 10°C (45°F), neither can it be slow charged below 0°C
(32°F). Some industrial chargers adjust the charge rate to
prevailing temperatures. Price sensitivity on consumer chargers
does not permit elaborate temperature control features.

Figure 4-7: Effects of temperature
on NiCd charge acceptance.
Charge acceptance is much reduced
at higher temperatures. NiMH cells follow a similar pattern.
The lead acid battery is reasonably forgiving
when it comes to temperature extremes, as in the case of car
batteries. Part of this tolerance is credited to the sluggishness
of the lead acid battery. A full charge under ten hours
is difficult, if not impossible. The recommended charge rate
at low temperature is 0.3C.
Figure 4-8 indicates the optimal peak
voltage at various temperatures when recharging and float
charging an SLA battery. Implementing temperature compensation
on the charger to adjust to temperature extremes prolongs
the battery life by up to 15 percent. This is especially
true when operating at higher temperatures.
An SLA battery should never be allowed to freeze.
If this were to occur, the battery would be permanently damaged
and would only provide a few cycles when it returned to normal
temperature.
|
|
| |
0°C
(32°F) |
25°C
(77°F) |
40°C
(104°F) |
|
|
| Voltage
limit on recharge |
2.55V/cell |
2.45V/cell |
2.35V/cell |
| Continuous
float voltage |
2.35V/cell
or lower |
2.30V/cell
or lower |
2.25V/cell
or lower |
|
|
Figure 4-8: Recommended voltage limits
on recharge and float charge of SLAs.
These voltage limits should be applied
when operating at temperature extremes.
To improve charge acceptance of SLA batteries
in colder temperatures, and avoid thermal runaway in warmer
temperatures, the voltage limit of a charger should be compensated
by approximately 3mV per cell per degree Celsius. The voltage
adjustment has a negative coefficient, meaning that the voltage
threshold drops as the temperature increases. For example,
if the voltage limit is set to 2.40V/cell at 20°C, the setting
should be lowered to 2.37V/cell at 30°C and raised to 2.43V/cell
at 10°C. This represents a 30mV correction per cell per 10
degrees Celsius.
The Li-ion batteries offer good cold and
hot temperature charging performance. Some cells allow charging
at 1C from 0°C to 45°C (32°F to 113°F). Most Li-ion
cells prefer a lower charge current when the temperature gets
down to 5°C (41°F) or colder. Charging below freezing must
be avoided because plating of lithium metal could occur.
|