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Some charger manufacturers claim amazingly short
charge times of 30 minutes or less. With well-balanced
cells and operating at moderate room temperatures, NiCd batteries
designed for fast charging can indeed be charged in a very
short time. This is done by simply dumping in a high charge
current during the first 70 percent of the charge cycle.
Some NiCd batteries can take as much a 10C, or ten times the
rated current. Precise SoC detection and temperature monitoring
are essential.
The high charge current must be reduced to lower
levels in the second phase of the charge cycle because the
efficiency to absorb charge is progressively reduced as the
battery moves to a higher SoC. If the charge current remains
too high in the later part of the charge cycle, the excess
energy turns into heat and pressure. Eventually venting occurs,
releasing hydrogen gas. Not only do the escaping gases deplete
the electrolyte, they are also highly flammable!
Several manufacturers offer chargers that claim
to fully charge NiCd batteries in half the time of conventional
chargers. Based on pulse charge technology, these chargers
intersperse one or several brief discharge pulses between
each charge pulse. This promotes the recombination of oxygen
and hydrogen gases, resulting in reduced pressure buildup
and a lower cell temperature. Ultra-fast-chargers based on
this principle can charge a nickel-based battery in a shorter
time than regular chargers, but only to about a 90 percent
SoC. A trickle charge is needed to top the charge to 100 percent.
Pulse chargers are known to reduce the crystalline
formation (memory) of nickel-based batteries. By using these
chargers, some improvement in battery performance can be realized,
especially if the battery is affected by memory. The pulse
charge method does not replace a periodic full discharge.
For more severe crystalline formation on nickel-based batteries,
a full discharge or recondition cycle is recommended to restore
the battery.
Ultra-fast charging can only be applied to healthy
batteries and those designed for fast charging. Some cells
are simply not built to carry high current and the conductive
path heats up. The battery contacts also take a beating if
the current handling of the spring-loaded plunger contacts
is underrated. Pressing against a flat metal surface, these
contacts may work well at first, and then wear out prematurely.
Often, a fine and almost invisible crater appears on the tip
of the contact, which causes a high resistive path or forms
an isolator. The heat generated by a bad contact can melt
the plastic.
Another problem with ultra-fast charging is servicing
aged batteries that commonly have high internal resistance.
Poor conductivity turns into heat, which further deteriorates
the cells. Battery packs with mismatched cells pose another
challenge. The weak cells holding less capacity are charged
before those with higher capacity and start to heat up. This
process makes them vulnerable to further damage.
Many of today’s fast chargers are designed
for the ideal battery. Charging less than perfect specimens
can create such a heat buildup that the plastic housing starts
to distort. Provisions must be made to accept special needs
batteries, albeit at lower charging speeds. Temperature sensing
is a prerequisite.
The ideal ultra-fast charger first checks the
battery type, measures its SoH and then applies a tolerable
charge current. Ultra-high capacity batteries and those that
have aged are identified, and the charge time is prolonged
because of higher internal resistance. Such a charger would
provide due respect to those batteries that still perform
satisfactorily but are no longer ‘spring chickens’.
The charger must prevent excessive temperature
build-up. Sluggish heat detection, especially when charging
takes place at a very rapid pace, makes it easy to overcharge
a battery before the charge is terminated. This is especially
true for chargers that control fast charge using temperature
sensing alone. If the temperature rise is measured right on
the skin of the cell, reasonably accurate SoC detection is
possible. If done on the outside surface of the battery pack,
further delays occur. Any prolonged exposure to a temperature
of 45°C (113°F) harms the battery.
New charger concepts are being studied which
regulate the charge current according to the battery's charge
acceptance. On the initial charge of an empty battery when
the charge acceptance is high and little gas is generated,
a very high charge current can be applied. Towards the end
of a charge, the current is tapered down.
Newer battery systems demand more complex chargers
than batteries with older chemistries. With today’s charge
IC chips, designing a charger has been simplified. These chips
apply proven charge algorithms and are capable of servicing
all major battery chemistries. As the price of these chips
decreases, design engineers make more use of this product.
With the charge IC chip, an engineer can focus entirely on
the portable equipment rather than devoting time to developing
a charging circuit.
The charge IC chips have some limitations, however.
The charge algorithm is fixed and does not allow fine-tuning.
If a trickle charge is needed to raise a Li-ion that
has dropped below 2.5V/cell to its normal operating voltage,
the charge IC may not be able to perform this function. Similarly,
if an ultra-fast charge is needed for nickel-based batteries,
the charge IC applies a fixed charge current and does not
take into account the SoH of the battery. Furthermore, a temperature
compensated charge would be difficult to administer if the
IC chips do not provide this feature.
Using a small micro controller is an alternative
to selecting an off-the-shelf charge IC. The hardware cost
is about the same. When opting for the micro controller, custom
firmware will be needed. Some extra features can be added
with little extra cost. They are fast charging based on the
SoH of the battery. Ambient temperatures can also be taken
into account. Whether an IC chip or micro controller is used,
peripheral components are required consisting of solid-state
switches and a power supply.
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