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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.
Charge IC Chips
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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