|
There are three types of chargers for nickel-based batteries.
They are:
Slow Charger — Also known as ‘overnight
charger’ or ‘normal charger’, the slow-charger
applies a fixed charge rate of about 0.1C (one tenth of the
rated capacity) for as long as the battery is connected. Typical
charge time is 14 to 16 hours. In most cases, no
full-charge detection occurs to switch the battery to a lower
charge rate at the end of the charge cycle. The slow-charger
is inexpensive and can be used for NiCd batteries only. With
the need to service both NiCd and NiMH, these chargers are
being replaced with more advanced units.
If the charge current is set correctly, a battery
in a slow-charger remains lukewarm to the touch when fully
charged. In this case, the battery does not need to be removed
immediately when ready but should not stay in the charger
for more than a day. The sooner the battery can be removed
after being fully charged, the better it is.
A problem arises if a smaller battery (lower
mAh) is charged with a charger designed to service larger
packs. Although the charger will perform well in the initial
charge phase, the battery starts to heat up past the 70 percent
charge level. Because there is no provision to lower the charge
current or to terminate the charge, heat-damaging over-charge
will occur in the second phase of the charge cycle. If an
alternative charger is not available, the user is advised
to observe the temperature of the battery being charged and
disconnect the battery when it is warm to the touch.
The opposite may also occur when a larger battery
is charged on a charger designed for a smaller battery. In
such a case, a full charge will never be reached. The battery
remains cold during charge and will not perform as expected.
A nickel-based battery that is continuously undercharged will
eventually loose its ability to accept a full charge due to
memory.
Quick Charger — The so-called quick-charger,
or rapid charger, is one of the most popular. It is positioned
between the slow-charger and the fast-charger, both in terms
of charging time and price. Charging takes 3 to 6 hours
and the charge rate is around 0.3C. Charge control is required
to terminate the charge when the battery is ready. The well
designed quick-charger provides better service to nickel-based
batteries than the slow-charger. Batteries last longer if
charged with higher currents, provided they remain cool and
are not overcharged. The quick-chargers are made to accommodate
either nickel-based or lithium-based batteries. These two
chemistries can normally not be interchanged in the same charger.
Fast Charger — The fast-charger offers
several advantages over the other chargers; the obvious one
is shorter charge times. Because of the larger power supply
and the more expensive control circuits needed, the fast-charger
costs more than slower chargers, but the investment is returned
in providing good performing batteries that live longer.
The charge time is based on the charge rate,
the battery’s SoC, its rating and the chemistry. At a
1C charge rate, an empty NiCd typically charges in a little
more than an hour. When a battery is fully charged, some
chargers switch to a topping charge mode governed by a timer
that completes the charge cycle at a reduced charge current.
Once fully charged, the charger switches to trickle charge.
This maintenance charge compensates for the self-discharge
of the battery.
Modern fast-chargers commonly accommodate both
NiCd and NiMH batteries. Because of the fast-charger’s
higher charge current and the need to monitor the battery
during charge, it is important to charge only batteries specified
by the manufacturer. Some battery manufacturers encode the
batteries electrically to identify their chemistry and rating.
The charger then sets the correct charge current and algorithm
for the battery intended. Lead Acid and Li-ion chemistries
are charged with different algorithms and are not compatible
with the charge methods used for nickel-based batteries.
It is best to fast charge nickel-based batteries.
A slow charge is known to build up a crystalline formation
on nickel-based batteries, a phenomenon that lowers battery
performance and shortens service life. The battery temperature
during charge should be moderate and the temperature peak
kept as short as possible.
It is not recommended to leave a nickel-based
battery in the charger for more than a few days, even with
a correctly set trickle charge current. If a battery must
remain in a charger for operational readiness, an exercise
cycle should be applied once every month.
|
