Types of battery chargers
Simple
A simple charger works by connecting
a constant DC power source to the
battery being charged. The simple
charger does not change its output
based on time or the charge on the
battery. This simplicity means that
a simple charger is inexpensive, but
there is a tradeoff in quality.
Typically, a simple charger takes
longer to charge a battery to
prevent severe over-charging. Even
so, a battery left in a simple
charger for a long period will be
weakened or destroyed due to
over-charging.
Trickle
A trickle charger is a kind of
simple charger that charges the
battery slowly, at the
self-discharge rate. A trickle
charger is the slowest kind of
battery charger. A battery can be
left in a trickle charger
indefinitely. Leaving a battery in a
trickle charger keeps the battery
"topped up" but never over-charges
the battery.
Timer-based
The output of a timer charger is
terminated after a pre-determined
time. Timer chargers were the common
type for high capacity Ni-Cad cells
in the 1990s for example
(low-capacity consumer Ni-Cad cells
were typically charged with a simple
charger).
Often a timer charger and set of
batteries could be bought as a
bundle and the charger time was set
to suit those batteries. If
batteries of lower capacity were
charged then they would be
overcharged, and if batteries of
higher capacity were charged they
would be only partially charged.
With the trend for battery
technology to increase capacity year
on year, an old timer charger would
only partially charge the newer
batteries.
Timer based chargers also had the
drawback that charging batteries
that were not fully discharged, even
if those batteries were of the
correct capacity for the particular
timed charger, would result in
over-charging.
Fast
Fast chargers make use of control
circuitry in the batteries being
charged to rapidly charge the
batteries without damaging the
cells' elements. Most such chargers
have a cooling fan to help keep the
temperature of the cells under
control. Most are also capable of
acting as a standard overnight
charger if used with standard NiMH
cells that do not have the special
control circuitry. Some fast
chargers, such as those made by
Energizer, can fast-charge any NiMH
battery even if it does not have the
control circuit.
Intelligent
Output current depends upon the
battery's state. An intelligent
charger may monitor the battery's
voltage, temperature and/or time
under charge to determine the
optimum charge current at that
instant. Charging is terminated when
a combination of the voltage,
temperature and/or time indicates
that the battery is fully charged.
For Ni-Cad and NiMH batteries, the
voltage across the battery increases
slowly during the charging process,
until the battery is fully charged.
After that, the voltage decreases,
which indicates to an intelligent
charger that the battery is fully
charged. Such chargers are often
labeled as a ΔV, or "delta-V,"
charger, indicating that they
monitor the voltage change.
However, the magnitude of "delta-V"
can become small or even nonexistent
if (very) high capacity rechargeable
batteries are recharged. This can
cause even an intelligent battery
charger to not sense that the
batteries are actually already fully
charged, and continue charging.
Overcharging of the batteries
result.
A typical intelligent charger
fast-charges a battery up to about
85% of its maximum capacity in less
than an hour, then switches to
trickle charging, which takes
several hours to top off the battery
to its full capacity.
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